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Service
Model 8901B
Type
of
Processing
High-Pass Filters
Low-Pass
Filters
Selections Available
<20 Hz
(through path)
50
Hz
300
Hz
3
kHz
15
kHz
>20
kHz (low
ringing)
>200 kHz (260
kHz
LPF)
FM
De-emphasis Networks
FM
De-emphasis Selection
25
ps
50
ps
75
ps
750
ps
None
I
Pre-display
On
Pre-displav
Off
I
Signal Polarity
Relative Gain
L
Inverting
Non-inverting
0 dB
20
dB
40
dB
The Audio Range Detectors are used to determine the audio gain (the Audio Range Level line) and to
sense audio overloading (the Audio Overvoltage Status line).
Volt meter
The demodulated signal is detected by both the Audio Average Detector and the Audio Peak Detector.
The detector outputs are two of several Voltmeter inputs switched by the Input Selector. The Voltmeter
consists of a Voltage-to-Time Converter whose output is applied to the Counter. The Voltage-to-Time
Converter produces a Stop-Count pulse with an interval between pulses proportional to the dc input
voltage. The pulse gates the Counter, which counts the 10 MHz time base reference. The count
accumulated during the gate interval is proportional to the input voltage.
Other inputs into the Voltmeter include:
0
RF Peak Level
0
IF Peak Level
0
IF Average Level (normally equal to the ALC reference)
0
Audio Range Level
0
RF Average Power Level
0
AM Calibration
0
Audio Distortion (from an audio rms-to-dc coverter)
0
Various service-related voltages not shown.
The output from the Input Selector is always present at the rear-panel RECORDER OUTPUT
connector. Normally, the output
is
constantly switching between inputs, but by means of special
functions any of the measurement results can be selected and held. This feature
is
especially useful for
making precise RF power measurements.
8D-4
Service Sheet BD1
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