Variable Frequency Oscillator Module A9

consisting of capacitors C10 and

network contributes a stability f actor about

C11 and inductor L1 prevent rf

40 times greater than that of a free-run-

leakage into the +10-volt dc supply.

ning oscillator.

The rf output from Q2 can be meas-

a. Sidestep Oscillator Module.

ured at test jack J2.

(1) The audio signal from speech am-

b. Quarter-Wave Network Module A19.

plifier limiter module A22 (para

(1) The quarter-wave network is made

12) is coupled to the sso through

up of two pi-networks: one for the

coupling capacitor C6 and isolating

high band and one for the low band.

resistors R8 and R7, across diode

Each network has a crystal which

CR1. Diode CR1 is a variable ca-

is used to control the sso fre-

pacitance diode. When the audio

quency. BAND switch S4 (fig. 88)

signal is applied across the diode,

on the front panel s e 1 e c ts the

the effective capacitance of CR1

proper network.

changes as the voltage across it

(2) E a c h network is an artificial

varies. These capacitance changes

quarter-wave line. An impedance

are coupled through capacitor C4,

or reactance atone end is reflected

which blocks the audio signal,

as the exact opposite at the other

across the sso tuned circuit con-

end. This permits the use of a low-

sisting of inductor T1 and capaci-

impedance crystal with the high-

tor C4. The effective capacitance

impedance tank circuit in the sso.

of the tuned circuit, and the output

If the crystal were connected di-

frequency, varies at the audio rate;

rectly into the tuned circuit, devia-

therefore, the output frequency of

tion of the sso frequency would not

the sso is frequency-modulated.

be possible. Inductors L1 and L2

(2) The ac component of the emitter

are used to control the amount of

current flows through part of T1,

coupling between the crystal and

coupling capacitor Cl, and swamp-

the tuned circuit.

ing resistor R2 to the emitter of

Q1. This signal induces an in-phase

voltage at the top of T1 which is

15. Variable Frequency Oscillator Module A9

coupled through coupling capacitor

(fig. 10)

C2 to the base of Q1 to sustain os-

cillation. Resistors R3 and R4 pro-

Variable frequency oscillator (vfo) Q1

vide base-to-emitter bias for Q1.

functions as the local oscillator during re-

ception and as the master oscillator during

Resistor R1 is the emitter load

resistor. Resistors R5 and R6 es-

transmission. The vfo is a modified Hartley

tablish a dc bias for CR1. Capacitor

type that generates a signal in the fre-

C5 is an rf bypass capacitor for

quency range of 41.45 mc to 64.45 mc. The

tank circuit for Q1 receives an automatic

the audio input circuit.

phase control (ape) dc voltage from module

(3) The frequency-modulated rf output

from the oscillator tuned circuit

All (para 29). The apc voltage is applied

to capacitance diode CR1. The effective

is coupled from a tap on T1 through

capacitance of CR1 varies with the voltage

capacitor C7 to the base of buffer

applied to it. Capacitance diode CR1 is

Q2. The buffer stage isolates the

connected across the vfo tank. Any vfo out-

sso from load variations. The buf-

fer output is developed across the

put frequency deviation will develop an er-

ror signal and cause a change in the apc

tuned circuit consisting of trans-

voltage that will change the effective ca-

former T2 and capacitor C8. Re-

pacitance of CR1; this condition causes

sistors R9 and R10 establish the

the vfo output to return to its proper fre-

base-to-emitter bias for Q2. Re-

sistor R11 is the emitter voltage

quency. The vfo has outputs to the fss buf-

fer, the receiver mixer, and transmitter

dropping resistor; capacitor C9 is

the emitter bypass. A pi-type filter

mixer. The fss buffer provides gain,