HP 8901A User Manual

HP 8901A Modulation Analyzer

H

150 kHz to 1300 MHz

Product Overview

Measurements*

Frequency

Range: 150 kHz to 1300 MHz
Resolution:

10 Hz below 1000 MHz,
100 Hz above 1000 MHz

Input Level:

Automatic Mode: −20 dBm to
+30 dBm
Manual Mode: Typically

−

60 dBm to +30 dBm

Power

Display: Peak envelope power
Range: 1 milliwatt to 1 watt
Accuracy: Typically ±1.5 dB

Input Power Protection:

>25 watts

Outstanding signal
characterization

Applications

Mobile Radio

The HP 8901A Modulation Analyzer combines all the capabilities
necessary for making accurate transmitter measurements. It
can be used in making all the tests listed below. It can also be
used to test modules and subassemblies from either the trans­mitter or receiver. For reliability, input power protection reduces
the chance of accidentally damaging the modulation analyzer by
connecting it directly to a high power transmitter.

• Carrier power

• Carrier frequency and stability

• AM depth

• FM deviation

• Hum and noise

• Incidental AM or FM

• Modulation limiting

- Instantaneous

- Steady state

• Audio frequency response

Modulation

Frequency Modulation

Rates: 20 Hz to 200 kHz
Deviations: to 400 kHz
Accuracy: ±1% of reading ±1 digit

for rates 30 Hz to 100 kHz

Amplitude Modulation

Rates: 20 Hz to 100 kHz
Depths: to 99%
Accuracy: ±1% of reading ±1 digit

for rates 50 Hz to 50 kHz and
depths >5%

Phase Modulation

Rates: 200 Hz to 20 kHz
Deviations: to 400 radians
Accuracy: ±3% of reading ±1 digit

Maintenance and Metrology

Accurately measuring modulation has long been a problem for
metrology laboratories. The HP 8901A Modulation Analyzer
helps solve this problem two ways. First, it provides an extremely
accurate method of measuring AM depth and FM deviation, and
it recovers the modulation with little degradation for making
measurements such as modulation distortion. Second, the optional
calibrators provide a precise modulation source for calibration.
There are many metrology and maintenance uses for the modula­tion analyzer. They include:

• Signal generator calibration

• Modulation calibration standard

• VCO and VCXO characterization

*All parameters describe performance in automatic operation or properly set manual conditions. Specifications
describe the instrument’s warranted performance. Supplemental characteristics (shown in italics) are intended to
provide information useful in applying the instrument by giving typical, but non-warranted, performance parameters.

FeaturesApplications, cont.

Research and
Development

The accuracy and versatility
of the modulation analyzer
make it a very useful labora­tory instrument for charac­terizing a wide variety of
devices and assemblies.
When used with a modulated
signal source the modulation
analyzer can make stimulus­response measurements for
direct measurement of a
device’s effects on such
signals. Because of its wide
frequency range, it can
measure the performance
of both RF and IF assemblies.
Characterizing modulated
sources for sensitivity,
distortion, and incidentals,
and measuring the noise
performance of local
oscillators is also possible.

• Mixer and Amplifier com­pression

• Local oscillator residual FM

• Modulator characterization

• Incidental AM or FM

• RF and IF characterization

Complete Signal
Characterization

The HP 8901A Modulation Analyzer
brings together in one instrument
several RF signal measurement
capabilities. It is more than just a
high quality modulation meter. It
accurately measures carrier fre­quency and peak input power in
addition to completely characteriz­ing modulation. This unique combi­nation of capabilities makes the
HP 8901A Modulation Analyzer an
extremely powerful tool for analyz­ing signals. In normal use it elimi­nates the need to frequently connect
and disconnect several instruments,
such as counters and power meters.
The modulation analyzer is fully
programmable and can be used as
part of an automatic system to make
all these measurements under
remote control.

Single Key Measurements

The HP 8901A Modulation Analyzer
features easy to use controls. In
automatic operation, all major func­tions are selected by pushing a
single key. No manual tuning or
range selection is needed. The modu­lation analyzer automatically tunes
to the input signal, adjusts for proper
signal level, selects the appropriate
measurement range, makes the mea­surement, and displays the result.

High Performance

The HP 8901A Modulation Analyzer
offers a significant advancement in
modulation measurements. Modula­tion depth and deviation are mea­sured with unprecedented 1 percent
accuracy. To complement this capa­bility, optional calibrators with 0.1
percent depth or deviation accuracy
can be included in the analyzer.

A significant improvement has also
been made in the area of residual
noise. The extremely low internal
noise of the modulation analyzer
makes possible residual AM and FM
measurements even on very stable
signals. Accurate measurement of
signals with small incidental AM
or FM modulation are now easily
performed. Residual AM in a 50 Hz
to 3 kHz bandwidth is <0.02 percent,
and residual FM in the same band­width is <8 Hz at 1300 MHz
decreasing to <1 Hz below
100 MHz.

For certain measurements manual
operation may be more desirable.
When selecting a specific signal in
the presence of others or for special
applications, the operator can easily
use the modulation analyzer’s key­board to set any or all measurement
parameters. Functions not selected
manually remain in automatic mode.
This greatly increases the modulation
analyzer’s versatility.

2

The HP 8901A Modulation Analyzer
is a complete measurement system for
accurately characterizing signals in the
150 kHz to 1300 MHz frequency range.

It can make more than just a
single form of measurement; it
combines the capabilities of
three separate instruments. The
HP 8901A Modulation Analyzer
has the capability of a frequency
counter for measuring carrier
frequency. It can measure RF
peak power with typical measure­ment accuracy of ±1.5 dB. It can
also accurately measure modula­tion and recover the modulating
signal. This allows the user to
make those measurements most
commonly needed to totally char­acterize a signal. The modulation
analyzer can measure a signal’s
frequency, frequency drift, peak
power level, AM, FM, or φM, and
AM and FM noise components.
It recovers the modulating signal
with very low added distortion for
audio analysis.

Besides combining several mea­surements in one instrument the
HP 8901A Modulation Analyzer
makes a second contribution to
signal analysis—extremely pre­cise modulation measurements.
Its ability to make highly precise
depth and deviation measure­ments coupled with very low in­ternal noise enables the analyzer
to characterize very accurate
signal sources. Modulation depth
or deviation accuracy is generally
<1 percent of reading. Residual
noise in a 50 Hz to 3 kHz band­width is 0.02 percent for AM and
<8 Hz for FM at 1300 MHz carrier
frequencies, decreasing to <1 Hz
below 100 MHz.

The modulation analyzer is fully
automatic and all major measure­ments can be made by pushing a
single key. The modulation
analyzer’s large digital display
shows measurement results with
excellent resolution and is easy
to read. All measurements can
be easily controlled remotely
and data transferred via the
Hewlett-Packard Interface
Bus (HP-IB).*

Frequency Measurements

In automatic operation the
HP 8901A Modulation Analyzer
has the performance of a high
quality 150 kHz to 1300 MHz
frequency counter. Resolution
is 10 Hz below 1000 MHz and
100 Hz above 1000 MHz. Sensi­tivity is −25 dBm (12 mVrms)
below 650 MHz and −20 dBm
(22 mVrms) above 650 MHz.

Besides normal frequency
measurement capabilities the
analyzer’s counter has several
additional capabilities. Like most
frequency counters it will mea­sure signals over a wide dynamic
range, >50 dB (22 mVrms to
7 Vrms), and is protected from
damage for signals up to 35 Vrms.
Unlike many frequency counters,
however, it automatically adjusts
itself as the input level changes.
There is no need to manually set
or adjust any input attenuator.
Because the modulation analyzer
is usually used to measure modu­lated signals, its frequency
counter accurately measures
signals with significant levels
of AM modulation.

The modulation analyzer uses an
indirect technique for measuring
RF frequencies. Instead of count­ing directly, the input signal is
down-converted to an interme
diate frequency (IF) using a mixer
and a local oscillator (LO). By
counting the frequency of both
the IF and LO and calculating
their difference, the modulation
analyzer can determine the
frequency of the input signal.
In automatic operation the ana­lyzer automatically tunes to the
largest input signal and measures
its frequency.

In manual operation the user de­termines the frequency to which
the modulation analyzer tunes.
When more than one signal is
present it is thus normally pos­sible to select which signal is
counted. Entering the approxi­mate frequency on the keyboard
causes the IF filter to eliminate
all but very close interfering sig­nals. This allows the modulation
analyzer to selectively count sig­nals other than the largest. Also,
because of the large IF gain of the
modulation analyzer, it is possible
to measure very low-level signals.
In manual operation the modula­tion analyzer has sensitivity of

0.22 mV
of >90 dB (0.22 mV

* HP-IB is Hewlett-Packard's implementation

of IEEE standard 488.

, and dynamic range

rms

to 7 V

rms

rms

).

3

RF Power Measurements

The HP 8901A Modulation Ana­lyzer uses a diode detection cir­cuit to measure RF input power.
This technique measures peak
voltage and is calibrated from
1 mW to 1 W for sine wave inputs.
In the case of amplitude modu­lated signals, the modulation ana­lyzer measures the peak envelope
power with ±1.5 dB accuracy,
thus eliminating the need for a
power meter in most applications.

The modulation analyzer is
equipped with input power protec­tion to prevent damage from the
accidental application of exces­sive power. This is a common
cause of damage in equipment,
such as the modulation analyzer,
used to measure transmitters.
The modulation analyzer is tested
for inputs up to 25 watts. Protec­tion is provided by limiting diodes
and an RF relay. When excessive
power is applied the relay opens
and protects sensitive compo­nents, and the analyzer displays
an error message. The circuit
automatically resets whenever a
key is depressed. This technique
is superior to fuses which in many
cases are too slow for adequate
protection and require replacement
each time an overload occurs.

In addition to normal RF level
measurements made directly on
the input signal, the modulation
analyzer can measure the signal
level in the constant-gain IF filter
pass band. This is the Tuned RF
Level function. In this mode the
analyzer accuracy is degraded
from normal RF measurements,
but relative power measurements
at a single frequency can be made
with increased resolution. Because
the IF filter allows some selectiv­ity, one signal can be measured
even when others are present.

Modulation Measurements

In AM, high accuracy and low
noise are coupled with resolution
of 0.01 percent below 40 percent
depth and 0.1 percent resolution
to over 100 percent. AM signals at
rates up to 100 kHz can be mea­sured and the modulation accu­rately recovered. AM signals with
significant levels of FM can be
measured because of excellent
FM rejection.

Most AM depth measurements can
be made with accuracies better
than 1 percent of reading. This is
made possible by very linear am­plifiers and detectors. Because
these amplifiers and detectors
are also low noise, residual AM
in a 50 Hz to 3 kHz bandwidth is
<0.02 percent rms.

FM deviation can be measured
with accuracy of 1 percent and
displayed with resolution ranging
from 1 Hz for deviations below
4 kHz, to 100 Hz for deviations
greater than 40 kHz. Modulation
is recovered with less than 0.1
percent distortion, and most
AM is rejected.

The ability to measure low
residual FM is one of the key
contributions of the modulation
analyzer. A low noise local
oscillator in combination with a
low noise discriminator allows
residual FM measurements of
<8 Hz at 1300 MHz and <1 Hz
below 100 MHz. This is low
enough to allow the direct
measurement of residual FM
of such low noise sources as
crystal oscillators.

For all AM depth and FM deviation
measurements the user can select
from three detectors. Both posi­tive and negative peak (trough for
AM) can be measured. The modu­lation analyzer also has an aver­age-responding detector which is
RMS sine wave calibrated. This
type of detector is useful for
determining the residual noise
on a signal where the RMS value
and not the peak is generally the
desired measurement.

The modulation analyzer also has
a Peak Hold function that is used
with either the positive or nega­tive peak detectors. This function
captures and displays the maxi­mum peak modulation of a signal
and is ideal for making measure­ments such as modulation limiting
on mobile radios. Peak Hold can
capture even very short tran­sients and display their peak
value indefinitely. Measurements
can be made for any length of
time and either the largest posi­tive or negative peak that occurs
will be measured. Pushing the
Peak Hold key resets the
display and initiates a new
measurement cycle.

4

Post Detection Audio
Filters

The modulation analyzer has two
high-pass and three low-pass post­detection audio filters for filtering
the recovered modulation. These
filters can be selected individu­ally or in combination. Their cut­off frequencies have been chosen
to match those needed for appli­cations such as transmitter or
signal generator testing. The
>20 kHz filter is a Bessel filter.
It minimizes overshoot for
square-wave modulation so that
this type of modulating waveform
can also be accurately measured.

The modulation analyzer contains
four de-emphasis networks that
can be used in addition to the
audio filters. These are the ones
commonly used in FM communi­cations—25, 50, 75, and 750 µs.
When selected, the de-emphasis
networks always affect the de­modulated output. The user can
select whether or not the de-em­phasis network affects the devia­tion measured and indicated by
the display. The ability to select
either the actual or “de-empha­sized deviation” increases the
usefulness of the modulation
analyzer in many applications.

Modulation Calibrators

One of the most difficult problems
involved in making very accurate
measurements of AM depth or FM
deviation is generating a pre­cisely modulated signal to use as
a calibration standard. By order­ing Option 010 a precise AM and
FM modulation standard is in­cluded in the HP 8901A modula­tion analyzer.

The AM standard is generated by
summing two identical 10 MHz
signals. When one of the signals
is switched on and off at a 10 kHz
rate, the result is 33.33 percent
AM depth. By internally measur­ing any slight difference in the
levels of the 10 MHz signals the
analyzer is able to determine the
actual depth to ±0.1 percent accu­racy. To further improve the
modulation envelope the rise and
fall transitions are smoothed to
eliminate ringing that might
otherwise occur when this signal
is measured.

The FM standard is generated by
square-wave modulating a VCO
with a nominal 33 kHz peak devia­tion. By using the internal counter
to measure the upper and lower
frequency of this signal, the ac­tual peak deviation is determined
to ±0.1 percent accuracy. To
prevent ringing, the square
wave is modified to a round
edge trapezoid.

Because the modulation stan­dards are internal to the analyzer,
there is little need for metrology
laboratories to purchase separate
calibration standards. Also, be­cause of the technique used, it is
easy to verify that the calibrators
are operating properly.

5

Operation

Often instruments with state-of­the-art accuracy require tedious
setup or highly skilled operators
in order to be used. This is not the
case with the HP 8901A Modula­tion Analyzer. It provides excel­lent accuracy while remaining
easy to use. The front panel is
simple, uncluttered, and easy
to understand.

The user need only select the
measurement to be made. There
is no need to tune, adjust levels,
or select the appropriate range;
the internal microprocessor does
all this quickly. Because the mi­croprocessor determines the best
instrument settings, most mea­surements require only a single
keystroke.

For those applications requiring
tuning to a specific frequency,
automatic tuning may be overrid­den. This feature allows a single
signal to be selected in the pres­ence of others but retains the
speed and convenience of the
rest of the automatic functions.

The user can also make measure­ments relative to either a mea­sured value or one entered from
the keyboard by using the ratio
keys. Relative measurements can
be expressed in either dB or per­cent. This means that when test­ing FM mobile transmitters a user
could enter 3 (kHz), depress the
dB key, and make measurements
in dB relative to 3 kHz deviation.
Similarly, in broadcast FM appli­cations, deviation could be dis­played in percent relative to
75 kHz deviation where 75 kHz is
defined as 100 percent. The user
can also enter a measurement
limit on the keyboard which will
cause the modulation analyzer to
signal whenever the measured
value exceeds the value entered
as a limit.

Special Functions

The modulation analyzer can do
more than is apparent from the
front panel. This capability is
accessed by using the data keys
and a special function key or
remotely via HP-IB. They fall into
three categories: manual control
of instrument functions, instru­ment operation verification, and
service aids.

An example of the type of special
function found in the manual
control category is the auto
tune-track mode. This mode is
accessed by entering 4.1, then
pressing the special key. Once
the modulation analyzer has been
placed in track mode the analyzer
will continuously track the signal
as it changes frequency. This
eliminates the delays caused by
the analyzer searching for the
signal each time the signal’s
frequency changes. Using this
special function, a user could

continuously monitor modulation
accuracy on a signal generator
while tuning across the signal
generator’s frequency band.
Auto tune-track makes it possible
to tune the analyzer three ways:
automatically, manually by
entering the frequency on the
keyboard, and track mode.

Special functions can also be used
to set any measurement range or
instrument function. They can be
used to select either of two inter­nal IF’s, the one normally used for
frequencies above 10 MHz or a
narrow IF where rates and devia­tions are more restricted but
selectivity is increased. All instru­ment functions not set using these
special functions remain in auto­matic mode. This allows the user
to select any combination of
manual or automatic operation.
By depressing the special key, the
display shows an eight-digit num­ber that indicates which functions
are in automatic and the state of
those manually set.

There are also numerous special
functions that can be used in veri­fying that the instrument and its
various sections are operating
properly. These, along with ser­vice functions used in diagnosing
and repairing the analyzer, make
repairs much faster and easier.
An additional service aid is the
built-in ability to use HP signature
analysis instrumentation. This
allows a technician with little
knowledge of digital circuits to
rapidly troubleshoot a failure in the
digital portion of the instrument.

Those special functions that are
most commonly used in operating
the analyzer are described on the
pullout information card under the
front panel.

6

Programmability

The HP 8901A Modulation Ana­lyzer is completely programmable
via the Hewlett-Packard Interface
Bus (HP-IB). This, coupled with
the ability of the modulation ana­lyzer to make several measure­ments, the speed with which these
measurements can be made, and
the flexibility of the special func­tions, makes the modulation ana­lyzer ideal for systems
applications. In many instances
it can reduce the number of in­struments in a system, speed
measurements, reduce complex­ity, and improve accuracy.

When the analyzer is in remote,
the front panel annunciators make
it very easy to determine what
state the analyzer is in.

Applications

The HP 8901A Modulation Ana­lyzer is a useful tool for analyzing
many types of signals. Often it
can provide needed information
that has been difficult to obtain
such as incidental FM or residual
FM. It can replace large, complex
test systems, and speed and sim­plify measurements. The modula­tion analyzer is superbly capable
of measuring transmitters used in
mobile communications. This single
instrument can be used in making
most of the measurements made on
transmitters.

The modulation analyzer can be
equally useful for other types of
transmitters. For avionics appli­cations it can be very useful in
measuring navigation signals. In
testing ILS transmitters the ana­lyzer can be used to very accu­rately measure depth of
modulation. For broadcast AM and
FM it can be used to measure AM
depth or FM deviation, and it can
accurately recover the modula­tion for making measurements
such as stereo separation.

The modulation analyzer’s capa­bilities exceed those required to
verify many signal generator
specifications. Besides improving
the accuracy of these measure­ments it greatly reduces the time
involved in making measure­ments. Also, the optional calibra­tors provide a new level of
modulation standard accuracy
and help ensure accurate mea­surements. They also enhance
the calibration of the modulation
analyzer itself .

Because the modulation analyzer
is useful for characterizing all
types of signals in general, it is
very useful in research and devel­opment laboratories. It can be
used for characterizing VCO’s,
measuring residual noise on
crystal oscillators, measuring
incidental modulation, measuring
frequency on low level signals,
etc. When used with a signal
source it can be used to charac­terize RF and IF designs, evaluate
modulators, and test individual
IC’s or modules.

With its accuracy the modulation
analyzer makes an excellent addi­tion to a metrology laboratory. An
example of its usefulness is in
calibrating signal generators.

7

Options

HP 8901A Modulation Analyzer
Option 001: Rear panel instead of front panel connections

for input, modulation output, calibrators.

Option 002: lx10−9/day internal reference oscillator.
Option 003: Rear panel connections which allow use

with an external local oscillator.

Option 004: Operation from 48 to 440 Hz power line.
Option 010: AM and FM calibrators.
Option 907: Front panel handle kit.
Option 908: Rack mounting flange kit.
Option 909: Front panel handle plus rack mounting flange kit.
Option 910: Extra manual.
Option 915: Service manual.

Related Literature:

Technical Specifications .................. 5968-1286E

Price List ......................................... 5968-1285EUS

Warranty Information

This Hewlett-Packard instrument product is warranted against defects in material and workmanship
for a period of one year from date of shipment. During the warranty period, Hewlett-Packard
Company will at its option, either repair or replace products which prove to be defective.

For warranty service or repair, this product must be returned to a service facility designated by HP.
Buyer shall prepay shipping charges to HP and HP shall pay shipping charges, duties, and taxes for
products returned to HP from another country.

HP warrants that its software and firmware designated by HP for use with an instrument will execute
its programming instructions when properly installed on that instrument. HP does not warrant that
the operation of the instrument, or software, or firmware will be uninterrupted or error free.

Limitation Of Warranty

The foregoing warranty shall not apply to defects resulting from improper or inadequate mainte­nance by buyer, buyer-supplied software or interfacing, unauthorized modification or misuse,
operation outside of the environmental specifications for the product, or improper site preparation
or maintenance. No other warranty is expressed or implied. Hewlett-Packard specifically disclaims
the implied warranties of merchantability and fitness for a particular purpose.

For more information about Hewlett­Packard test and measurement products,
applications, services, and a current
sales office listing, visit our web site:
http://www.hp.com/go/tmdir

You can also contact one of the follow­ing centers and ask for a test and
measurement sales representative.

United States:

Hewlett-Packard Company
Test and Measurement Call Center
P.O. Box 4026
Englewood, CO 80155-4026
(tel) 1 800 452 4844

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Mississauga, Ontario
L4W 5G1
(tel) [phone]

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(tel) (81) 426 56 7832
(fax) (81) 426 56 7840

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U.S.A.
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(fax) [phone]

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ã 1982 Hewlett-Packard Co.
Data subject to change
Printed in U.S.A. 7/98
5968-1287E