Figure 2-21. 3.975 MHz oscillutor circuit simplified schematic diagram.

TM 11-6625-467-34

Figure 2-21. 3.975 MHz oscillutor circuit simplified schematic diagram.

detector output. Capacitor C18, and resistor R25

the emitter of Q1. The voltage at this point is in

form a decoupling network along with RF choke L1.

phase with the voltage at the collector and thereby

sustains oscillators in the circuit. Capacitors C2 and

2-27. 30 MHz and 68 MHz Oscillators

C3 shunting the 3.975-MHz crystal of C4 in series

with the crystal also have a slight effect upon the

The 30-megahertz and 68-megahertz oscillators are

frequency. LOW FREQ AMPL control R1 varies the

identical except for component values. For the pur-

dc voltage at the collector of the oscillator. This

voltage directly controls the output amplitude of the

poses of this discussion, only the 30-MHz oscillator is

oscillator. Capacitor C5 couples the oscillator output

considered. The oscillator circuit is a Colpitts-type

to the input of emitter follower Q2. Resistors R6 and

with collector-to-emitter feedback using a crystal in

R7 form a voltage divider network that educes the

the feedback path. Inductor L1 and capacitors C3

amplitude of the signal at the base of Q2. The emit-

and C4 in the collector of Q1 are resonant at 30

ter follower circuit current-amplifies the signal and

megahertz. Crystal Y1 couples the output from the

provides a low-impedance output. A level detector

tuned circuit back to the emitter circuit of Q1. The

consisting of diode CR1, resistor R5, and capacitor

crystal acts as a series-tuned circuit and offers

minimum impedence to a 30-megahertz resonant fre-

C6 samples the output of the oscillator and produces

a dc level that is proportional to the output ampli-

quency. At frequencies above or below 30-mega-

hertz, Y1 is reactive and causes a phase shift in the

tude of the oscillator. Resistors R8, R9, and R29

form an adjustable voltage divider for the detector

feedback voltage appearing at the emitter of Q1. At

a frequency of 30 megahertz, the reactive com-

output. The divider output connects to the meter

ponents of Y1 cancel and the feedback voltage to the

circuit in the test set. Resistor R9 calibrates the

2-62