Agilent
8645 Signal Generator
Communication
Product Note 8645-2
A catalog of 8645A information
Table of contents
Operation related topics. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
Block diagram and theory of operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
Timebase configurations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Internal audio source. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Frequency sweep capabilities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Externally doubled outputs to 2060 MHz . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Operation as a phase noise reference . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Programming with HP-SL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Command sequence independence using HP-SL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Performance related topics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Phase noise performance. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Spurious performance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Third order intermodulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
Divided outputs below 515 MHz . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
Stereo separation quality . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
Minimizing fan noise . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
Frequency agility . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
Functional description of frequency agile operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
Faster frequency switching using multiple agile generators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
Frequency accuracy of agile outputs. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
Relating phase error and frequency accuracy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
Amplitude dynamic range while frequency hopping . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
Amplitude shaping of agile outputs. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
Modulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
High rate, high deviation FM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
Simultaneous modulations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
Digitized FM operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
AC coupled FM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
Special capabilities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
Tailored operation through special functions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
Protecting classified instrument settings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
Storage registers and sequential recall . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
Offsets and multipliers of frequency and amplitude . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
Built-in calibration functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
Finding failures with internal diagnostics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
2
This product note is actually a
compilation of many brief product notes, each concerned with a
particular aspect of the 8645A
agile signal generator. Included
in these pages are explanations
of how this unique signal generator operates, the capabilities it
has to offer and the performance
it can provide. The objective of
this product note is to be a reference guide for the owner of a
8645A, to help maximize the usefulness and performance of this
agile signal generator in the
intended application. While none
of the topics are covered in great
detail and other literature may
offer a more thorough treatment
of a subject, these summaries
should provide sufficient information to help in many situations.
Operation related topics
Block diagram and theory
of operation
The basis of the 8645A is a single fractional N loop controlling
a VCO operating in the frequency range of 515 to 1030 MHz.
The reference signal for this
phase lock loop originates from
either an internal 10 MHz oscillator or an external input. An
extensive divider section at the
output of the phase lock loop
provides coverage down to
252 kHz and a doubler in the
output section extends the frequency range to 2060 MHz. All
four modulation types are implemented in the 8645A with either
the internal 400 kHz synthesizer
integrated circuit providing the
modulation waveform or an
external input. Frequency modulation uses two techniques
including an analog signal
summed into the VCO tuning
input and a digitized FM technique that directly modifies the
fractional N number of the phase
lock loop. Phase modulation signals are summed directly into
the fractional N phase lock loop.
Pulse modulation occurs directly
after the divider section.
Amplitude modulation is accom-
plished in the output section
through control of the Automatic
Level Control (ALC). The AM signal is summed together with the
level DAC which sets the amplitude level that reaches the attenuators. The combination of the
level DAC, the AM signal, and
the attenuators (up to 120 dB of
attenuation) determine the actual output level of the 8645A. The
Reverse Power Protection (RPP)
prevents the output circuits from
damaging signals externally
input through the generator’s
output. Controlling all of this
hardware in the many states the
user can set up is a Motorola
68000 microprocessor.
The basic block diagram summarized above produces all the traditional functions of a signal
generator. For the applications
intended for the 8645A, the
phase noise and spurious signals
must be very low at offsets
greater than approximately
10 kHz. A major advantage of
the block diagram is that a
clean-up loop based on a delay
line and a phase detector can be
added in parallel to the fractional N phase lock loop. The 70 nsec
delay line in the clean-up loop of
the phase noise enhancement
section decreases the phase
noise and spurious signals to
levels required by communications hardware tests.
Besides high performance outputs for traditional applications,
the 8645A is designed to provide
sequences of many frequencies
in rapid order. Frequency
switching is specified as fast as
15 usec between frequencies. To
accomplish this switching speed,
the fractional N phase lock loop
is opened and replaced by a
delay line frequency lock loop.
Phase noise and spurious signals
on the VCO output are again
decreased by the delay line and
phase detector in the fast hop
enhancement section. VCO settings learned before fast hop
operation begins are sent to the
VCO through a pretune DAC in
the order of the output frequencies the user wants and at the
rate programmed. Amplitude
information is simultaneously
sent to the level DAC. A hardware state machine programmed
by the microprocessor provides
all the fast control signals needed while fast hop operation is
underway.
Many of the operational areas
briefly discussed on this page
are covered more thoroughly in
other parts of this product note.
Refer to the table of contents for
a listing of the topics.
3
4
Timebase
configurations
The frequency stability of the
8645A depends a great deal on
the reference oscillator in use.
The standard internal timebase
is a non-ovenized 10 MHz crystal
oscillator with a typical aging
rate of ±2 ppm per year. With
this timebase, a 1 GHz output of
the signal generator would not
vary more than ±2 kHz in a year
due to timebase aging. However,
the frequency drift due to temperature changes may be twice
this amount because this oscillator is not ovenized. Although the
8645A has several design features to minimize internal temperature fluctuations, the
standard timebase could drift by
as much as ±4 ppm over a temperature range variation of 0 to
+55 degrees centigrade.
Option 001 of the 8645A adds a
more stable 10 MHz ovenized
timebase to the instrument. The
aging rate is specified to be better +0.0005 ppm or a 0.5 Hz variation of a 1 GHz output in 24
hours after a 10 day warm-up.
Frequency drift due to a ambient
temperature change of 0 to +55
degrees centigrade is typically
less than +0.006 ppm. The frequency of this timebase can be
mechanically adjusted through a
hole in the rear panel using a
tweaker. Voltage control of the
timebase frequency is available
using the Electronic Frequency
Control (EFC) input. The maximum ±10 volt EFC input signal
will produce a ±1 Hz frequency
change of the 10 MHz output.
The output of this optional high
stability timebase is only routed
to the rear panel of the instrument as the oven ref output. An
external jumper cable is used to
input this reference signal at the
ref in port for routing into the
frequency synthesis circuits.
When this jumper cable is connected, the instrument will
sense the presence of a reference
signal at the ref in input and utilize it automatically. Without a
signal present at the ref in
input, the 8645A will use the
standard timebase as its reference oscillator.
To allow other instruments to
use the timebase signal from the
8645A, the rear panel 10 MHz
ref out output provides an output of either the standard or
optional timebase that is currently in use. The signal generator can also utilize an external
10 MHz timebase that would be
input at the ref in input.
Activating special function 161
will provide a readout indicting
whether the 8645A is utilizing
the standard timebase or a signal entering the ref in input.
5
8645A
Rear panel
Typical connection
Standard
reference
oscillator
(10 MHz)
Optional
high stability
oscillator
(10 MHz)
Switch
control
To synthesizer
Ref in
Level
detector
Oven ref
output
EFC input
10 MHz ref out
8645A internal timebase configuration
Internal audio source
The internal audio source in the
8645A can generate four basic
waveforms of sine, sawtooth,
square, and white Gaussian
noise. Waveforms are generated
by a numerical synthesis technique. The heart of the synthesizer is a Digital Waveform
Synthesis Integrated Circuit
(DWSIC). The DWSIC generates a
continuous stream of numbers
that represents instantaneous
levels of the waveform. This “digital” waveform is then converted
to an analog signal by a
digital-to-analog converter. The
analog signal is conditioned by
conventional analog circuitry
and routed to various parts of
the signal generator. The conditioning circuits include a sample
and hold to remove DAC switching noise, filters to remove quantization noise, and amplifiers to
boost the output.
The internal audio source is
used in the signal generator for
modulation, sweeping, calibration, and diagnostics. To the
user, the source appears like an
internal function generator used
to modulate the carrier with the
four basic waveforms. It is also
used as a ramp voltage into the
FM circuitry during phase continuous sweep that disallows
internal modulation being active
this sweep mode. This source is
used as an accurate DC reference to calibrate FM deviation
and AM depth when these modulations are active. The built-in
diagnostics use the source for
DC and AC signals to test various modules in the instrument.
And of course the audio signal is
available at the front panel audio
output with programmable waveforms, amplitude, and frequency.
The type of waveform produced
can be selected by activating
special function 130 or via GPIB
with the command
LFS:Waveform <type> where
<type> is sin, square, saw, or
WGN (for white Gaussian noise).
The frequency can be selected
over a range of 0.1 Hz to 400
kHz. Sawtooth and squarewave
rates should be limited to less
than 50 kHz because the output
circuitry degrades the performance at higher rates. Frequency
accuracy is equal to the internal
timebase accuracy of the instrument. Frequency switching
speed of the source is typically
less than 30 msec. Output level
is programmable and ranges
from 1 mV to 1 Vrms into a 600
ohm load with a specified accuracy of ±20 mV. Adjusting the
output level will effect the
amount of internal modulation
present such that a decrease in
output level will proportionately
decrease the amount of internal
modulation. This feature can be
used to increase the amount of
external modulation allowed
during simultaneous internal
and external modulation. The
sum of the internal and external
voltages should not exceed
1.4 Vpeak during simultaneous
modulation or clipping distortion may occur.
6
Frequency sweep
capabilities
The 8645A was designed to have
three different types of frequency sweep operation to accommodate a wide variety of applications.
As is evident from the descriptions that follow, the wide deviation FM capabilities and the fast
hop operation offer unique
sweep capabilities not present in
the typical RF signal generator.
The most useful sweep for finding the frequency response of
narrowband devices is the phase
continuous frequency sweep.
The instrument uses the wide
deviation FM circuitry to create
a phase-continuous output over
spans as wide as twice the maximum FM deviation available for
that carrier frequency range. In
the main VCO band of 515 to
1030 MHz the maximum span is
20 MHz. This range is decreased
by half for each divider band
below this main carrier band. A
sweeptime range of 10 msec to
10 seconds is allowed for any
span that is chosen. Only a linear frequency sweep is allowed.
Another capability that offers
very high accuracy of each frequency point of the sweep is the
digitally stepped frequency
sweep. The instrument will step
the synthesizer across any span
set by the user in a linear or log
frequency spacing. The number
of discrete points output will
depend on the span and sweeptime that is set. Sweeptime can
range from 0.5 to 1000 seconds
with each discrete point requiring typically 90 msec to complete. To reduce the amount of
switching transients spurs due
to each frequency change, the
output level is reduced approximately 60 dB between each discrete frequency. This amplitude
blanking may cause dropouts on
the displayed frequency
response. Due to these dropouts
it may be more useful to specify
a fast hop sweep for wide frequency spans as the following
describes.
A unique frequency sweep capability of the 8645A is the fast
hop sweep. Utilizing the frequency agile capability, large frequency spans with 1000 discrete
frequency steps in as little as
100 msec per sweep. The number of frequency steps varies
according to the sweeptime and
frequency range selected with
each discrete step taking 30
microseconds for outputs from
128 to 2060 MHz. The user can
set a sweeptime range from
10 msec to 100 seconds.
Although the output is blanked
between each frequency step as
in digitally stepped sweep, the
duration of the blanking is so
short that the detector used to
measure the frequency response
will typically not show the
dropout on the oscilloscope or
network analyzer. Either a linear
and log distribution of frequency
steps can be selected.
Each of the three types of frequency sweep described above
can be operated in a continuous
repetitive output or a single
sweep output triggered by the
press of a key or an HP-SL command. Additionally the digitally
stepped and fast hop sweep
types can be operated manually
using the front panel knob or
up/down arrow keys. Up to three
markers can be entered for output during a sweep. When the
sweep reaches the marker frequency a 0 volt signal is output
from the Z axis port on the rear
panel. The Z axis output is
+1 volt during a sweep and
+5 volts during retrace to blank
the CRT of an oscilloscope. The
X axis output of 0 to 10 volts
matched to the progress of the
frequency sweep.
7
Externally doubled
outputs to 2060 MHz
For applications requiring outputs above the 1030 MHz maximum frequency of the standard
8645A, consideration can be
given to either ordering the
8645A with the optional internal
doubler, installing the 11867A
retrofit kit or using an external
doubler. This technical brief
summarizes the capabilities and
performance the user can expect
while using one such external
doubler, the 11721A frequency
doubler, to increase the frequency
range of the 8645A to 2060 MHz.
The 11721A frequency doubler is
a passive, full-wave rectifying
doubling circuit that was
designed to minimize conversion
loss over a wide frequency
range. Its output frequency
range is 10 MHz to 2560 MHz. At
input levels above +13 dBm the
doubler has an almost constant
conversion loss of approximately
11 dB. This typical conversion
loss after the +16 to +18 dBm
maximum output of the 8645A
results in an output signal level
of +5 to +7 dBm for the average
11721A external doubler. The
harmonic and spurious content
of the output is almost completely a function of the input signal.
Note that harmonics input to
doubler (specified at <–30 dBc
for the 8645A below 1030 MHz)
will increase approximately 6 dB
due to the doubling function.
The same 6 dB increase will be
present on the phase noise of
the carrier. Any frequency modulation at the input to the doubler will double in deviation
also.
For frequency agile signals, the
11721A has no measurable affect
on the frequency switching time
up to the fastest time of 15 usec
available on the 8645A.
By using special function 111
frequency multiplier with an
entered multiplier of 2, the display of the 8645A will represent
the signal at the output of the
doubler as a convenience.
Simultaneously, the doubler’s
conversion loss can be entered
as an amplitude offset to calibrate the display for the actual
amplitude at the doubler’s output.
More general information about
the 11721A frequency doubler in
use with the 8662A synthesized
signal generator is available in
application note 283-2 (literature number 5952-8217).
8
Operation as a phase
noise measurement
reference
Among several techniques for
measuring the phase noise of a
source is the method of using a
second source to demodulate the
phase instability using a phase
detector. Commonly referred to
as the phase detector method,
this process requires that the
second source or reference
source have as good or better
phase noise performance than
the source being tested. It is also
required that one of the sources
have an FM capability in order
to maintain phase quadrature at
the output of the phase detector.
These needs for good phase
noise performance and FM capability often result in a generic
signal generator being used as
the reference source of a phase
noise measurement system. The
subject of this brief product note
is how to optimize use of the
8645A as a reference source for
phase noise measurements. More
information on the measurement
technique itself can be found in
literature related to products
such as the 11729C carrier noise
test set or 3048A phase noise
measurement system.
Several features of the 8645A
make it a good choice for use as
a phase noise measurement
source. These include the wide
carrier frequency range, an output power of +16 dBm and a
large FM deviation range. The
phase noise of the 8645A’s output is very low at offsets greater
than 10 kHz from the carrier, as
is commonly required for testing
channelized communication
devices or systems. The 8645A
has very few spurs on its output
which simplifies the detection
and interpretation of spurs from
the test source. The typical
phase noise and spurious performance is indicated in the
graph included in the “phase
noise performance” summary of
this product note.
As with any reference source
used in the phase detector
method, only as much FM deviation as required to establish the
phase lock loop for the measurement should be used. Minimizing
the FM deviation decreases the
noise contribution of the FM circuits and reduces the potential
for an unstable Phase Lock Loop
(PLL). The design of the 8645A
uses two different FM implementations that the user should
choose between according to the
FM deviation range required.
The standard FM is recommended for phase noise measurements that use a PLL bandwidth
of less than 1.6 kHz. Variations
in the group delay of the FM circuits for deviation settings to
support more than 1.6 kHz could
cause inaccurate measurements
or loop oscillations. For situations that require more loop
bandwidth, it is recommended
that the fast hop mode be activated for the measurement. In
the fast hop mode the group
delay of the FM is very low and
remains constant at higher FM
deviations. Although the phase
noise at low offsets increases in
this mode, it is generally acceptable as sources that require
more FM range to maintain
quadrature also have higher
phase noise to be measured.
One other unique characteristic
of the 8645A is that several circuits internally are reset whenever the center frequency setting
is changed so the output is not
phase continuous during these
changes. The output is decreased
by more than 60 dB during these
resets so that the unspecified
output during the transition will
not affect the user’s device. This
transition period lasts less than
85 msec typically. While this
operational characteristic will
not affect a phase noise measurement in progress, it will be
apparent when the center frequency of the 8645A is being
tuned as the beatnote disappears
momentarily with each change.
This signal interruption will
cause the PLL to momentarily
break lock. Activating special
function 105 amplitude muting
disables this amplitude blanking
but the unspecified transitions
of the output signal could still
result in perturbations while
tuning frequency.
9
Programming with
HP-SL
Hewlett-Packard Systems
Language (HP-SL) is the programming language for instrumentation adopted by Agilent
Technologies. This language uses
standard GPIB hardware and
will be used in many new Agilent
products. The 8645A is the first
signal generator to implement
HP-SL. HP-SL uses self-explanatory commands and is flexible
for beginning and advanced programmers. Programs written in
HP-SL for the 8645A will be compatible with the other generators
with the exception of commands
associated with unique functions
of the signal generator such as
fast hop capabilities. This is
intended to minimize software
modifications by the customer
when hardware is upgraded or
replaced.
Many Agilent divisions have contributed to the development of
HP-SL and will use it as part of
an interface system that conforms to the new IEEE 488.2
standard. The advantage of the
new IEEE standard is that it
defines common global commands such as for the instrument preset function, as well as
hardware and protocol that is
compatible with previous standards. In the short term HP-SL
will be easier to learn and self
documenting and in the long
term, HP-SL will provide a more
common language to reduce the
cost of software support.
A simple example shows how
HP-SL commands are self
explanatory and what a typical
program for the 8645A could
look like. The following program
lines will perform an instrument
preset on the signal generator,
set the RF frequency to 500 MHz
and the amplitude to 10 dBm,
and turn the RF output on.
100 Output 719; “*RST”
200 Output 719; “Frequency:CW 500 MHz”
300 Output 719; “Amplitude:Level 10 dBm”
400 Output 719; “Amplitude:State on”
This example programming can
be further simplified because
with HP-SL commands can be
combined in a single output
statement without regard for the
order in which the instrument
will execute the commands. This
means that HP-SL instruments
will take in the full command
“message” of a single line of programming before executing any
of the contents. The command
message defines the final instrument state that is wanted without regard for the order of
commands given. This eliminates
the problem of programming an
unallowed instrument state such
as increasing FM deviation
before increasing carrier frequency. However care must be
taken that each individual message only defines one final
instrument state and not several.
With this HP-SL capability, the
previous example can be
changed to the following:
Since “*RST” defines a complete
instrument state on its own, it
cannot be combined with the
other commands or it will be
uncertain which state will result.
This example also shows the use
of the short form of commands
as well as implied commands
and implied units. The semicolon
is used to separate commands in
a single output, and the colon is
used to separate words in a single command. The commands
with asterisks are used with all
IEEE 488.2 compatible instruments that can execute that
function. In HP-SL commands,
spaces should be between words
and arguments but not before or
after punctuation. Much more
information on HP-SL programming with the 8645A is provided
in the HP-SL Programming
Guide (literature number
5951-6710).
10
100 Output 719; “*RST”
200 Output 719; “Freq 500 MHz;Ampl:Lev 10; Stat on”
Command sequence
independence using HP-SL
A current problem with instrument programming is that each
command that is received by an
instrument is executed immediately. When the user is trying to
set up a complete instrument
state, the order in which the
commands are sent must be correct so that each intermediate
state is valid. Implementing
HP-SL on the 8645A has eliminated this command order
dependence through the creation
of command “messages”. A message contains all of the instrument commands that will result
in the desired instrument state.
None of the commands are
implemented by the instrument
until the complete message is
received. In this structure, the
order of the commands in the
message is irrelevant. The programmer constructs messages to
describe the final instrument
state that is needed without worrying about the way the instrument gets to that state.
For example, suppose a signal
generator had the following
capability dependencies between
carrier and FM deviation range:
Carrier range FM deviation range
100 MHz to 1 GHz 1 MHz to 10 MHz
10 MHz to 100 MHz 100 Hz to 1 MHz
With the previous control structure, it is impossible to serially
change the frequency and FM
deviation because either command to go to another range will
cause an error as the other
parameter is out of range. The
programmer would have to create an intermediate state such as
turn FM off before changing the
frequency so that all intermediate states were valid. The
Performance Signal Generator
(PSG) implementation of the
IEEE 488.2 standard eliminates
this problem because only the
final state need be valid. The
message of “Freq 200 MHz; FM
8 MHz” would put the instrument right to the new state that
is wanted.
It is important that the user
does not define an ambiguous
state within a message by modifying the same function more
than once in a single message. It
is uncertain (and undefined)
what the final instrument state
would be if the 8645A received
the following messages:
Freq:step 10 HZ;:Freq up;:Freq:step 100
HZ;:Freq down
As the frequency is repetitively
changed in a single message the
final frequency of the instrument will depend on the execution order of the commands,
which is not defined.
*RST;Freq 123 MHz;FM:State on
In this case the *RST command
could be executed after the other
commands, canceling their
effects. The command *RST
defines a complete instrument
state by itself and so should be
sent alone.
FM:State on;:AM:State on;:Mod:State
off;:Freq 100 MHz;:Mod:State on
In this case the user has specified conflicting states for the
mod:state command.
If the execution order of a group
of commands is important, the
user must send a separate message for each command.
11
Performance related
topics
Phase noise
performance
The 8645A agile signal generator
was designed to minimize the
phase noise of its signal at offsets corresponding to typical
channel spacings of communication systems. These offsets of
interest are generally greater
than 10 kHz from the carrier.
Simultaneously, the close-in
noise was reduced to assure low
residual FM for receiver testing.
The following summarizes the
phase noise performance of the
8645A.
For offsets less than 100 Hz, the
primary contributor of phase
noise is the fractional N synthesis circuitry in a single phase
lock loop if FM is not active. A
typical level is –80 dBc (in a
1 Hz noise bandwidth) at 100 Hz
offset for carriers in the main
band of 515 to 1030 MHz. The
phase noise at offsets between
100 Hz and 10 MHz is determined primarily by a frequency
discriminator inside a frequency
locked loop. The typical phase
noise level at a 20 kHz offset is
–133 dBc in the main band.
Beyond 10 MHz the phase noise
is that of the VCO or output section divider noise floor at
approximately –150 dBc. These
phase noise levels at offsets less
than 10 MHz will decrease by
approximately 6 dB each time
the carrier frequency is reduced
by half due to the dividers in the
block diagram. This phase noise
reduction continues until the
dividers’ noise floor of approximately –150 dBc/Hz is reached.
At offset frequencies of 20 kHz
or greater phase noise does not
increase when FM is active as
long as the deviation used is less
than approximately 5% of the
maximum FM deviation allowed
at that carrier frequency. For
example, in the main band of
515 to 1030 MHz the maximum
available deviation is 10 MHz,
but the phase noise performance
at a 20 kHz offset remains the
same as in CW operation if
500 kHz or less of FM deviation
is set by the user. If the full 10
MHz deviation is used, the phase
noise at this 20 kHz offset typically increases by 17 dB to
–116 dBc.
As with any signal generator,
close-in phase noise of the
8645A goes up as the FM deviation increases. This is because
the internal FM circuits contribute more noise as the deviation (gain) increases. For
example, the phase noise level is
approximately –80 dBc at a 10 Hz
offset for a main band output
from 515 to 1030 MHz with FM
deviation set to 100 Hz. If FM
deviation is set to 100 kHz the
phase noise at this offset
increases by 35 dB to typically
–45 dBc. Using the full 10 MHz
deviation, the maximum available in this main band, phase
noise will go up another 40 dB
to approximately –5 dBc at a
10 Hz offset.
12
There is no degradation of phase
noise at offsets greater than
10 kHz if special function 120
“Linear DCFM” is activated. At
smaller offsets however ,linear
FM operation will result in up to
35 dB less noise for operation
using high FM deviations. For
example, at a 10 Hz offset and
with 10 MHz FM deviation set,
the phase noise level in the
default FM mode is typically
–5 dBc while the linear FM level
will be at –35 dBc. Similarly for
a 1 kHz offset the levels are typically –51 dBc and –77 dBc. At a
FM deviation of approximately
20 kHz the level of phase noise is
about equal for the two FM
modes while at smaller deviations the default digitized FM
actually exhibits lower phase
noise. In general, the linear FM
special function may improve
phase noise performance at offsets less than 10 kHz when FM
deviations greater than 20 kHz
are in use.
In summary, the 8645A will add
the least amount of phase noise
to the carrier if the lowest FM
deviation necessary for the
application is used.
Phase noise levels in fast hop
operation are degraded approximately 3 dB from non-agile levels due to a reduction of filtering
on the VCO pre-tune lines. Less
filtering is necessary, as the signals on these lines require a
higher bandwidth during fast
hop operation. The typical level
of phase noise for frequency
agile signals between 515 and
1030 MHz is –130 dBc at a
20 kHz offset.
8645A signal generator typical phase noise
and spurs at 1 GHz
13
Spurious performance
The spurious performance of the
8645A is quite good, but there
are still spurs to be found. This
product note describes the
sources of potential spurs and
where in the spectrum they can
be found.
The harmonically related spurs
are caused by nonlinear operation of amplifiers in the RF path.
The specification for harmonics
of the carrier for carrier frequencies below 1030 MHz is
–30 dBc. Typically, they are better than –35 dBc. Subharmonics
typically are caused by a divider
in the Phase Lock Loop (PLL)
signal path that affects the main
VCO output and amplification in
the output section. The dominant subharmonic is at 0.5 * the
VCO frequency. This spur in the
main band (515 to 1030 MHz) is
less than –70 dBc, in the doubled band (1030 to 2060 MHz) it
is less than –45 dBc, and below
515 MHz it is practically non
existent.
Nonharmonically related spurs
are caused by a number of
things. These include the power
supply, microphonics, and digital
circuits. The power supply spurs
are all input line frequency related and are typically less than
–60 dBc in the main band. A
careful design of the regulators
and power distribution circuits
keeps the power supply ripple in
the instrument very low. The
front panel display circuitry can
produce a spur at an offset of
approximately 1.5 kHz. Its level
is less than 20 dB above the signal generator’s phase noise
measured in a 1 Hz noise bandwidth. Microphonics is another
source of spurs that depends on
how severe the signal generator
is being vibrated. One inherent
source is the fan, with the location of the spur dependent on
fan speed which in turn is a
function of the instrument’s temperature. The fan spur is usually
less than 20 dB above the noise.
A spur can be produced in the
output when external modulation (i.e., FM) is enabled and the
internal audio source is active.
Its location will be at the audio
source frequency. The level will
depend on the amount of FM
deviation programmed. For
example, if 1 MHz deviation
using an external FM source is
set and the internal audio oscillator is at 100 kHz, it will cause
a spur at approximately –80 dBc.
It is recommended that the
audio oscillator be turned off
when not in use.
One final type of spur to be mentioned is due to the fractional-N
circuitry in the PLL. When the
output frequency in the main
band is not an integer multiple
of 400 kHz, a spur will be produced. This spur is caused by
the PLL divider alternating
between two divider numbers
(integers) such that the average
frequency is the desired frequency. Compensation in the PLL circuitry keeps these spurs to less
than 25 dB above the phase
noise of the output signal (in a
1 Hz noise bandwidth). The fractional-N spur frequency in the
main band will be at half the difference between the closest integer multiple of 400 kHz and the
instrument’s output frequency.
14
Third order
intermodulation
Third Order Intermodulation
(TOI) products result when the
outputs of two signal generators
are summed together in a combining network. These spurious
signals occur at frequencies
2*F1-F2 and 2*F2-F1, where F1
and F2 are the output frequencies of signal generators 1 and 2
respectively. The unwanted
intermodulation signals are the
result of the Automatic Level
Control (ALC) loops in the output sections of each generator
“seeing” the other generator’s
signal and responding to it as if
it were unwanted modulation on
the desired output signal. If the
frequency difference between
the desired output signal and the
other generator’s signal is less
than the bandwidth of the ALC
loop, the loop can respond to the
signal’s presence. In trying to
remove this single sided “modulation”, the loop inadvertently
produces modulation sidebands
of its own. This unfortunate
process is also occurring in the
other generator’s ALC loop at
the same time. The overall result
is third order intermodulation
products accompanying the two
test signals at the output of the
combiner.
Since signal generator outputs
are usually combined to provide
a stimulus to test the TOI performance of receiver front ends,
it is important that the TOI
products caused by the signal
generators be well below those
expected from the device under
test. One way to reduce the TOI
products from the signal generators is to use a directional coupler rather than a resistive
summer to combine the two signal generator outputs. Another
way to decrease TOI products is
to reduce the bandwidths of the
signal generator ALC loops well
below the frequency spacing
used. This approach can be
taken to the extreme of entirely
opening (or disabling) the ALC
loops, since an open loop can be
thought of as an infinitely narrow (0 Hz) bandwidth because
the ALC will not respond to a
signal at any frequency.
The 8645A has five different
ALC loop bandwidths: 200 kHz,
50 kHz, 5 kHz, 60 Hz, and 0 Hz.
The instrument automatically
selects the optimum bandwidth
for lowest AM distortion, fastest
amplitude switching speed, and
lowest TOI. The three widest
bandwidths are used only when
AM is enabled. In that case, the
bandwidth selected is a function
of carrier frequency. The 60 Hz
bandwidth is used whenever AM
is turned off. At this bandwidth,
the TOI level produced by two
signal generators with a frequency difference of 25 kHz and with
output levels of +8 dBm is typically less than –55 dB. For better
performance with smaller frequency differences, the 0 Hz
bandwidth can be selected using
special function 104.
15
Divided outputs
below 515 MHz
To create signals below the main
VCO frequency range of 515 to
1030 MHz, the 8645A divides the
VCO into lower frequency
octaves using digital dividers
switched into the signal path. A
total of 11 divide bands extend
the frequency coverage down to
252 kHz. This technique is very
good for spectral purity as with
each division of the signal the
phase noise and spurs are
decreased approximately 6 dB.
This reduction in phase noise
continues until the noise floor of
the dividers is reached which is
typically approximately –150
dBc. Residual FM is also reduced
as the carrier frequency is divided down. However, there are
other consequences that must be
dealt with as the following
describes.
When FM is applied to the VCO
the amount of deviation that is
present in a divided output is
divided by the same number as
the carrier. For the lowest or
11th divider band this division
equals 211 or a divisor of 2048.
This large divisor is one of the
reasons the 8645A has a maximum of 10 MHz of FM deviation
in the main band. With this large
deviation available in main band
there is still 10 MHz / 2048 or
4.8 kHz of deviation available in
the 252 to 503 kHz band. FM
rate also decreases with each
successive divide band because
each band has 2 half-octave low
pass filters present to reduce the
level of harmonics at the output.
Phase continuous frequency
sweep is also reduced by the
action of the divider circuits.
The actual frequency change in
this function is an FM operation
using the full FM deviation available in the main band to get a
20 MHz span (±10 MHz). The
available span width is reduced
by half with each successive
divide in the same way FM
deviation is.
The AM bandwidth is always
limited to something much less
than the carrier frequency
because the level detector
(which is designed to follow the
AM envelope or any level variation and not the RF) would start
detecting the RF waveform if the
bandwidth were too wide. In a
divided output, the AM is
applied to the divided RF output
(not the main VCO signal) and
therefore the AM bandwidth
must be less than the band’s
lowest RF signal so as not to
react to the carrier. But as AM
bandwidth is reduced, amplitude
switching time gets longer and
AM distortion is worse. In the
8645A three AM bandwidths are
used to optimize the AM performance and still allow fast
amplitude transitions as is necessary for fast hop operation.
Over the frequency range of 128
to 2060 MHz a 100 kHz bandwidth is used, a 50 kHz bandwidth is active for signals down
to 8 MHz and a 5 kHz bandwidth
limits signals for outputs to
252 kHz. In fast hop operation,
the amplitude is decreased by
approximately 30 dB by the AM
circuitry during each frequency
change of the main VCO. The
reduction of AM bandwidth for
the lower divide bands which
causes slower amplitude switching time is the only reason the
fast hop switching time is longer
for low carrier frequencies.
16
Stereo separation quality
Stereo separation is a measure
of a receiver’s ability to separate
the left and right channel of a
stereo signal. To a listener, this
is a measure of the receiver’s
ability to recreate the spatial
impression of a stereo signal. In
an FM system the audio information is received as a left + right
signal and a left-right signal. The
receiver decodes the left channel
by adding the two signals and
decodes the right channel by
subtracting the two signals. The
separation of the channels
depends on the cancellation of
the right channel during the
addition and cancellation of the
left channel during subtraction.
For this to happen properly, the
relative phase and amplitude of
the two original signals must be
kept equal.
In a signal generator the FM linearity and group delay flatness
determines whether the relative
phase and amplitude of the
stereo signal is preserved.
Typically, test signals near 1 kHz
and 38 kHz are used to modulate
the signal generator to test
stereo separation in a receiver.
The quality of the test signal,
and therefore the measurement,
will depend on the FM linearity
and group delay at these frequencies.
Radio manufacturers specify
stereo separation in dB as the
amplitude difference between a
desired signal in one channel
and an undesired signal in the
other channel. The desired signal
is a known test signal used to
stimulate one channel. The undesired signal is the unwanted
“leakage” or coupling of the test
signal into the other channel.
Radio manufacturers typically
specify 40 dB separation which
is beyond most listener’s ability
to detect distortion or “crosstalk”
between the channels.
The 8645A in the fast hop operation uses linear FM and has low
group delay. Typical stereo separation is greater than 55 dB in
this mode and is sufficient to
test most consumer radio equipment. Using digitized FM in standard operation of the 8645A will
result in poor stereo separation
due to variations in the group
delay of the FM signal path.
Activating special function 120
linear FM with AC coupling set
provides separation similar to
the fast hop mode but has the
disadvantage that signals below
20 Hz cannot be used. Linear FM
with DC coupling also has good
stereo separation but poor center frequency resolution.
Therefore, for the best stereo
separation with the 8645A the
user should activate the fast hop
mode with DC coupling.
17
Minimizing fan noise
Increasing concern over the level
of audio noise coming from test
instruments has resulted in several design features to minimize
noise from the Performance
Signal Generator (PSG). The
objectionable noise from test
instruments comes from the fans
used to create the internal airflow to cool the electronics and
prevent heat related failures.
These modifications from what
was done in the past are related
to careful fan selection, fan
speed and rear panel fan cover.
An overriding consideration was
to maintain the high reliability
design goal for the PSG by ensuring sufficient airflow for cooling
components.
A number of fans were evaluated
for use in PSG. Along with being
of the right physical size and
pushing enough air, the noise
level when running was considered. Of the fans that would
meet the cooling requirements,
the one with lowest noise level
was chosen. The next step was
to evaluate the noise contribution the fan cover was responsible for. The shape of the grill
work of the cover changes the
noise level due to the fan blades
passing close to it in their rotation. An analysis of noise
sources of various grill shapes
with the blade shape of the low
noise fan led to making several
grills to try out. The combination
of the grill and low noise fan
that produced the lowest noise
was chosen for implementation
in PSG.
The final step taken to reduce
the noise of the PSG produced
the biggest benefit for the average user. Minimum airflow
required for high reliability operation is calculated assuming the
ambient temperature at the maximum operating temperature
specified. For PSG, this temperature is 55 degrees centigrade
(131 degrees fahrenheit). Fan
rotation speed is set to provide
enough airflow at this high environmental temperature. At lower
temperatures, less airflow is
needed to keep internal components at their specified operating temperature so the fan speed
could be reduced. In most previous instruments the fan speed is
held constant at the highest airflow needed for high ambient
temperatures. In PSG instruments, the fan speed is controlled by a temperature sensor
to vary airflow as needed to
maintain as much as possible
constant internal temperature
over the full environmental
range of 0 to 55 degrees centigrade. Since the average user
has the instrument in environments much less than 55 degrees
centigrade, the fan speed is
much slower than the maximum
it could do. As fan noise is
directly related to the fan speed,
in typical use the PSG instruments are much quieter than
previous signal generators. This
provides a much more pleasant
environment for the operator of
a performance signal generator.
18
Frequency agility
Functional description of
frequency agile operation
The frequency agile operation of
the 8645A is unique in both its
capabilities and its operation.
The following describes what the
instrument is actually doing
while in fast hop operation.
Entering the fast hop mode
Either pressing the fast hop
mode select key or sending the
counterpart HP-SL command
will put the instrument in the
fast hop synthesis mode. The
instrument’s output frequency is
no longer phase locked. Instead
the frequency accuracy depends
on an extremely stable VCO and
a frequency locked loop. At this
point the fast hop subsystem is
set to idle allowing parameters
such as output level and FM
deviation to be programmed the
same as in non-agile operation.
Entering channel information,
hop rate, and dwell time
As the user enters each frequency and amplitude into a channel
location, the information is put
into non-volatile memory for use
during the learn operation that
precedes frequency hopping. If a
channel sequence is entered for
the channel numbers, this information is also put in this memory. A hop rate and dwell time
are always in memory and are
modified according to any new
values entered. Any conflict
between the hop rate and dwell
time is not checked until the frequency learn operation is initiated.
Initiating the learn operation
The learn operation recalls each
frequency and amplitude stored
in each channel location and
sets the phase locked synthesizer
and the ALC of the output to
each value. At each setting the
instrument records the VCO tuning voltage and the ALC amplifier gain. The output is turned off
while this process is underway.
The hop rate and dwell are also
verified that they will not conflict for the frequencies (and
associated switching time) in the
channel table. The only channels
that are part of these operations
are those in the current
sequence table. If the user did
not specify a sequence table, the
8645A creates one that reflects
the number and order of the
entries in the channel table. The
instrument does not program
frequencies and amplitudes of
any channels that are repeated
in the sequence table, rather the
VCO and ALC settings already
learned are copied into memory.
The learn operation always lasts
a minimum of 10 seconds to
ensure that the hopping circuits
are exercised sufficiently to stabilize any thermal changes in the
transition from the idle state. As
more unique frequencies are
included in the channel table it
takes longer to set up each state
to record the settings and so the
learn time required increases.
For 2400 channels, learn time is
approximately 1 minute. This
time doubles to 2 minutes if FM
is active.
Initiating hop operation
When the 8645A begins frequency hopping a unique “fast controller” takes control of the VCO
and ALC. The data contained in
the fast hop memory is presented to the hardware to duplicate
each channel in the order it
appears in the sequence table.
Depending on which fast hop
mode is active, the fast controller may cycle through the
sequence table at a programmed
rate or enable external inputs to
trigger a hop to the next channel
or to select which sequence location to output based on the input
at the fast hop bus. The instrument will remain in the hop
state until the idle or learn operation is selected or a function is
changed that would invalidate
the data in the fast hop memory
(such as changing the FM deviation). Rate and dwell can be
changed without having to learn
again.
19
Faster frequency
switching using multiple
agile generators
The 8645A can provide frequency agile outputs with hop rates
of up to 50,000 hops/second
depending on the carrier frequencies above 128 MHz,
11,000 hops/second above 8
MHz, and 2,000 hops/second
above 252 kHz. While these hop
rates are fast enough for the
majority of agile applications,
sometimes higher hop rates may
be needed. The 8645A has been
designed to make it easy to synchronize and combine the outputs of multiple units in order to
create agile signals at higher hop
rates. This product note explains
how to configure multiple units
to work together.
Frequency agile operations that
include interfacing with a radio
usually involve control of the following parameters: frequency
selection, data valid, hop triggering, dwell time, modulation and
amplitude. Of these parameters
only data valid and hop triggering require extra attention when
synchronizing the outputs of two
or more 8645A’s. In a typical
instrument set-up, each 8645A
will have loaded into memory
identical channel and sequence
tables of all the frequencies and
amplitudes to be output. Dwell
time for each hop frequency will
be constant and controlled by
the instrument’s internal timers.
The modulation waveform to be
placed on the carrier would be
input to both external FM inputs
with the same FM deviation set
on each generator. The frequency control word to select each
channel to be output according
to its location in the sequence
table would also be input to the
fast hop bus of both instruments
simultaneously. The RF output
of each generator would be
brought together with a combiner for input to the device under
test. All of these control inputs
and instrument settings are
identical to that required for
operating a single 8645A.
By alternating which signal generator receives the data valid
and hop trigger signals it is possible to give one 8645A time to
switch frequencies while a second unit is producing the needed
output. Then while the second
unit is changing frequencies the
first can provide the next output. In this way the combined
agile output can be switched at
much higher rates than are possible with a single 8645A. Each
unit gets a trigger signal at half
the hop rate of the combined
output will be. The data valid
input to clock in each frequency
word occurs at the same time as
the hop trigger but is input to
the opposite instrument.
Consequently the same trigger
signal (approximately +5 volts
for 1 usec) can be used for both
the hop trigger of one unit and
the data valid input of the other
unit since the two signals occur
simultaneously but are just routed to two different inputs.
Although both generators get the
modulating signal, only the generator that is presently outputting a signal will carry the
modulation since the output of
the other generator is decreased
by over 60 dB while it is changing frequencies. Also the frequency word that goes to both
fast hop bus inputs is ignored by
the generator that does not also
get a hop trigger to implement
the word.
20
The diagram that follows illustrates the connections that will
alternate outputs from two agile
signal generators to produce a
hopped signal at rates above
50,000 hops/second for carrier
frequencies above 128 MHz. Note
that two pulse generators (such
as an 8116A), one with a
delayable trigger (such as an
8013B), are used to provide a
hop trigger alternately to each
generator. Frequency selection is
controlled by the internal
sequence table of each generator
so the frequency control word
and data valid inputs are not
needed. The two outputs are
combined using a power splitter
such as the 11667A. With this
configuration a maximum hop
rate of 93 kHz can be produced
with a switching time of 4.3
microseconds between channels
and a dwell time of 6.4 microseconds per frequency. By increasing the dwell time of each agile
output the switching time can be
reduced. To calculate the combined output switching time, it is
only necessary to subtract the
dwell time from the 8645A’s
specified switching time for the
carrier frequency range in use
and divide by two. For example,
if the signals were at carrier frequencies below 8 MHz with a
specified switching time of
500 usec and the desired dwell
time was 300 usec, the switching
time of the combined output
would be (500 usec minus 300
usec)/2 or 100 usec. These signals would have the same specifications of ±2 ppm frequency
accuracy and ±1 dB amplitude
accuracy as an 8645A operating
by itself would have.
21
Multiple signal generators provide faster frequency switching
Pulse
generator
Delayed trigger
HOP
trigger
8645A 8645A
Data
source
FM RF FM RF
Output
Pulse
generator
with delayable
trigger
HOP
trigger
Power
combiner
Frequency accuracy of
agile outputs
The frequency accuracy and stability of the 8645A is directly
related to the 10 MHz timebase
used as a reference for non-agile
operation. The output accuracy
is a direct multiple of the timebase error. For the high stability
timebase specified at 0.0005
parts per million (ppm) aging
rate per day, the worst case
error of a 1 GHz output after 10
days would be 5 Hz assuming no
initial inaccuracy.
For frequency agile outputs, the
8645A specifies a maximum
error of ±2 ppm. There are two
contributors to the frequency
error: timebase error and temperature related drift. Timebase
error is a factor because during
the “learn” operation the 8645A
briefly synthesizes each output
frequency using the phase lock
loop circuits and reads voltage
levels of the VCO tuning line.
While hopping this tuning voltage is sent back to the VCO to
create the output signal very
rapidly. Any error of the timebase will be reflected in the tune
voltage sent to the VCO used for
fast hop signals.
Temperature related frequency
errors result from a change in
the operating temperature of the
components in the agile signal
path. Several steps have been
taken to reduce the temperature
variations within the instrument
such as providing constant-temperature heating of the delay
line and temperature regulating
the fan speed that provides cooling. These design features and
the large thermal mass of the
instrument greatly reduce the
sensitivity of the 8645A’s agile
frequency accuracy to ambient
temperature changes. In any
case, it is recommended that the
user re-learn the hop frequencies before beginning a frequency agile test. Each learn
operation will remove any temperature related offset between
the fully synthesized calibrating
signal and the agile output. Also,
as noted in the specifications
table, having the unit plugged-in
for 24 hours and operating for a
minimum of 2 hours before the
learn operation and frequency
hopping begin, will ensure the
heating elements and the thermal mass of the instrument are
at a stable operating temperature.
The typical worst case frequency
error of ±1 ppm for agile frequency outputs can be significantly improved if the test
parameters for the application
are within a certain criteria that
minimizes the minute thermal
variation of the agile components themselves. For example,
tests conducted during the
8645A’s design show that the
frequency error is reduced for
agile test sequences of less than
approximately 60 unique frequencies with hop rates of
greater than 10 hops/second.
The same improvement occurs
for internally controlled
sequences having up to the maximum number of 2400 unique
frequencies as long as the
distribution of how often each
frequency is output is pseudorandom. The average frequency
error measured under these conditions was less than ±0.3 ppm.
22
Relating phase error and
frequency accuracy
The quality of a signal can be
specified in many ways including
amplitude accuracy, spectral
purity, output level, modulation
distortion, etc. A common specification of many sources is frequency accuracy or phase error.
While many signal generators are
specified in terms of frequency
accuracy, other types of sources
are just as commonly described
with a phase error. Since the signal generator is often used to
simulate or substitute for a
source specified in terms of
phase error, it becomes necessary to convert between frequency accuracy and phase error to
determine if performance is sufficient. This calculation is of critical importance for frequency
agile sources that are specified to
be within a stated frequency
accuracy or phase error in a certain amount of time after a frequency change is triggered. This
product note discusses the conversion between phase error and
frequency accuracy.
Converting a phase error specification to a frequency accuracy
number is based on a key
assumption: that the phase settling characteristics are approximately linear. Since the rate of
phase change of a signal is related to its frequency, knowing that
a source’s output is uniformly
approaching the desired final
phase state allows a calculation
of the corresponding frequency
change. If the phase settling
departs significantly from linear,
the calculated frequency error
will be too low for faster phase
settling and too high for slower
settling signals. The following
examples use typical characteristics for a frequency agile local
oscillator to illustrate the phase
error to frequency accuracy conversion process.
Example 1: Converting phase
error to frequency accuracy
A local oscillator switching in
the range of 800 to 1000 MHz is
specified to be within 0.1 radian
of final phase 20 usec after the
frequency change trigger is
received. The signal settles to the
final phase during the 5 usec
duration of the output.
Converting the accumulated
phase error over the 5 usec
duration results in a calculation
of (0.1 radians)/(5 usec) equaling 20,000 radians/second that
the signal is changing. This
equals a frequency error of
(20,000 rad/sec)/(2*pi rad/sec)
or 3183 Hz. The frequency accuracy of the 8645A is specified at
±2 ppm of the carrier which
translates to a maximum of 2 kHz
frequency error for the 800 to
1000 MHz frequency range. In
this application the 8645A can
substitute for the local oscillator
as far as the frequency accuracy
requirement is concerned.
If in the previous example the
signal settled to its final phase
faster than 5 usec, the calculated
frequency error would be correspondingly higher. Another
example follows which converts
the frequency accuracy of the
8645A to its corresponding
phase error for the previous
example.
Example 2: Converting
frequency accuracy to
phase error
The frequency accuracy of the
8645A is specified at +2 ppm of
the carrier frequency. For a 1 GHz
output this equals a maximum
error of 2 kHz. Converting 2 kHz
to a phase error equals (2000
Hz)*(2*pi rad/sec) or 12,566
radians/sec. In the 5 usec duration of the signal described in
example 1, the maximum accumulated phase error is (12566
rad/sec)*(5 usec) or 0.0628
radians.
If the phase error is specified in
terms of degrees the values
given in radians in the above
examples should be multiplied
by (2*pi). In example 2 the
8645A’s phase error of 0.0628
radians translates to 0.3946
degrees.
23
Amplitude dynamic range
while frequency hopping
The available output level
dynamic range in fast hop mode
is a function of the amplitude
switching time required. The frequency switching time of the
8645A is not controllable by the
user and is always typically
9 microseconds, the speed of the
fundamental VCO. The specified
switching times in the data sheet
are actually amplitude switching
times, that is the time it takes
for the power to rise to 90% of
its final value at the new frequency. This “power rise time” is
also not directly controllable by
the operator, but he can effect it
by utilizing the amplitude hopping capability of the 8645A.
The 8645A hops with the ALC
loop closed unless the operator
selects [open loop hop “on”],
special 202. For [open loop hop
“on”], amplitude switching time
does not degrade as a function
of amplitude range utilized, but
level accuracy does degrade. For
[open loop hop “off”], special
202, the default mode, the available amplitude range is a function of the amplitude switching
time required. This range can be
used for either programming different levels or for shaping the
power envelope (using the AM
port) while hopping. Both capabilities can be used simultaneously as long as the total range
is within the limit described in
the graph below. Note that the
amplitude accuracy is also a
function of the range utilization
and ALC special selected. The
level accuracy degradation will
occur at the lower amplitude
output.
The typical, quantifiable, degradation in amplitude switching
time and level accuracy is
described in the graph below.
Amplitude variations greater
than 20 dB may be programmed,
but amplitude inaccuracy could
become much greater than 5 dB
in some cases.
The reason for these interrelationships is, the same ALC loop
is used simultaneously for DC
coupled AM and level setting. To
provide specified 90% AM with
adequate design margin, 30 dB
of ALC range is required at the
worst case frequencies. The
instrument was designed to give
the user the option of using this
“excess” range to control level
while fast hopping. At the lower
portions of this “excess” ALC
range, the ALC loop bandwidth
will decrease, increasing the
amplitude settling time.
24
Amplitude shaping of
agile outputs
As part of their frequency
switching algorithms, many frequency agile radios reduce the
RF carrier power when switching between frequencies. Since
controlling the characteristics of
the amplitude transitions while
switching is critical to proper
hopped operation, the 8645A
was designed to allow the user
to shape the amplitude transitions of the RF carrier while in
fast hop operation.
If the amplitude transitions are
very sharp in nature, a frequency agile carrier (when viewed at
one specific frequency) will have
the same sin (X/X) spectral signature as a pulsed RF carrier. As
with a pulsed RF carrier, the
energy of the carrier will be distributed throughout the lobes of
the sin (X/X) envelope. This
energy distribution is typically of
no concern in pulsed applications but in frequency agile
applications the distributed
energy can fall into adjacent
communication channels, causing disruption of communications in those channels. To avoid
this problem, frequency agile
radios that reduce the RF carrier
power when switching also
shape the RF power transitions
to minimize the spectral splatter
associated with sharp (pulse
like) amplitude transitions.
When in fast hop operation, the
8645A automatically ‘softens’
the amplitude transitions to
decrease the spectral splatter. As
power is being shut off at a specific hop frequency, timers built
into the instrument send a negative step to the ALC loop so that
the amplitude drops at a rate
that is consistent with the ALC
loop bandwidth. After several
microseconds the ALC loop will
have decreased the output amplitude by approximately 30 dB. At
this point the pulse modulator is
activated to get an additional 35
dB or more of amplitude
decrease. This timing sequence
is reversed when the power is
being brought up at the new frequency, with the pulse modulator being turned off first and
then the ALC loop being allowed
to return to its pre-shutoff level.
This sequence greatly reduces
the spectral splatter from what it
would be if only the pulse modulator were used. If the user activates special function 202 ALC
off, to get slightly faster frequency switching speed, the ALC is
not used to decrease the output
power. Only the pulse modulator
would be used so spectral splatter increases somewhat due to
the more abrupt amplitude transition. Additionally the power is
only decreased approximately 35
dB between frequency hops.
The operator that would like to
use the 8645A to emulate a
transmitter that uses a rigorous
amplitude shaping technique, or
who needs to decrease spectral
splatter for other reasons, can
use the external DC AM port to
shape the amplitude transitions
during fast hop operation. The
shaping signal, such as a raised
cosine wave, when input into the
external DC AM port controls
the ALC loop to implement the
amplitude shaping. To shape the
amplitude rise and fall characteristics of a hopped signal, the
shaping signal must be synchronized to the hop trigger or dwell
time control (available on the
rear panel of the 8645A). It
should be noted that external
shaping, as well as the automatic
shaping previously described,
use some of the available ALC
range. This means that the
amount of amplitude variation
available while hopping is
decreased. Elsewhere in this
product note the interaction
between amplitude hopping
range, frequency switching
speed, and amplitude shaping is
discussed as “Agile Amplitude
Dynamic Range”.
25
Modulation
High rate, high deviation
frequency modulation
The 8645A has overcome many
of the previous barriers to provide FM with high deviation in
an RF signal generator. By using
digital techniques, deviations as
high as 10 MHz are possible for
carrier frequencies in the main
carrier band of 515 to 1030 MHz.
Accompanying this breakthrough
in high deviation FM is a similar
increase in the maximum rate of
FM to 10 MHz. One reason for
this extra FM performance is to
ensure sufficient usable capability after the many divider stages
that extend frequency coverage
down to the minimum carrier
output of 252 kHz.
In the 8645A, the amount of FM
deviation selected determines
the length of the delay line used
in the delay line discriminator
placed around the VCO to
reduce phase noise. At higher
selected FM deviations the delay
line is shortened so that it does
not reduce FM sensitivity in the
deviation range that is set. With
a shorter delay line the phase
noise from the VCO increases,
but generally this is acceptable
for applications needing high FM
deviations. The annunciator for
the mode 1 key on the front
panel lights up when the shorter
delay line is in use. As FM deviation is reduced below approximately 17% (1.76 MHz in the
main band) of the maximum
allowed at each carrier band, the
extra delay line is automatically
placed in the VCO signal path
unless the user has specifically
locked the instrument into mode
1 using the mode select keys.
Phase noise and spurs are
reduced for operation using the
smaller FM deviation and the
mode 2 indicator is lit. In this
way, the best spectral purity is
provided for any FM deviation
that is selected.
The 8645A has two different
types of FM referred to as digitized FM and linear FM, both of
which can be used with the
internal modulation oscillator
and/or an external source at the
front panel. Digitized FM is the
default type and utilizes an A/D
converter to translate the modulating waveform into digital
information that is used to modulate the fractional-N divider
number. This provides FM at
rates from DC up to the phase
locked loop bandwidth along
with the capability for high deviations. A wideband, high slew
rate analog path sums the modulation signal onto the VCO tune
line to allow FM at rates from
the PLL bandwidth up to
3.75 MHz and typically to
10 MHz while retaining the high
deviations.
Linear FM is activated using special function 120. In this FM
operation the digital path is
switched out leaving just the
analog path for improved flatness and stereo separation. In
mode 2 with linear FM, the PLL
is not used and linear DCFM is
available through the frequency
locked loop. During calibration,
offsets are nulled in this path to
improve frequency accuracy in
fast hop operation which always
uses linear FM. Group delay is
less than 1 usec in linear FM
with a typical value of 0.1 usec.
The maximum deviation available is the divided result of the
amount available in the main
band of 515 to 1030 MHz. The
maximum of 10 MHz deviation in
this main band becomes 5 MHz
in the first divide band of 207.5
to 515 MHz. The minimum deviation that can be set is 100 Hz in
the main band and also gets
halved by each divider band to a
minimum of 1 Hz. The maximum
available FM rate is also reduced
by each divider section due to
the half-octave filters present in
each divider section to reduce
the level of harmonics generated.
26
Simultaneous modulation
The 8645A has a wide range of
combinations available for simultaneous modulation. Various
combinations of amplitude, frequency, phase and pulse modulation are provided as follows:
With AM: FM, phase, pulse, FM
and pulse, phase and pulse
With FM: AM, pulse, AM and
pulse
With phase: AM, pulse, AM and
pulse
With pulse: AM, FM, phase, AM
and FM, AM and phase
In addition to these combinations, the modulating waveform
can be provided from either the
internal modulation source or
from an external source via a
front panel input The internal/
external status of a given modulation type can be set independent from any other modulation
type that may also be active. The
following combinations of internal and external modulation
waveform source are available
for each modulation:
AM: Internal, external
FM: Internal, external, internal
and external
Phase: Internal, external,
internal and external
Pulse: External only
In simultaneous internal and
external FM or phase modulation, an external signal of typically 30% or more of full scale
input can be applied simultaneously with a full scale internal
signal without any limiting
occurring.
When using the internal modulation source, rates from .1 Hz to
400 kHz are available with a resolution of .1 Hz. Therefore the
entire bandwidth of AM
(100 kHz for carrier frequencies
greater than 128 MHz) and
phase modulation (150 Hz) as
well as a major portion of the
FM bandwidth can be covered
with the internal modulation
source. The level of the internal
modulation signal can be adjusted with a resolution of .2% of full
scale to provide improved resolution of level of various modulation types. The internal
modulation source can also provide complex waveforms such as
sawtooth and squarewave at
rates up to 50 kHz and white
Gaussian noise of constant
amplitude from .1 Hz to 400 kHz.
The external modulation source
input of all modulation types
except pulse can be set to AC
coupling as an alternative to DC
coupling. In AC coupling all DC
drifts and biases up to ±10 volts
from external sources are
blocked without degradation of
performance. The lower 3 dB
bandwidth in AC coupling for all
modulation types is typically
20 Hz.
The input impedance of all the
external modulation ports is
600 ohms, except the FM port.
The external FM input impedance is 50 ohms to allow external modulation sources to
provide signals up to 10 MHz
with low loss. By activating special function 123 the user can
route modulation signals from
the phase modulation input to
the FM circuitry. Since the phase
modulation input impedance is
600 ohms, this capability provides higher input impedance for
FM operation. The upper 3 dB
FM bandwidth using this phase
modulation input is approximately 2 MHz.
27
Digitized FM operation
Synthesized signal generators
have traditionally generated FM
with a phase lock loop dedicated
for this purpose. For AC coupled
FM, this loop remains locked
while the frequency modulating
signal is injected into the loop to
FM the output. For DC coupled
FM, the loop must be unlocked
so that low modulating frequencies are not canceled by the loop
feedback. However, unlocking
the loop for DC FM allows the
output frequency to drift, which
can be a problem for some applications. The 8645A has this traditional implementation of FM
operation as a special function
(120). The standard FM implementation for the 8645A is one
that removes the frequency drift
problem of DC FM.
As standard operation, the
8645A uses a digital FM signal to
modify the instantaneous synthesizer divide values in the
fractional-N circuit. This method
greatly reduces the frequency
drift and offset that is usually
associated with DC FM operation. The only remaining drift is
related to the Analog to Digital
Converter (ADC) which is typically much less than the drift of
an open loop VCO. This modulation technique is also very accurate at low FM rates because the
output of an ADC can be much
more precise than the analog
tuning curve of a VCO. A side
benefit of major importance is
that this technique does not
introduce out-of-channel spurs.
In a traditional signal generator
block diagram, the FM loop
needs to sum into the main loop
with the unavoidable introduction of crossing spurs.
There are some disadvantages to
this digitized FM technique. One
is that the digitization and summation of the modulating signal
takes time, resulting in approximately 30 usec of group delay of
the modulating signal at rates
inside the PLL bandwidth. The
non-flat group delay may cause
distortion of the modulating signal, and in some instances may
actually cause the PLL to unlock.
Another implication of this modulation scheme is that when digital FM is being used in a
feedback loop, the quantization
steps of the ADC may cause
phase discontinuities. For these
reasons some applications may
require use of the traditional FM
implementation available as special function 120.
The following describes the performance that results from activating each type of FM in the
8645A.
Center frequency accuracy and
temperature stability:
For the digitized FM mode, the
initial center frequency accuracy
is typically 0.1% of the FM deviation set (typically 1% if in fast
hop mode). Non-digitized or linear FM (special 120) has an offset of typically 1 kHz for any
deviation in the 515 to 1030 MHz
carrier band (offset divides for
lower carrier bands.) The carrier
frequency temperature drift with
digitized FM active is typically
less than 0.1% of the set deviation over the full operating range
of 0 to 55 ˚C. The center frequency accuracy in linear FM
will vary approximately 1 kHz/
˚C in the 515 to 1030 MHz carrier range.
Phase noise variations at low
offsets:
For deviations less than 5% of
the maximum deviation available
at any carrier frequency, digitized FM operation negligibly
affects the phase noise performance of the output. The noise
increases with higher FM deviations. For linear FM, the phase
noise at low offsets goes up by
20 dB when activated but does
not increase as much for higher
deviations as it does in digitized
FM. In general, special 120 linear
FM may improve phase noise
performance over the digitized
FM at offsets less than 10 kHz
when deviations greater than
20 kHz are in use. The topic
“Phase Noise Performance” in
this product note has more
information on this subject.
FM deviation accuracy:
For modulation rates <1 kHz,
deviation accuracy for the digitized FM is a function of the
ADC, typically <1% of the set
deviation. For linear FM it is
approximately 5%. At higher
rates, the deviation accuracy is
dependent on analog factors
which makes it the same for
either FM technique.
Square wave or digital
modulation waveforms:
For linear FM the group delay is
very flat and typical of other signal generators. The group delay
of digitized DC FM is a function
of the modulating rate. For single-tone modulation signals or if
the spectral energy is primarily
below 10 kHz, group delay in digitized FM operation is constant
and will not affect the output
signal. For high rate or digital
modulating signals, the variable
group delay of digitized FM
could cause serious distortion so
linear DC FM should be selected.
28
AC coupled FM
This product note builds on the
information given in the digitized FM operation product note
to explain the operation and
resulting performance of AC
Coupled FM (ACFM).
The 8645A has digitized and linear ACFM capabilities. Digitized
ACFM is simply AC coupling of
the digitized DCFM described in
the “Digitized FM Operation”
product note. The default digitized DC Coupled FM (DCFM)
utilizes an Analog-to-Digital
Converter (ADC) to digitize the
incoming modulating signal as it
occurs and uses the digitized
information to modify the synthesis dividers. Low frequency
3 dB bandwidth of digitized
ACFM is approximately 7 Hz.
This function should satisfy
most applications if the slight
frequency inaccuracy of approximately .1% of the programmed
deviation can be tolerated. Care
must be exercised that the complexity of the modulating waveform does not cause unlocking
or distortion problems as
explained in the “Digitized or
Linear FM” product note. Due to
the very low drift of the digitized
DCFM operation in the 8645A,
there is less use for digitized
ACFM. It should be noted that
the 50 ohm input impedance to
ground of the external FM input
will be still there for ACFM
signals.
The 8645A also offers special
function 120 to get linear ACFM.
This mode is more typical of the
ACFM in other signal generators
with the modulating signal input
directly to the VCO except that
the phase lock loop that is modulated has a much wider bandwidth than is typical. Because of
this, the 3 dB low frequency corner is either 300 Hz or 3 kHz,
depending on whether mode 2 or
1 respectively of the mode select
keys is lit. If the user can tolerate this relatively high 3 dB frequency corner, the benefits this
operation includes minimal
group delay and precise center
frequency. The applications
where linear ACFM may be useful are when any frequency inaccuracy can’t be tolerated, or the
group delay characteristics of
the digitized FM can’t be tolerated, or unlocking occurs due to
square waves at the FM port.
29
Special Capabilities
Tailored operation through
special functions
In addition to the features
directly available to a user
through the front panel keys, the
8645A has a number of capabilities called “special functions”.
These functions are accessed
with the [Special] key in the utility field on the front panel and
allow a user to customize operation of the instrument for a specific application.
Special functions or “specials”
are functionally grouped by
number as follows:
To find a particular special in the
list, the user can scroll through
the list by pressing [Special] and
either turning the knob or pressing the up or down arrow keys.
When the display shows the
desired special, pressing [Enter]
will access that special. A user
may also press [Special] followed
by the number of the special and
[Enter] to access it. Pressing
[Special] [Enter] will re-access
the last special displayed. Once a
special is accessed, it can be controlled with the same keys used
for control of standard functions.
When displaying a special, the
light above the special key will
be lit if it is not at its default setting (the special is active). While
scrolling through the specials as
described above, the light will
also come on when displaying
the name of an active special.
When no special is being displayed and standard functions
are being used or displayed, this
light will be lit if there are any
specials active. Pressing the
[Display] key followed by the
[Special] key will show a list of
all active specials.
30
100-109: Amplitude & ALC functions
110-119: Carrier control (phase, frequency multiplier, sweep mode)
120-129: FM functions
130-159: Audio source control
160-169: Frequency reference
170-189: Tests, calibration, security, volt/power meter functions.
190-199: Serial number, display control
200-210: Fast hop functions
Protecting classified
instrument settings
The 8645A incorporates a number of functions to prevent the
unauthorized exposure of classified instrument settings and
readouts. They range from blanking the displayed readouts, to an
automatic memory erasure if
power to the instrument is interrupted.
The simplest function which has
been present on many instruments for some time now is display blanking. With the 8645A,
the entire display or specific
functional readouts such as frequency or modulation can be
blanked either from the front
panel by activating special functions or using HP-SL commands
via GPIB. This blanking includes
all annunciator lights on the
function specific keys as well as
the alpha-numeric display. This
will prevent the casual observer
from seeing the instrument settings. To prevent an operator
from unblanking the display by
deactivating the special function
another security-related capability, the local lockout can be initiated over GPIB. With local
lockout active the instrument
keyboard will not respond to the
user until the controller removes
the local lockout over the GPIB
or until power is cycled. If power
is cycled the machine returns to
whatever state it was in before
power was turned off, except
that local lockout will be disabled.
A function called RAM wipe was
created to allow the user to
erase all user entered parameters and operation specific calibration data from the internal
RAM. When this special function
172 is activated, a power-on
reset is performed on the instrument. All RAM locations are
cleared and tested with checkerboard patterns, and then cleared
again. A side effect of this operation is that the instrument must
spend a few minutes re-calibrating itself in order to restore the
calibration coefficients in RAM.
All storage registers are cleared,
all fast hop sequence and channel data are also cleared. This
feature satisfies the requirements set forth in Mil Std
380-380.
The three functions described so
far, blanking, local lockout, and
RAM wipe, can be used in combination with the security mode
function for the highest level of
security. Security mode special
function 173, is like a one way
operation in that no reduction of
security is allowed, only increases. Activating security mode sets
a flag in the instrument
firmware such that all or part of
the display can be blanked but
not unblanked. If an instrument
preset is performed, the instrument will remain in its current
state of operation, with displays
blanked and keyboard disabled if
previously set, and the security
mode still active. Security mode
can be deactivated by turning
special function 173 off or
through an GPIB command.
However, leaving security mode
either way automatically and
immediately initiates a RAM
wipe operation. If power is
cycled with security mode active
a RAM wipe will be performed
immediately when power is
restored and the security mode
will be deactivated. With security mode active the user can still
gain control of the instrument,
but not with any classified data
parameters still present on the
display or in memory.
31
Storage registers and
sequential recall
Storage registers are used to
store the current state of the
machine for later recall. There
are 50 storage registers provided
in the 8645A. The storage registers use non-volatile memory so
power interruptions won’t affect
the contents. Instrument preset
has no effect on the storage registers, but activating the RAM
wipe special function to remove
all user-entered data from the
instrument will erase them.
Storage registers 0 through 9
store the entire machine state
except for the fast hop channel
data. (The fast hop channel
information is held in
non-volatile memory also, so
power interrupts won’t force a
reload.) Registers 10 through 49
store only the current frequency
and amplitude (frequency offsets/multipliers or amplitude offsets are not included). Any of
the 50 storage registers can be
recalled explicitly by specifying
the register number.
Machine states are saved using
the front panel keys by pressing
[Save] <register number>
(Enter]. To recall a register,
press [Recall] <register number>
[Enter]. All of the storage registers can be erased at once by
pressing [Clear all] [Enter].
Registers can also be recalled in
a user-defined sequence that has
the instrument repetitively stepping from one register to another. Registers 0 through 9 can be
included in the sequence list
that can be up to 10 registers
long including any repeated registers. A sequence is entered by
pressing [Set seq] <first register
number.> [Enter] <second register number> [Enter] ... continue
to ... <last register number>
[Enter]. There are several ways
to recall registers from a
sequence list. To repetitively
recall the entire sequence, press
[Auto seq]. Pressing [Off] stops
this sequentially recall. To recall
each sequence entry individually, either press [Seq] on the front
panel or provide a +5 volt trigger
signal to the Seq input on the
rear panel. The entire sequence
list in memory can be displayed
by pressing [Display] [Seq].
Each of the functions described
above has its counterpart HP-SL
command for control over GPIB.
32
Offsets and multipliers of
frequency and amplitude
The 8645A has several features
which allow the user to streamline use of the instrument in
applications where the frequency or amplitude of the instrument is scaled or offset by the
user. This change of amplitude
or frequency may be due to an
amplifier, attenuator, cable,
mixer, multiplier, divider, etc.
For some applications, it may be
more useful to control and display the frequency and amplitude in terms of a device’s
output rather than the RF output of the signal generator.
Amplitude offset allows the user
to program into the 8645A the
amount of loss or gain in the
connecting device. For example,
to control and display the amplitude at a device’s input after a
connecting cable loss of 1.2 dB,
the user would press [Amptd
ofs] <–1.2> [dB]. If the display
previously read 0 dBm it will
now read –1.2 dBm with an
additional indicator “Offset”
showing that an offset is in use.
An offset to reflect the output of
a 20 dB amplifier would be
entered [Amptd ofs] <20> [dB].
The 0 dBm display would change
to +20 dBm by these entries. All
subsequent amplitude settings
entered into the instrument will
be displayed according to the
new amplitude reference point.
For example, entering an amplitude value of +5 dBm into the
8645A with a +20 dB offset
active to reflect the output of an
amplifier would actually produce
–15 dBm at the RF output of the
signal generator. In all cases,
losses are entered as negative
numbers and gains as positive
entries. The maximum amplitude
offset range is ±50 dB. Pressing
[Amptd ofs] [Off] turns off an
amplitude offset.
Frequency offset allows the user
to enter a frequency shift
between the RF output and the
display of the 8645A to ref lect
the result of the signal passing
through an external device. The
operation of a frequency offset is
similar to that of an amplitude
offset. For example, if the user
wants to produce an output that
will be translated by a mixing
process upward by 2 GHz and
have the display reflect the
result of the frequency translation, pressing [Freq ofs]
<2> [GHz] will do it. All subsequent frequency entries will
reflect the offset, ie. entering a
3 GHz frequency setting with the
2 GHz offset active will result in
a 1 GHz signal coming from the
RF output of the signal generator. A display that previously
read 1 GHz will then read 3 GHz
to reflect the output of the mixing process and the “Offset” indicator will be lit. Negative entries
can be used to shift the RF output for a down-converting
process. Frequency offsets are
limited to ±50 GHz. To turn off a
frequency offset, press [Freq ofs]
[Off].
Frequency multiplier allows the
user to enter a multiplier or divisor to modify the frequency output of the 8645A to reflect the
action of a multiplier or divider
connected to the signal generator. Frequency multipliers are
entered using special function
111. For example, if an external
device multiplies the signal by
four, then pressing [Special]
<111> [Enter] <4> [Enter] will
produce an RF output that when
multiplied by four will equal the
entered frequency setting. The
frequency display will show the
frequency at the output of the
multiplier. An “Offset” indicator
is lit below the frequency readout. Any subsequent frequency
entries will be in terms of the
entered multiplier and not the
actual output of the generator,
ie. entering 1 GHz with a multiplier of 4 active will produce a
250 MHz signal at the RF output
for multiplication to 1 GHz.
Similarly, negative entries reflect
a divider’s action. A multiplier
range of ±10 can be entered and
only integer values are allowed.
To disable the multiplier function, press [Special] <111>
[Enter] <1> [Enter] which resets
the frequency display for a multiple of 1.
When using a frequency multiplier, the FM deviation is scaled in
the same way as the carrier frequency since an external multiplier or divider also multiplies
or divides the amount of FM
deviation. Therefore, if a multiplier of four was active in the
8645A, an entered FM deviation
of 10 MHz would actually produce a 2.5 MHz deviation on the
actual output of the signal generator and a displayed value of
10 MHz. Frequency offsets have
no effect on FM.
Offsets and multipliers may be
used simultaneously. When
using both a frequency multiplier and a frequency offset, the
displayed frequency equals (the
output frequency) times (the
multiplier) plus (the offset).
33
Built-in calibration
functions
The 8645A was designed with no
manual internal adjustments. All
adjustments are under the control of the instrument firmware
responding to a variety of sensors placed at strategic locations
to monitor signal levels. This
allows the instrument to adjust
its signal parameters for an optimum output whenever the user
wants. With this capability, the
instrument can effectively calibrate itself by making adjustments to signal levels and
recording the settings that result
in the optimum and specified
output signal. This calibration
data is stored in battery back-up
RAM so that the user will not
have to repeat the calibration
whenever the instrument is
turned on. The self-calibration
process typically takes approximately 2 minutes.
If the user suspects that the
instrument is not performing
correctly, a re-calibration of the
instrument may be initiated by
pressing [Special] <171> [Enter]
[On] from the front panel or by
sending the “*CAL?” HP-SL common query over GPIB. The
instrument will display a result
code of 0 if no errors occurred
during the calibration operation.
The internal temperature is constantly monitored by the instrument to insure the optimum
calibrated output. Since portions
of the calibration data are temperature sensitive, the instrument will store the current
temperature when calibrating
along with the calibration data.
If the temperature changes
enough (approximately 10
degrees centigrade) to potentially invalidate the calibration
data, a warning will be placed in
the message queue to notify the
user. If the user feels that the
instrument performance is not
satisfactory, the instrument may
be re-calibrated at the new temperature using the special function.
Under certain conditions the calibration data may be lost or
become corrupt, such as due to
the following events:
1. Disabling the RAM using a
switch on the Digital Control
Unit (DCU).
2. Executing the RAM wipe
special function.
3. Turning off the security
function (which executes the
RAM wipe function).
4. Failing a self-calibration or
diagnostic test.
5. Removing the DCU from the
motherboard.
If the instrument powers up
without valid calibration data, it
will automatically initiate a
self-calibration. The exception to
this is when the service mode
switch on the DCU is enabled to
prevent nuisance calibrations
while working on an incomplete
instrument.
34
Finding failures with
internal diagnostics
The 8645A contains diagnostic
routines in the instrument
firmware which test the instrument hardware and will report
over 90% of all instrument failures that could occur. These
diagnostic routines will also isolate the cause of the failure to a
replaceable hardware module or
a cable. The diagnostic routines
are activated from the front
panel using an instrument special function or via GPIB commands and only run when
initiated by the user.
There is separate circuitry built
into the instrument that continually monitors high-level functionality (such as the phase lock
loop being locked) and will put a
message in the instrument message buffer should a problem be
detected. However, this message
is not specific enough to determine which module is the cause
of the problem. When the user
reads the message, the diagnostic routines can be activated to
determine which hardware module is at fault.
There is special hardware in the
instrument that makes these
diagnostics possible. The digital
controller contains a voltmeter
with AC and DC measurement
capability and each of the hardware modules contains at least
one 8 channel multiplexer
through which critical points on
the modules can be measured by
the voltmeter. A typical diagnostic routine would set a module to
a specific operating mode and
then set the multiplexer on that
module so a critical point on the
module can be measured. The
routine would compare the voltmeter reading with the normal
value. If the reading is not within preset limits, the routine will
terminate testing and a result
code is displayed on the front
panel of the instrument indicating where in the test sequence
the failure occurred. If the reading is normal, the routine continues to set the module to all its
operating conditions and measures critical circuit points for
each set of conditions. A typical
routine makes 100 measurements on a module.
The diagnostic routines can find
almost all-functional failures,
however, the diagnostics are limited in their ability to find fail-
ures where a performance value
is just out of spec. The internal
voltmeter has an accuracy of
approximately 2% and this is not
accurate enough to measure
some internal signals to determine if the specification is being
met. However, these ‘out of spec’
problems are historically a small
part of total instrument failures.
Another limitation is there are
usually only 8 points on a module where measurements can be
made. These 8 points were
selected to maximize the amount
of circuitry that can be tested,
but there is still some circuitry
on most modules that cannot be
tested. In general the diagnostics
will not find problems with
phase noise, spurs, output level
accuracy, high deviation FM and
fast switching.
The service documentation available with the PSG products consists of a diagnostics manual and
a service manual. The diagnostics manual is needed to use the
diagnostics. Its purpose is to
guide the user to finding the bad
module and replacing it with
another module. The service
manual contains schematic diagrams and uses the result of the
diagnostics to guide the user in
repairing the bad module to the
bad component.
35
Agilent Technologies’ Test and
Measurement Support, Services,
and Assistance
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value you receive, while minimizing your
risk and problems. We strive to ensure
that you get the test and measurement
capabilities you paid for and obtain the
support you need. Our extensive support
resources and services can help you
choose the right Agilent products for your
applications and apply them successfully.
Every instrument and system we sell has a
global warranty. Support is available for at
least five years beyond the production life
of the product. Two concepts underlie
Agilent's overall support policy: "Our
Promise" and "Your Advantage."
Our Promise
Our Promise means your Agilent test and
measurement equipment will meet its
advertised performance and functionality.
When you are choosing new equipment,
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Agilent instruments and systems, and
obtain dependable measurement accuracy
for the life of those products.
By internet, phone, or fax, get assistance
with all your test & measurement needs
Online assistance:
www.agilent.com/find/assist
Phone or Fax
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(tel) 1 800 452 4844
Canada:
(tel) 1 877 894 4414
(fax) (905) 282-6495
China:
(tel) 800-810-0189
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Product specifications and descriptions
in this document subject to change without notice.
© Agilent Technologies, Inc. 2001
Printed in USA July 18, 2001
5951-6712
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