Oscillators

audio amplifier V16B, which produces the power to

first mixer V13B. The output frequency of this oscil-

operate a speaker, headphones, or a phone patch.

lator is always 3.155 MHz higher than the lower edge of

the desired band. This high-frequency injection signal

3-17. Oscillators

is the crystal fundamental frequency for all desired

The transceiver contains the tone oscillator, the beat-

signals below 12 MHz. For operating frequencies above

frequency oscillator, the variable-frequency oscillator,

12 MHz, the crystal frequency is doubled in the plate

the high-frequency oscillator, and the crystal calibrator.

circuit of the oscillator. Instructions for calculating

a. Tone Oscillator. The tone oscillator operates

crystal frequencies for the desired bands are given in

when the EMISSION switch is at LOCK, TUNE, or

paragraph 3-13a

CW.

It is a phase shift oscillator operating at

e. Crystal Calibrator. The output of crystal

approximately 1,750 Hz. Its output is fed to the

calibrator V12B is coupled to the antenna and applied to

transmitter audio circuits for cw operation. Some of the

the receive circuits. The output is used when calibrating

output from the tone oscillator is applied to the receiver

the tuning dial.

audio circuits for sidetone monitoring in cw operation.

3-18. Vox and Antivox Circuits

Because of the 1,750 Hz tone applied to the balanced

Audio output voltage from second microphone am-

modulator during cw operation, the actual transmitted

plifier V1B is coupled through the VOX GAIN control

signal will be 1,750 Hz above the Collins KWM-212A

to vox amplifier V14B. The amplified voltage is applied

dial reading.

to vox rectifier V14A. The positive d.c. voltage output

b. Beat-Frequency Oscillator. The bfo is crystal-

of the vox rectifier is applied to vex retry switch V4B,

controlled at either 453.650 or 456.350 kHz, depending

causing it to conduct and actuate vox relay K2.

on whether Y16 or Y17 is selected by the EMISSION

Contacts of K2 switch the antenna connection to the

switch. The unused crystal is shorted out by the switch.

transmit circuits, the other relay coils, and bias voltage.

These crystal frequencies are matched to the passband of

Other relays switch the metering circuits from receive to

mechanical filter FL1 so that the carrier frequency is

transmit, the low plate voltages from receive to transmit

placed approximately 20 db down on the skirts of the

tubes, and the AVC and ALC leads. The antivox circuit

filter response. This 20-db carrier attenuation is in

provides a threshold voltage to prevent loudspeaker

addition to the 30-db suppression provided by the

output (picked up by the microphone circuits) from

balanced modulator.

tripping the Collins KWM-2/2A into transmit functions.

c. Variable-Frequency Oscillator. The vfo uses

Some of the receiver output audio voltage is applied

fixed capacitors and variable inductance to tune the

through the ANTIVOX GAIN control to antivox

range of 2.5 to 2.7 MHz. Its output is applied direct to

rectifier V14A. Negative d.c. output voltage from the

second receive mixer V17B, and is applied to second

antivox rectifier, applied to vox relay switch V4B,

transmit mixer V6 through vfo cathode follower V2A.

provides the necessary antivox threshold. The

d High-Frequency Crystal Oscillator. High-

ANTIVOX GAIN control adjusts the value of the

frequency crystal, oscillator V 13A is crystal-controlled

antivox threshold so that the loudspeaker output will not

by 1 of 14 crystals selected by bandswitch S2. The

produce enough positive d.c. output from the vox

output from the high-frequency crystal oscillator is

rectifier to exceed the negative d.c. output from the

applied to the transmitter second mixer and to crystal

antivox rectifier and cause V4B to actuate K2.

oscillator cathode follower V12A. The cathode follower

However, speech energy into the microphone will cause

provides isolation and impedance match between the

the positive vex voltage to overcome the negative

crystal oscillator and receiver

antivox voltage and produce the desired action of K2.

Section V. OPERATOR'S MAINTENANCE INSTRUCTIONS

NOTE

The instructions in paragraphs 3-23 and 3-

Refer to TM 750-244-2 for proper

24 are applicable for the operator's

procedures for destruction of this

maintenance portions of chapter 4 through

equipment to prevent enemy use.

a. Operator/crew preventive maintenance is the

3-19.Scope of Operator's

systematic care, servicing

and inspection of

equipment to prevent the occurrence of trouble to

Maintenance

reduce downtime and to maintain equipment in

The maintenance duties assigned to the operator of

serviceable condition. To be sure that your

the RT-718/FRC-93 follows:

3-16