Beat frequency oscillator. Unijunction transistor A1A3Q1 forms a voltage-controlled relaxation oscillator, the
frequency of which increases as the voltage from BFO control R2 is increased. Positive 20 volts dc, applied through
bias resistor A1A3R1, back biases the PN diode junction that effectively exists between the emitter (E) and base 1
(B1). Capacitor A1A3C1 charges towards the positive voltage from BFO control R2 through voltage divider A1A2R6,
A1A2R7. When the voltage across capacitor A1A3C1 reaches the firing potential of the unijunction transistor, the E-
B1 diode becomes forward biased and the capacitor discharges through the transistor and load resistor A1A3R2.
This results in a positive-going spike across A1A3R2, which is applied through coupling capacitor A1A2C6 to the
base of driver A1A2Q3. Charging time of capacitor A1A3C1 depends on the magnitude of the voltage applied to it;
with a higher voltage applied, less time is required to reach the firing potential of the unijunction transistor and the
oscillator output frequency is increased. A thermostatically controlled heater maintains the unijunction transistor at a
uniform temperature, to minimize drift in the oscillator output frequency.
Driver and dividing circuits. Driver A1A2Q3 amplifies the oscillator output and couples the resulting negative-
going spike waveform to divide-by-2 multivibrator A1A2Q4, Q5. Divideby-2 multivibrator A1A2Q4, Q5 is a
conventional bistable multivibrator which generates a square wave output at one-half the input (oscillator) frequency.
The multivibrator output is fed through coupling capacitor A1A2C12 and Isolating resistor A1A2R18 to the base of
mixer Injection amplifier A1A2Q6, which amplifies the 3,805-to 5,805-Hz signal and applies it to receiver mixer
injection amplifier A4A1Q1.
85-Hz diversity mode of operation of Modem. During the 85-Hz diversity mode of operation, the 3,805to 5,805-Hz
frequency is not generated because no operating voltage is supplied to the BFO driver and multivibrator circuits from
MODE SELECTOR switch S4. Instead, a 2,380-Hz signal from transmitter module A3 is fed through pin 10 of
connector A1J1, coupling capacitor A1A2C11, and isolating resistor A1A2R17 to the base of mixer injection amplifier
A1A2Q6. This signal is not generated during the 850-Hz mode of operation.
+20 Volt Regulator Circuit. Although physically located in audio module A1, the +20 volt regulator circuit Is
functionally a part of the power supply circuits. Consequently, the circuit is discussed In paragraph 1-66.
1-55. RECEIVER MODULE A4
The receiver module consists of those mixing, switching, and demodulating circuits necessary to convert received
tty tone signals into mark and space pulses for use by loop battery module A5. The circuits are divided into categories
and are discussed separately. The mixer and amplifying circuits are discussed below.
Tty Channel 1 . Nsk or fsk audio from fsk and nsk filtering is fed through pin 12 of connector A4J1 to a mixer
circuit consisting of diodes A4A1CR1, CR2; capacitors A4A1C2, C3; resistors A4A1R5, R6; and transformer A4A1T1.
Diodes A4A1CR1, CR2 provide the nonlinear circuit elements necessary for mixing. A 2,380-Hz (during 85-Hz diversity
mode of operation) or 3,805to 5,805-Hz (during 850-Hz mode of operation) signal from receive audio module A1 (para 1-
54) is amplified by mixer injection amplifier A4A1Q1 and coupled through capacitor A4A1C4 to the mixer circuit. The
resulting mixer products sum and difference are applied to nsk and fsk filters. All products, except 2,805-Hz ± 42.5 (nsk)
or 2,805-Hz ± 425 (fsk), are filtered at this point. After filtering, the signal is returned through pin 7 of connector A4J1,
amplified by amplifier A4A1Q2, and limited by A4A1Q3. Limiter amplifier A4A1Q3 assures that the signal level applied
to the fsk nsk discriminator through discriminator driver A4A1Q4 remains constant despite varying input signal levels.