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General Information
Model 8902A
The demodulated signal is amplified and filtered in the Audio Filters and Gain Control circuitry. The
filters are selected from the front panel, and for
FM,
the filtering may also include de-emphasis. The
processed signal is passed to the front-panel MODULATION OUTPUT/AUDIO INPUT connector
and the voltmeter.
The audio signal from the Audio Filters and Gain Control
is
converted to a dc voltage by the Audio
Peak Detector, the Audio Average Detector or the Audio RMS detector. The Audio Average and
RMS Detectors are used primarily for measuring noise. The output from the detectors is routed into
the Voltage-to-Time Converter.
The Voltage-to-Time Converter within the voltmeter converts the dc input into a time interval.
During the interval, the 10 MHz Time Base Reference is counted by the Counter, and the resultant
count represents the dc voltage. Other inputs to the voltmeter, which are not shown, include outputs
from an audio level detector and the AM calibrator.
The Distortion Analyzer measures the distortion of either the internal demodulated signal o r an
audio signal applied externally to the MODULATION OUTPUT/AUDIO INPUT connector. The
frequency of the input signal must be either 1 kHz or
400
Hz. The distortion on the signal is deter-
mined by measuring the amplitude of the signal before and after a notch filter that is set to 1 kHz or
400 Hz. The two ac signals are converted to dc
by
a the Audio RMS Detector and then measured
by the voltmeter. Distortion is computed as the ratio of the voltage out of the notch filter t o the
voltage into the filter. (The Audio RMS Detector can also be used to measure the demodulated AM,
FM, or <PM internally or the ac level of an external audio signal applied to the MODULATION
OUTPUT/AUDIO INPUT connector.)
The frequency of the audio signal at the MODULATION OUTPUT/AUDIO INPUT connector,
whether internal or external, is measured by a reciprocal-type Audio Counter. In the Audio Counter,
the input signal is used to gate the 10 MHz Time Base Reference into the main Counter. (This gating
function is also used by the Voltage-to-Time Converter.) The number of time base pulses received
during the count is read by the Controller which computes and displays the signal frequency.
The AM and FM Calibrators provide a nominal 10.1 MHz signal with a precisely known amount
of AM or FM. When this signal is applied to the instrument's RF INPUT connector (either directly
or via the Sensor Module), the modulation is measured and the calibration factor of the AM or
FM Demodulator is computed and displayed. Related front-panel functions are automatically set
for proper demodulation of the calibrator signal.
1-1 1. MODULATION BASICS
The Measuring Receiver can demodillate and memire three types of modulation: amplitude mod-
ulation (AM), frequency modulation (FM), and phase modulation (QM). In general, modulation is
that characteristic of a signal which conveys the information.
A
signal without modulation is said
to be a continuous-wave (CW) signal. C W signals contain two information-carrying parameters: am-
plitude and frequency. These two parameters, however, are static (time invariant). Consequently,
the information conveyed by them is scant-you
know only that a signal is present at a certain
frequency. When one or both of these parameters is altered as a function of time, the signal is said
to be modulated.
The
RF
signal which is modulated is called the carrier. The modulating signal is referred to as the
baseband signal and can be of any arbitrary form (for example, voice, tone, noise). Demodulation is
the process of recovering the baseband signal from the modulated carrier. The Measuring Receiver
can measure the modulation on carriers in the range of 150 kHz to 1300 MHz. Measurement accuracy
is specified for modulation rates generally between 20 Hz and
100
kHz. The demodulated signal is
present at the MODULATION OUTPUT connector.
Amplitude Modulation
As
the name implies, a carrier is amplitude modulated when its amplitude is varied
as
a function
1-12
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