Crystal Oscillator Circuit

TM 11-6625-486-14&P

NOTE

INDICATES EQUIPMENT MARKING.

EL6625-486-14-TM-11

Figure 5-3. FR-149A/USM-159 and FR-149B/USM-159, crystal oscillator, schematic diagram.

parallel resonant circuit with maximum impedance at

5-2. Crystal Oscillator Circuit

resonance.

If either crystal is removed and an

(fig. 5-2 and 5-3)

equivalent resistance (approximately 400 ohms) is

a. The crystal oscillator provides crystal check

substituted, the circuit will oscillate at its free-running

frequencies at either 1- or 2.5-MHz intervals throughout

frequency.

the fundamental ranges of the meter. Use of these

signals with associated checkpoints printed on the

c. The collector of Q201 receives voltage through

filmstrip permits correction of the dial to the original

switch S701 (CHK position), decoupling resistor R204,

calibration settings. Two crystals are used in the

and coils L201 and L202. Bypass capacitor C206 keeps

oscillator circuit. Switching coil L202 in or out of the

radiofrequency out of the power supply and grounds one

tank circuit permits selection of either the 1- or 2.5-MHz

side of the tank circuit for rf. The base of Q201 is

frequency by use of the RANGE SW. The 1-MHz

forward-biased by the current flowing through voltage

crystal (Y202) is used in the tank circuit, formed by

divider R204, R201, and R202.

Capacitor C201

capacitors C202 and C203 in parallel with L201 and

decouples the base and (in AN/USM-159 only) stabilizes

L202. In the 2.5-MHz operation, the 2.5-MHz crystal

collector current. Resistor R203 and capacitor C204

(Y201) is used with capacitors C202 and C203 in

stabilize emitter current and provide impedance

parallel with L201. The coils are slug-tuned and permit

matching between the emitter-follower output circuit and

amplitude peaking. Trimmer capacitors C209A and

buffer amplifier Q202. Amplifier Q202 isolates the

C209B (Not in AN/USM-159) allow adjustment of

oscillator and prevents circuit loading. Forward base

crystals Y201 and Y202, respectively, to the exact

bias is furnished by R205 and R206. Resistor R208

frequency.

supplies emitter bias, and capacitor C208 bypasses rf.

The output signal of Q202 appears at collector load

b. In figure 5-2 and 5-3, transistor Q201 operates

resistor R207 and is applied through C207 to the mixer

as a Colpitts oscillator. Oscillation takes place from the

circuit. The connection of resistor R308 to the RANGE

collector to the emitter through the tank circuit and

SW (range A) (in AN/USM-159 only) provides diode

emitter connection to the two crystals. The junction

biasing voltage in the mixer circuit.

connection of C202 and C203 serves the same purpose

as a center tap on the tank circuit. Each crystal forms a

5-4