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Service
Model 8901B
The audio output path to the Voltmeter (A5) is through the Inverting/Non-Inverting Amplifier. The
amplifier usually has a gain of -1, but when PEAK+ is selected or, for FM and QM, when the input
signal is down converted by the Input Mixer (A17), the gain is
+l.
By using PRE-DISPLAY, the input
to the amplifier can be selected to include the FM de-emphasis networks.
The Absolute Peak Detector, the Audio Overvoltage Detector, and the Voltmeter together sense the
audio signal level to determine the audio range. The Audio Overvoltage Detector compares the audio
voltage to a reference. If the audio level is too high, the Audio Overvoltage Detector reacts quickly to
set the audio gain to minimum and to set a status flag which can be read by the Controller. The output
of the Absolute Peak Detector (which detects the greater of the positive and negative peaks) is read by
the Voltmeter.
If automatic ranging has been selected, the Controller reduces the audio gain depending on the following
conditions governed by the Controller:
(1)
the magnitude of the positive or negative peak of the audio
signal or (2) the voltage read from the Audio Peak Detector of the A5 Voltmeter Assembly. Most often
the audio autoranging is determined by the voltage read from the Audio Peak Detector. However,
situations arise where the predominant component of the audio signal is filtered out by one of the
active filters in the audio chain (for example, a 10 kHz signal into the
3
kHz Low-Pass Filter). Having
the Absolute Peak Detector ahead of the active filters prevents such situations from overdriving the
audio circuits.
The front-panel MODULATION OUTPUT/AUDIO INPUT is driven by an inverting Output Amplifier
via the A52 Audio Counter/Distortion Analyzer Assembly. The output of this connector
is
always
affected by audio filtering and FM de-emphasis, when selected.
Voltmeter Assembly (A5)
The Voltmeter consists of an average detector, a peak detector, and a voltage-to-time converter.
The Audio Average Detector consists of a precision Half-Wave Rectifier and a Summer and Filter. The
summer amplifier adds the input signal, weighted by a factor of one, to the inverted and half-wave-
rectified input, weighted by a factor of two. The resultant sum is a full-wave-rectified output. After
filtering, the output dc voltage is equal to the signal's rectified average.
The Audio Peak Detector captures the positive, ac peak. The Sample-and-Hold Switch controls the
transfer of the detector's output to the Voltage-to-Time Converter (via the Buffer Amplifier) and also
controls the discharging of the Audio Peak Detector. Special F'unction 5 controls the discharge rate.
The voltage is actually measured by the Voltage-to-Time Converter. The Input Selectors select one of
many dc inputs into the converter. The output of the selectors is a reference input to a Comparator. The
Comparator's other input is a linear ramp. As the ramp (initiated by the Counter) rises, the Counter
counts the time base reference (10 MHz). When the ramp voltage equals the level of the Comparator's
other input, the Comparator signals the Counter to stop counting. The accumulated count represents
the dc voltage. Ground is measured separately and subtracted from the Voltmeter measurement.
Special Functions 49 and 50 allow direct access and display of the Voltmeter readings.
Audio Counter/Distortion Analyzer (A52)
Inputs to the Audio Counter/Distortion Analyzer come from either the internal audio circuits via
the Output Amplifiers in the A3 Audio De-emphasis and Output Assembly or from the external
MODULATION OUTPUT/AUDIO INPUT as determined by the Internal/External Source Switch.
The distortion (plus noise) on a 400 Hz
or 1
kHz audio signal is measured by comparing the level
of the input signal with the level of that same signal which has had the fundamental removed by a
notch filter. The distortion measurement result (which also includes noise)
is
the ratio of the signal,
with fundamental removed, to the unfiltered signal. In both measurements, the signal level is measured
by the RMS-to-DC Converter which is then read by the Voltmeter. The voltage from the RMS-to-
DC Converter can be measured and displayed by means of Special Function 30, which automatically
switches the Input/Output Switch to the external, audio input position.
8D-38
Service Sheet BD4
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