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RECEIVER MODULE A4, OUTPUT STAGES
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TM-11-5805-387-34-2 Modem Radion Teletypewriter MD-522A/RC (NSN
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DIVERSITY SWITCHING, RECEIVER MODULE A4
TM 11-5805-387-34-2
1-58. DIVERSITY SWITCHING
During 85-Hz diversity operation, the diversity switching circuit automatically selects the tty channel with the
greater signal strength and squelches the weaker channel. However, if the levels of both channels are within 3 db, the
outputs of both channels are combined. During 850-Hz operation, the circuit squelches channel 2, and during 85-Hz and
85-Hz plus voice modes, squelches channel 1. Operation of both channels is similar. Channel 1 is described below, and
differences between channels are noted where necessary.
Resistors A4A1R43, A4A2R5, and A4A2R32 form a voltage divider for the amplified tty channel 1 signal. A portion
of the signal that appears across potentiometer A4A2R5 is applied through coupling capacitor A4A2C4 to channel 1
detector network A4A2CR3, A4A2CR4, A4A2C5, and A4A2R6. The detector network provides a positive dc voltage at
the junction of A4A2C5 and A4A2R6 that is proportional to the input signal. This dc voltage is applied to pin 1 of
differential amplifier A4A2Z2. The dc voltage from the channel 2 detector network is applied to pin 6 of the differential
amplifier.
Differential amplifier A4A2Z2 is an integrated circuit differential amplifier. When the dc level applied to pin 1
exceeds that applied to pin 6 by approximately 3 db, the dc level of output pin 8 will approach the + 10.7 volt dc supply
voltage and the dc level of output pin 10 will be low. The opposite condition exists if the dc level on pin 6 (channel 2)
exceeds that on pin 1. With pin 8 high, threshold zener diode A4A2VR4 conducts and the voltage drop across bias
resistor A4A2R16 causes channel 2 switch A4A2Q4 to conduct.  The low impedance of the conducting transistor
effectively grounds the base of limiter amplifier A4A2Q5, blocking the channel 2 output. The low level at pin 10 of
differential amplifier A4A2Z2 prevents threshold zener diode A4A2VR5 from conducting, and keeps channel 1 switch
A4A2Q3 cut off.
Channel 2 is squelched during 850-Hz operation by +22 volts dc applied to the base of channel 2 switch A4A2Q4
through bias resistor A4A2R39. This biases the transistor on. Similarly, channel 1 is squelched during the 85-Hz and 85-
Hz plus voice modes by +22 volts dc applied through bias resistor to channel 1 switch A4A2Q3 through A4A2R40.
1-48

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