AC Amplifier

TM 11-5820-69-35

Q14 goes to ground through R63 which is also

and the signal from the reference crystal oscillator.

connected to the 'base circuit of Q12 through R58 to

provide a dc bias at Q12. Bypass capacitor 062

(1) Both the primary and secondary of

prevents positive feedback to Q12. The output of Q14

T11 are tuned to 70 MHz. Connector J4 (TP4 DISCR) is

is developed across R62 and is dc coupled to the base

used for observing the response during alignment.

of emitter follower Q15. Resistor R65 in the emitter

Tuning is accomplished by means of variable capacitors

circuit of Q15 and Q13 provides negative feedback to

C52 and C53. Q11 output is connected to the primary

Q13. The output of the agc amplifier appears at Q15

at terminal 3 through parasitic suppressor resistor R50,

emitter across R64 and R65, and is coupled to the

and to the secondary center tap (terminal '5) through dc

phase detector through C65 and R66. Power supply

blocking capacitor 051. At resonance (input signal at 70

decoupling is provided by L20 and C59.

MHz), the voltages across two halves of the secondary

are in guadrature with the voltage across the primary

Phase Detector. The output from the agc

and in opposition to each other. When the input signal

frequency is higher or lower than the resonant

amplifier is an ac square wave with the peak amplitude

frequency, the phase angle between one half of the

representing the frequency difference between the

secondary and primary will be greater than 90 degrees.

sampled modulator frequency and the reference crystal

The phase angle between the other half secondary and :

oscillator frequency. However, the signal does not

the primary will be less than 90 degrees, and as in the

identify whether the modulator frequency is high or low.

resonant conditions will be 180 degrees out of phase

This information is provided by the phase detector. The

with the other half secondary.

phase detector is comprised of transformer T2, detector

diodes CR12 and CR-13, and associated components.

(2) The signal at the anodes of CR10 and

CR11, are the vector sums of the primary and

(1) The Q15 output is applied to the

associated half secondary voltages. At resonance, they

primary of T2 where high frequency components are

will be of equal amplitude; in all other cases they will be

bypassed by capacitor C64. This signal is also applied

unequal. The circuit is arranged so that CR10 anode

to J5 (TP3) for monitoring. The drive to the secondary

voltage exceeds that at CR11 anode when the input

center tap is supplied by the Q4, Q5 multivibrator

frequency is below resonance, resulting in a positive

through R27 of emitter follower Q6, coupling capacitor

signal appearing at R,55. Each diode conducts when its

C31, and R36. Emitter follower Q6 is driven through

anode is positive with respect to its cathode. Thus

R74 from the square wave used to trigger the diode gate

CR10, conducts on one half cycle, while CR11,

which controls the modulator frequency sample input.

conducts during the next half cycle. Consequently, the

This signal also appears at J2 (TP) and is used as an

output from the discriminator is the difference between

oscilloscope sync for waveforms taken at J3, J4, and J5.

the two. At resonance, the levels are equal and the

The positive portion of the signal from Q6 is

output is zero. Above resonance the signal at CR11 is

synchronized with the half cycle from Q1,5 and

higher and therefore a negative signal is produced.

corresponds to the modulator frequency error signal.

Actually, because of the phase difference between the

two voltages, the output is a pulsating wave with a

(2) During the positive half cycle of the

repetition rate at the input frequency. This wave is

gating pulse, diodes CR12 and CR13, conduct. The

average by RC network ('R55, C56) to produce an

ground return for this current path is located in the

essentially dc level.

differential amplifier in the modulator module. When

the modulator error signal is positive during this period a

AC Amplifier. The discriminator square

current flows through resistor R70 from top to bottom.

Conversely, when the modulator error signal is negative,

wave output is amplified in an ac amplifier chain

the current flows from bottom to top of R70. When the

consisting of transistor stages Q12, Q13, Q14, and

gating signal goes negative, both diodes are back

Q115. Negative feedback is employed for stabilization.

biased and only a small current flows through R70.

The input appears at Q12 base through R57 and C57,

Thus, the phase detector output is a positive or negative

which, drives Q13, and Q12 emitter being tied directly to

square wave above or below the zero axis. The square

Q13 base. To prevent positive feedback, Q12 collector

wave is converted to a dc signal by RC networks in the

is isolated by R61 and bypassed by a capacitor C58.

mod-

The output of Q13 is developed across R59 and is dc

coupled to Q14 base. Bypass capacitor C60 removes

high frequency components from Q14. The input to

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