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.
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