Electronic Frequency Control 1A7

TM 11-5820-695-35

square wave amplifier Q12Q15. The square wave

frequency. The vco's center frequency can be adjusted

amplifier is a highly stable feedback amplifier which is

with front panel variable capacitor C15. The 1.50 MHz

ac coupled both at the output and the input. The phase

frequency modulated output of the vco is applied to

detector circuit receives the error signal and an input

amplifier stages Q3, Q4, Q8, and Q9 through emitter

from the multivibrator to determine the polarity for plus

follower Q2 which isolates the vco from the amplifier

or minus correction. When the sampled input from the

stages. The amplified frequency modulated 1150 MHz

modulator is lower than 70 MHz, the phase detector

signal is applied through matching pad R%3-R65 to

provides a negative correction voltage output to drive

frequency mixer 11A9.

the vco frequency higher. When the sampled input from

(2) The 150-MHz output signal is also applied

the modulator is higher than 70 MHz, the phase detector

to mixer circuit T3, CR7, CR8 where it is mixed with a

provides a positive correction voltage output to drive the

220 MHz signal. The 220 MHz signal is developed by

vco frequency lower. The center frequency of the phase

the 110 MHz crystal oscillator circuit Q5 and frequency

detector can be adjusted with phase detector balance

doubler circuit T2, CR5, CR6. The output of the mixer is

variable resistor R78. Emitter follower Q6 isolates the

applied to low-pass filter L10, C38, and C39. The 70

multivibrator from the phase detector. The correction

MHz difference signal produced by the mixing process is

signal (afc) is applied to differential amplifier A2-A and

passed by the filter and amplified by stages Q6 and Q7

A2-B in modulator 1A8. The differential amplifier

for application to electronic frequency control 1A7.

increases the sensitivity of the afc output circuit. The

c. Electronic Frequency Control 1A7. Electronic

afc signal applied to the 150 MHz vco provides -20 kHz

frequency control 1A7 receives the sampled frequency

center frequency stability. The EFC DISABLE switch on

modulated 70 MHz signal from radio transmitter

modulator 1A8 is used to disable the afc input signal.

modulator 1A8 and produces an afc signal to stabilize

When depressed, the EFC DISABLE switch shorts out

the center frequency of the 1150 MHz vco in 1A8. The

the afc input to modulator 1A8. When aligning the vco,

70 MHz sampled signal is applied to gate-A, CR4CR6,

the EFC DISABLE switch is depressed and front panel

through emitter follower Q3 in electronic frequency

variable capacitor C15 is adjusted.

control 1A7. A 70 MHz reference signal from reference

d. Meter Circuits

oscillator circuit Q1 is applied to gate-B, CR1-CR3,

through emitter follower Q2. The frequency of the

(1) The orderwire metering circuit in

reference oscillator can be adjusted using variable

attenuator assembly 1A3 provides an output to the

inductor L2. With switch S1 in the OPR position, free-

meter selector switch on meter panel assembly 1A15A8.

running multivibrator Q4 and Q5 alternately switches

The orderwire signal level may be monitored by setting

between gate A and gate B. Thus, the 70 MHz sample

the meter selector switch to the OW TEST position.

signal from 1A8 and the 70 MHz signal from the

(2) Various signals from radio transmitter

reference oscillator are alternately applied to emitter

modulator 1A8 and electronic frequency control 1A7 are

follower Q7. The emitter follower provides isolation

applied to the meter selector switch on meter panel

between the gate inputs and limiter stages Q8 and Q9.

assembly 1A15A8. The traffic input to 1A8 is applied

The limiter stages prevent any amplitude variations from

through emitter follower A2) (part of integrated circuit

reaching the discriminator. The limiter stages are tuned

A2) to the 48 channel traffic attenuator in attenuator 1A3

by adjusting the 1ST LIM LEVEL and the 2ND LIM

and then to the meter selector switch by alarm monitor

LEVEL controls. Emitter follower Q10 isolates the

1A5. Note that the 48/96 channel attenuator switch on

second limiter stage from discriminator driver Q11. The

1A3 must be set to the 48 CH position (para 2-10a(2)).

discriminator, T1, C51-C56, CR10 and CR11 is a

Variable resistor 1A15A8R7 mounted on the back of

conventional type, employing the phase shift principle.

meter panel assembly 1AllStA8 is used to calibrate the

Both the primary and secondary of T1 are tuned for a

T,RA'FFIC meter indication. The 150-MHz output and

linear S-curve centered about 70 MHz. The output of

70 MHz sample output signals from 1A8 are applied

the discriminator is a square wave with an amplitude

through meter circuits CR10 and CR9, respectively, to

proportional to the frequency difference of the signals

the meter selector switch. The center frequency (afc

(70 MHz modulated signal from the modulator and 70

signal) is applied from output electronic frequency

MHz reference signal). The difference (error) signal is

control 1A7 to the meter selec

applied to phase detector circuit T2, CR12-CR15 by

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