HP 8901B Operation And Calibration Manual page 21

Model 8901B General Information
High Selectivity Measurements.
Options 030-037 add selective power measurement capability to the
HP
8901B Modulation Analyzer. Used with a low-noise external LO, the HP 8901B performs fast,
accurate single-sideband (SSB) carrier noise measurements to 1.4 GHz.
RF Frequency Measurements
In automatic operation, the Modulation Analyzer has the performance of a high-quality, 150 kHz to
1300 MHz frequency counter. The frequency counter automatically adjusts itself as the input level
changes. There is no need to manually set or adjust the input attenuator. Because the Modulation
Analyzer is usually used to measure modulated signals, its frequency counter
also
accurately measures
signals with significant levels of AM.
Modulation Measurements
The Modulation Analyzer has extremely low internal noise. Incidental AM, FM, and QM can be
measured on a wide range of simple and complex modulated signals. To complement its modulation
measurement capability, the HP 8901B characterizes audio signal level, frequency, and distortion.
(These features are also available for external audio signals.)
Filters, De-emphasis Networks and Modulation Calibrators
Post-Detection Audio Filters.
The Modulation Analyzer has two high-pass and three low-pass post-
detection 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.
De-Emphasis Networks.
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
ps.
When selected, the de-emphasis networks always affect the demodulated output.
You can select whether the de-emphasis network affects the deviation measured. 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. In all instruments, a precise AM and FM modulation standard is included.
When the output of the calibrator is connected to the Modulation Analyzer's input, the amount
of modulation is measured to create a calibration factor. The calibration factor can be used to
automatically compensate
all
subsequent measurements. The calibration factor is the ratio of the
measured modulation to the internally-computed modulation of the calibrator, expressed in 7%.
Additional Features
Tuning Features.
In automatic operation, the Modulation Analyzer automatically tunes to the input
signal and measures it.
In manual operation, you can determine the frequency
to
which the Modulation Analyzer tunes.
Entering the approximate frequency on the keyboard causes all but very close interfering signals to be
eliminated. This allows the Modulation Analyzer to selectively measure signals other 'than the largest.
A track mode feature enables you to track a signal, as it changes frequency, from either automatic or
manual tune operation.
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