MIXER OSCILLATOR AND SWITCHING

TM 11-5805-387-34-2

The divide-by-10 circuit is used only during the 85-Hz, 85-Hz diversity, and 85-Hz plus voice modes of operation of

the modem. During the 850-Hz and voice modes of operation, no -20 volt dc operating voltage is supplied to the circuit

from front panel MODE SELECTOR switch S4F. The circuit consists of three bistable multivibrators and a gate arranged

in a divide-by-5 configuration followed by a divide-by-2 multivibrator. The preceeding figure shows the waveform

produced. The gate is formed by diodes A3A1CR16, CR17 and resistor A3A1 R42. As long as diode A3A1CR17 is

reverse biased, the trailing edge of the square wave output pulses from the divide-by-8 circuit will pass through diode

A3A1CR16, be differentiated by A3A1C16, R29 or A3A1C17, R34 and trigger bistable multivibrator A3A1Q11, Q12.

Each pulse from the divide-by-8 circuit is also applied (after differentiation) to the base of transistor A3A1Q7 to set

multivibrator A3A1Q17, Q18 so that the collector of A3A1Q17 is high and the collector of A3A1Q18 is low. After four

input pulses multivibrator A3A1Q15, Q16 toggles to the state where the collector of A3A1Q16 is low. This causes

multivibrator A3A1Q17, Q18 to change state, so that the collector of A3A10Q17 is low and the collector of A3A1Q18 is

high. The low level at the collector of A3A1Q17 forward biases diode A3A1CR17, which in turn reverse biases diode

A3A1CR16, preventing the fifth input pulse from being applied to multivibrator A3A1Q11, Q12. However, the fifth input

pulse is applied to the base of A3A1Q18, resetting multivibrator A3A1Q17, Q18 so that the output of A3A1Q17 is high

and the output of A3A1Q18 is low. This results in a negative-going (after differentiation) output pulse at the collector of

A3A1Q18 for every five input pulses. The output of multivibrator A3A1Q17, Q18 is divided by 2 by multivibrator

A3A1Q13, Q14, which results in a 2,762.5or 2,847.5-Hz (2,805 + 42.5 Hz) signal that is applied through coupling resistor

A3A1 R41 to the sine wave pass filter (para 1-50).

1-46. MIXER OSCILLATOR AND SWITCHING

(fig. FO-10)

The mixer oscillator and switching circuit provides mixer injection frequencies used to establish the center

frequencies of the 850-Hz mode and the lower band of frequencies used during 85-Hz diversity. The oscillator portion of

the circuit consists of an a stable multivibrator A3A2Q3, Q4 with a crystal feedback. This oscillator is identical with the

221.0-kHz oscillator described in paragraph 1-43 except that two separate feedback paths through the frequency

determining elements exist. Either path may be selected to determine the oscillator output frequency. The proper

feedback path is selected by a ground from front panel MODE SELECTOR switch S4G to switching diode A3A2CR5 or

A3A2CR6. Assume the MODE SELECTOR switch is set at 85 HZ DIVERSITY. Ground is applied to switching diode

A3A2RC6 through cathode resistor A3A2R32, enabling the 23,800-Hz feedback path. No feedback occurs through the

26,050-Hz path because switching diode A3A2CR5 is not forward biased. Capacitor A3A2C10 suppresses undesirable

modes of oscillations of the crystals.

Two outputs are obtained from the mixer oscillator circuit. The signal developed across collector load resistor

A3A2R35 is always applied to divide-by-10'driver A3A2Q5 (para 1-47). An additional output is applied to the anode of

divide-by-10 bypass switching diode A3A2CR10. With front panel MODE SELECTOR switch S4G at any setting except

850 HZ, diode A3A2CR10 is reverse biased. When the MODE SELECTOR switch is placed at 850 HZ, ground is applied

through isolator A3A2CR12 and resistor A3A2R59 to forward bias switching diode A3A2CR10. The oscillator signal is

then fed through coupling capacitor A3A2C11 and a filter, composed of A3A2R58, R57, and C17, to mixer A3A2T1 (para

1-48). Isolator A3A2CR12 prevents the 2,380-Hz signal from leaking through 2,000-Hz fsk pass filter FL2 in diversity

mode.

1-35