Multivibrator

TM 11-5820-6935

Q, while that of the tank is quite low, the crystal

at the output With CR4 anode low, the diodes are

frequency shift is very small compared to that of the

,reverse biased and no signal flows. b. Reference

tank. The crystal oscillator is dc coupled to emitter

Crystal Oscillator The reference

crystal oscillator

follower Q2 through resistor R75. The collector is

generates a precise 70 MHZ reference signal which is

isolated from +15 volts BY L3 and C11. This stage

compared with the sampled 70 MHz signal from radio

decouples the oscillator from the following circuit. The

transmitter modulator 1A8. The oscillator stage is

output is taken from the following circuit. The output is

comprised of transistor Q1, crystal Y1, tank circuit C7

taken from the junction of the bias resistors R14 and

and ,L2 bias resistor R1, R2, R3., R5 and R6, loading

R15 and is coupled to a diode gate through capacitor

resistor R4, rf choke L1, bypass capacitors C1, C2, C3

C12. The gate circuit consists of diodes CR1, CR2,

and C10, and coupling capacitor C4 and C6. The

CR3, coupling capacitors , C19, C20, load resistors

crystal operates

in series resonance on the fifth

R16, R17, and decoupling networks L5, C13, and L7,

overtone When voltage is applied, the initial transient

C18. The operation of this gate is identical to the CR4,

causes oscillations to start. The crystal signal at Q1

CR5, gate previously described.

emitter base an amplified signal to appear at Q1

Multivibrator.

The multivibrator is a

collector. The tank circuit which is tuned closely to the

symetrical, free running, mulitivibrator consisting of

crystal frequency, resonates and feeds a reinforcing

transistor stages Q4, Q5. Its function is to supply gating

signal back to the crystal through C6. is causes further

signals to the previously described diode gates. These

buildup in the signal level of the tank, and increased

signals consists of positive rectangular pulses appearing

feedback to the crystal. This increase in level continues

alternately at the Q4 and A5 collectors. When the pulse

until the transistor gain drops to unity, at which time a

is present at Q4 collector, diode gate CR1, CR2, CR3, is

sustained level is maintained.

enabled. During this period the pulse is absent at Q5

collector, so that is associated gate, CR4, CR5, CR6 is

Variable inductor L2 in the tank is used to adjust the

open. The mulivibrator drive to the gates is applied

frequency of oscillation by a small percentage. With the

through S1. When this switch is in the OPR position,

tank turned above the crystal frequency, the tank

the gates are under control of the multivibrator. In the

appears capacitive to the signal at the collector, causing

TEST position, the multivibrator is disconnected and

the feedback signal to lead the crystal frequency. The

CR1, CR2

crystal pulls to a higher frequency in order to restore a

zero phase condition. Since the crystal has a very high

2-69