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Operation
Model 8902A
THEORY
Residual noise is the short-term amplitude or phase (and thus frequency) instability inherent in
any CW signal source. In a measurement system composed of a signal source and the Measuring
Receiver, residual noise is contributed by both instruments. When modulation is measured with the
Measuring Receiver, both the intended and residual combined are measured. To precisely determine
the modulation index due to the baseband signal alone, the effects of residual noise must be factored
out of the measurement results.
Two noise components are commonly encountered in modulation measurements: periodic (often
line-related) and gaussian (random). Periodic noise and the baseband signal behave identically. The
Measuring Receiver measures the arithmetic sum of the peak or average levels of the two signals
(according to the detector selected). To determine the modulation index due to the baseband signal
alone, switch the modulation
o f f ,
measure the peak-residual noise on the same range as before,
and subtract the result from the original displayed modulation. The effects of random noise o n the
measurement system are not as straight forward. True random noise, when viewed in the frequency
domain, is a continuous spectrum of frequencies at various amplitudes. The frequency of the noise
spectrum is limited only by the bandwidths of the observing and/or generating devices. In the time
domain, noise of this kind appears as random amplitude spikes (or fuzz) riding on
top
of the recovered
baseband signal. The amplitude of these spikes
is
limited by the slow rate of the measuring and/or
generating devices. Peak detecting these spikes exaggerates the amount
of
energy present in the
noise spectrum so noise measurements are typically made with average-responding detectors and
with limited bandwidths.
The measurement problem arises because modulation index is typically expressed as a peak level.
To account for residual noise in these peak measurements, the actual effects of the noise o n the
Measuring Receiver's peak detector must be determined.
A
simplified diagram of the Measuring Receiver's peak detector is shown in the following figure.
Whenever the signal-plus-noise voltage into the comparator exceeds the voltage stored on the output
capacitor,
C,
the comparator closes switch S1. The capacitor is then charged via the path from +V
through resistor R. When the voltage across the capacitor exceeds the voltage of the incoming signal,
the comparator opens the switch again. This process continues until the voltage on the capacitor
is transferred to the voltmeter through the sample-and-hold switch, S2.
C is
then discharged by 53.
When very narrow noise spikes are imposed on the comparator's input, the charging circuit's RC
time constant will not allow the capacitor
to
fully charge before the noise peak has passed.
+v
C 0 M
PA
R
AT
0
R
To
Voltmetel
3-222
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