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Model 8901B
Service
Service Sheet BD4
BLOCK
a
Audio
PRINCIPLES OF OPERATION
General
The Audio Block Diagram documents the audio circuits which process the demodulated IF signal and
convert the audio signal to dc for measurement by an internal digital voltmeter. The audio circuits
include amplifiers, filters, attenuators, switches, ac to dc converters, an integrator, a counter, a distortion
analyzer, etc.
Audio Filters Assembly (A2)
The residual IF carrier on the demodulated AM or FM is filtered by the 260 kHz Low-Pass Filter in
each path. These filters determine the audio bandwidth when L P FILTER is set to >200 kHz. 20 dB
Attenuator 1 partially determines the audio gain in the FM and @Mmodes.
The demodulated signal passes through Amplifier 1, which has a gain of 8.9 dB,and when selected,
the 15 kHz or >20 kHz Low-Pass Filter further filters the signal. The 15 kHz Low-Pass Filter is
automatically selected for the 455 kHz IF. The >20 kHz Low-Pass Filter can also be selected. The 6
dB Attenuator in the through-path matches the 6 dB loss through the two filters.
Amplifier 2 has 13.7 dB of gain. 20 dB Attenuator 2 gives further audio-range control. Amplifier 3
has 20 dl3 of gain. Amplifiers 1, 2, and 3 distribute the audio gain for optimum noise reduction and
distortion. Special Function 2 controls the overall audio gain. Table 8D-12 lists the modulation ranges
and the associated attenuation. (The ranges in the table apply only for the PEAK+, PEAK-, and
AVG audio detectors.)
Table 80-12. Attenuation us. Modulation Range
AM
Depth
("/I
40
100
100
Modulation Range
FM
@M
Deviation
Deviation
(kHz)*
(rad)
4
4
40
40
400
400
20 dB Attenuator
1
2
out
out
out
in
in
in
With 750 ps FM DE-EMPHASIS and PRE-DISPLAY selected the FM
ranges are 0.4, 4, and 40 kHz.
Audio De-emphasis and Output Assembly (A3)
The Audio De-emphasis and Output Assembly contains audio filters, FM de-emphasis networks, a QM
integrator, audio output amplifiers, and two audio level detectors.
The 50 and 300 Hz High-Pass and 3 kHz Low-Pass Filters are active filters selected from the front
panel. The four FM de-emphasis networks are single-pole, low-pass filters with time constants of 750,
75, 50, and 25 p s . The 750 ps network also adds 20 dB of gain.
The phase deviation of the RF input signal is recovered by integrating the demodulated FM with the
Phase Modulation Integrator. In mathematical terms, the instantaneous phase deviation is the integral
of the instantaneous frequency deviation.
Service Sheet BD4
8D-37