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MIXER OSCILLATOR AND SWITCHING
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TM-11-5805-387-34-2 Modem Radion Teletypewriter MD-522A/RC (NSN
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TRANSMITTER MODULE MIXER CIRCUIT, SINE WAVE PASS FILTER AND ISOLATION AMPLIFIER
TM 11-5805-387-34-2
1-47. MIXER OSCILLATOR OUTPUT DIVIDER CIRCUITS
(fig. FO-10)
During the 85-Hz diversity mode of operation only, the output of mixer oscillator A3A2Q3, Q4 is divided-by-10
before being applied to mixer transformer A3A2T1. The divide-by-10 circuits consists of divide-by-10 driver A3A2Q5 and
divide-by-10 integrated circuit A3A2Z1.
A 23,800-Hz signal from mixer oscillator A3A2Q3, Q4 is applied to divide-by-10 driver A3A2Q5 when front panel
MODE SELECTOR switch S4G is set at 85 HZ DIVERSITY. Driver A3A2Q5 shapes the oscillator signal to the proper
waveform to trigger divide-by-10 integrated circuit A3A2Z1. The shaped signal is coupled through capacitor A3A2C7 and
developed across resistor A3A2R49. When in the 85-Hz diversity mode of operation, ground for A3A2Z1 is provided
through isolating diode A3A2CR11. In any mode except 85 Hz diversity, diode A3A2CR11 is ungrounded, biasing
A3A2Z1 off.
Divide-by-10 integrated circuit A3A2Z1 is a monolithic decade counter integrated circuit connected in a divide-by-
10 configuration. The circuit produces 1 output pulse for every 10 input pulses. The resulting 2,380-Hz square wave
signal is applied through coupling capacitor A3A2C14 and a filter composed of A3A2R55, R56, and C12 to mixer
A3A2T1. The signal is also coupled through resistor A3A2R61 and pin 9 of connector A3J1 for use as an injection
frequency in receive audio module A1 (para 1-54).
Note that although the 26,050-Hz signal generated in the 850-Hz mode of operation is also shaped by driver
A3A2Q5, no ground is applied through diode A3A2CR11 to divide-by-10 integrated circuit A3A2Z1. Consequently, the
output of driver A3A2Q5 is not used.
1-48. SINE WAVE PASS FILTER, ISOLATION AMPLIFIER AND MIXER
The sine wave pass filter, isolation amplifier, and mixer circuit provide the output frequencies listed for each mode
of operation of the modem in the chart below. The 2,805-Hz 42.5 output is an amplified 2,805-Hz 42.5 signal from the
mark and space oscillator output divide-by-10 circuit A3A1Q11Q18; the 425-Hz 42.5 signal is generated by
subtractively mixing the 2,805-Hz 42.5 signal with a 2,380-Hz signal from the mixer oscillator output divide-by-10 circuit
A3A2Z1. Subtractively mixing a 26,050-Hz signal from mixer oscillator output divide-by-10 bypass switch A3A2CR 10
and a 28,050Hz 425 signal from mark and space oscillator output divide-by-8 circuit A3A1Q5-Q10 develops the 2,000-
Hz 425 output.
MODE
OUTPUT FREQUENCY
850-Hz
2,000-Hz
425
85-Hz
2,805-Hz
42.5
85-Hz diversity
2,805-Hz
42.5, 425-Hz 42.5
85-Hz + voice
2,805-Hz
42.5
1-36

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