7-10 RECEIVE AUDIO MODULE
7-10-1 THEORY OF OPERATION (Refer to Figure 7-12 and Receive Audio
Circuit Schematic in Section 10)
A. GENERAL
Demodulated AM and FM audio signals from the Receiver/Generator
are fed into a mode selector which is controlled by the Modula
tion Mode Selector Switch on the Front Panel. The squelch gate
multiplexer, operated by the squelch level circuit, couples the
selected signal to three lowpass filters (80 kHz, 8 kHz and
250 Hz). The signals from the 80 kHz and 8 kHz filters pass
through a multiplexer, operated by the "FM WIDE" position of
the Modulation Mode Selector Switch, to an amplifier whose out
put goes three places: (1) out of the module, through the
VOLUME Control to the Generate Audio Module, to the Speaker;
(2) out of the module to the DEMOD jack on the Front Panel; and
{3) to a range control multiplexer in the Modulation Meter
driver circuit. Signals from the 8 kHz and 250 Hz filters pass
through a multiplexer controlled by the Variable Tone Generator,
to a multiplexer in the Generate Audio Module. This multiplexer
normally couples this signal to the AGC circuit in the Receive
Audio Module and on to the Audio Counter Circuit in the Digital
Module. However, when "SINAD" or
11
DIST" is selected on the
Modulation Meter Range Switch, this multiplexer disables the
received audio signal and couples a signal from the EXT MOD/
SINAD jack to the AGC circuit in the Receive Audio Module.
The signal selected by the Modulation Meter range control multi
plexer is rectified, then combined with a DC offset. The DC
offset level is controlled by the Modulation Meter Range Switch,
the Modulation Mode Selector Switch, and the operating mode
circuit in the Generate Audio Module. It is then available for
the function select multiplexer U3313 to couple to the Modula
tion Meter.
For sinad and distortion measurements, the output of the AGC
passes through a 1 kHz notch filter (very narrow band-reject)
and rectifier to the function select multiplexer U3313. A level
detector samples the AGC control level and pegs the meter if
the input signal is too low.
A branch of the demodulated AM signal is amplified and fed back
to the Receiver/Generator for automatic gain control. After
amplification, it also is applied to the function select multi
plexer U3313 to provide an indication of received signal
strength. This signal is also used by the squelch control.
The squelch control voltage and the demodulated and amplified
AM audio signal are compared by an op amp. When the audio
signal level exceeds the squelch voltage level, the amplifier
enables the squelch gate, which conducts the audio signal to
the bandpass filters.
7-57
Содержание FM/AM-500
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