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Adjustments
Model 8902A
Adjustment 21-Audio
True RMS Detector and Notch Filter Gain
Reference
Service Sheet 16.
Description
The offset of the audio true rms detector is first adjusted t o read zero with no audio input, then a
signal of known level is applied to the detector and the sensitivity of the detector is adjusted to read
the correct level. The gain of the audio notch filter is adjusted for a reading of 100% with an input
signal that is twice the frequency of the notch filter,
Equipment
Audio Oscillator
.................................................................
HP 8903B
Digital Voltmeter
................................................................
HP 3455A
OUTPUT
1 - 1
AUDIO
OSCILLATOR
1
HP 8902A
MEASURINQ RECEIVER
0
D I G I T A L VOLTMETER
o
0 0
oooooo
o
0000
0
OUTPUT/
AUDIO
INPUT
I
Figure
5-9. A
irdio True
RMS
Dt>kc*lor and Nolch Filttir
Gain
Adjirslmenl
TtW ScVirp
Procedure
1.
Remove any cable from the MODULATION OUTPUT/AUDIO INPUT connector of the
Measuring Receiver.
2.
On
the Measuring Receiver, press the blue key then press INSTR PRESET (the AUTOMATIC
OPERATION key) to preset the instrument. Key in 30.0 SPCL to set the instrument to measure
and display the voltage at the MODULATION OUTPUT/AUDIO INPUT connector with the
audio true rms detector. Key in 49.7 SPCL to measure and display the output of the audio true
rms detector. (The displayed voltage will be ten times the level displayed with Special Function
30.)
3.
Adjust A52R47 (RMS
OFS)
for a reading o n the Measuring Receiver's display between -0.0005
and 0.0005.
4. Key in 30.0 SPCL. The display should read 0.0000 V.
5. Connect the equipment as shown in Figure 5-9.
6. Set the audio oscillator for 3.9 Vrms into an open circuit at 2 kHz. Set the voltmeter to read
ac volts.
5-36
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