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Section VII. STAGE ANALYSIS, RECEIVE SIGNAL PATHS
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TM-11-5805-387-34-2 Modem Radion Teletypewriter MD-522A/RC (NSN
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RECEIVER MODULE A4
TM 11-5805-387-34-2
1-54. RECEIVE AUDIO MODULE A1
(fig. FO-12)
Audio Amplifier Circuit. In 85-Hz plus voice mode of operation, audio from an associated radio receiver is
applied through MODE SELECTOR switch S4E to receive voice filter FL5. After filtering, the audio is again applied
through the MODE SELECTOR switch and AUDIO GAIN control R1 to receive audio module A1. In the voice mode of
operation, audio from an associated receiver is applied directly to receive audio module A1 through AUDIO GAIN control
R1. In receive audio module A1, the audio input signal is amplified by driver A1A1Q1 and applied to push-pull audio
output amplifier A1Q2, Q3.  Potentiometer A1A1R10 allows balance of the bias of transistors A1Q2 and A1Q3 to
minimize output distortion. The output of the amplifier is coupled to audio transformer A1T1 and the transformer output
is coupled to attenuating network A1A1C8, A1A1R17, and A1A1R18, which provides a 2-watt output to a loudspeaker
and a 600-ohm, 10-mw output for driving a headset or handset.
Squelch Circuit. A portion of the audio input signal is applied to detector A1A1CR1, Q2 through SQUELCH
SENS potentiometer R9. With audio applied, detector A1A1CR1, Q2 is forward biased by the positive portion of the
audio signal. Inverse amplifier A1A1Q3 then conducts, biasing threshold zener diode A1A1VR1 below the firing point.
Thus, squelch switch A1AiQ4 remains turned off, and the circuit is unsquelched. When audio is removed, detector
A1A1CR1, Q2 cuts off. However, increase amplifier A1A1Q3 remains conducting because of the negative charge on
capacitor A1A1C7. Capacitor A1A1C7 discharges through the forward resistance of inverse amplifier A1A1Q3. When
capacitor A1A1C7 is fully discharged, inverse amplifier A1A1Q3 cuts off, firing threshold zener diode A1A1VR1 and
gating on squelch switch A1A1Q4. This effectively grounds the input to driver A1A1Q1 through capacitor A1A1C4, which
is a low impedance to noise at audiofrequencies, and the circuit is squelched. The circuit remains squelched until audio
is again applied, at which time detector A1A1CR1, A1A1Q2 is again forward biased and squelched switch A1A1Q4 is
turned off.
Mixer Injection Frequency Circuits.
General. Depending on the mode of operation of the modem, the mixer injection frequency circuit provides either a
frequency within the range from 3,805 to 5,805 Hz (850-Hz mode of operation) or an amplified 2,380-Hz frequency
(85-Hz diversityoperation). The output is applied to receiver mixer injection amplifier A4A1Q1.
850-Hz mode of operation of the Modem.
General. During 850-Hz mode of operation, the mixer injection frequency is developed by BFO A1A3Q1, amplified,
by driverA1A2Q3, divided by divide-by-2 bistable multivibrator A1A2Q4, Q5, and amplified by mixer injection
amplifier A1A2Q6. When front panel MODE SELECTOR switch S4 is set at 850 Hz, +20 volts dc is applied through
pin 1 of connector A1J1 to energize the BFO, driver and multivibrator circuits. The +20 volts dc is also applied to
BFO tune control R2 (fig. FO-8). A portion of the voltage across R2 is applied to the BFO through voltage divider
A1A2R6, A1A2R7.
1-41

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