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FREQUENCY DISCRIMINATOR AND SWITCHING, RECEIVER MODULE A4
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TM-11-5805-387-34-2 Modem Radion Teletypewriter MD-522A/RC (NSN
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RECEIVER MODULE A4, OUTPUT STAGES
TM 11-5805-387-34-2
1-57. RECEIVE MARK-SPACE PULSE GENERATION, STAGE ANALYSIS
The output stages of receiver module A4 form the receive mark-space pulse generation circuitry consisting of
resistive combiner network A4A1R22, R23, R26, R27; discriminator differential amplifier A4A1Z1; dc bistable
multivibrator A4A1Q5, Q6; and ac bistable multivibrator A4A1Q7, Q8. Paragraph 1-29 gives a detailed block diagram
discussion of this circuitry.
Resistive Combiner Network. The resistive combiner network isolates the output of tty channel 1 frequency
discriminator A4A1Q8, Q9 from that of tty channel 2 frequency discriminator A4A2Q5, Q6 in all modes except 85-Hz
diversity. In 85-Hz diversity, the outputs of tty channels 1 and 2 are combined at this point. The input to the resistive
combiner network is a varying dc from the frequency discriminator, the polarity of which depends on the frequency of the
input (that is, a mark or space tone). The difference of potential is about 2 volts, dropping to zero in the absence of
signal input.
Discriminator Differential Amplifier  Discriminator differential amplifier A4A1Z1 is an integrated circuit dc
.
amplifier connected as a differential amplifier.  Circuit details are shown in note 5 of the overall receiver module
schematic diagram (fig. FO-13). A voltage divider consisting of A4A1 R42, R28, and R29 supplies a +13.2 volt input to
terminal 9 of A4A1Z1. A +6.6 volt dc bias from the same voltage divider is applied to terminal A4A1Z1-2. Terminals 1
and 6 receive their bias from the 6.6 volt-source through resistors A4A1R24 and A4A1R25, respectively. Capacitor
A4A1d25 cancels noise and transients at the A4A1Z1 input. A mark tone input to the modem produces a discriminator
output so that discriminator differential amplifier terminal A4A1 Z1-1 is positive with respect to terminal A4A1Z1-6.
When this condition occurs, A4A1Z1 output terminal 10 is in high state (more positive than output terminal 8).
Conversely, a space tone input to the modem produces a discriminator output so that terminal A4A1Z1-1 is negative with
respect to terminal A4A1Z1-6. When this condition occurs, A4A1Z1 output terminal 8 is in high state. The output of the
discriminator differential amplifier can be sampled across DISCR AMPL jacks A4A1J2, J3.
Dc Bistable Multivibrator A4A1Q5, Q6.  Multivibrator A4A1Q5, Q6 is a conventional dc-coupled bistable
multivibrator. Resistors A4A1R45, R44, and R46 form a balance network for the multivibrator input. By adjusting
A4A1R44, compensation can be made for differences in level of the output of A4A1Z1. When the output of A4A1Z1-10
is positive, corresponding to a mark, A4A1Q5 is biased into a low state, resulting in a high state for A4A1Q6.
Conversely, a positive output from terminal A4A1Z1-8 switches A4A1Q6 to a low state, with A4A1Q5 at high state. Note
that A4A1Q6 can be locked in low state through tty squelch switch A4A1Q1 (mark hold condition) as described in
Ac Bistable Multivibrator A4A1Q7, Q8.  Multivibrator A4A1Q7, Q8 is a conventional ac-coupled bistable
multivibrator. When A4A1Q5 is in a high state (corresponding to a mark), A4A1Q7 is held in a high state through
A4A1C18. Thus, a positive voltage appears at receiver module output A4J1-24, corresponding to a normal mark. At the
same time, A4A1Q8 is held in a low state, so ground appears at receiver module output A4J1-23, corresponding to a
reverse mark. A space input to the modem would result in a ground at A4A1-24 and a positive voltage at A4A1-23.
1-46

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