The 150 MHz voltage controlled oscillator

TM 11-5820-695-35

characteristic, known as junction barrier potential.

while adjusting the 150 MHz voltage controlled oscillator

Essentially this means that, with forward voltage applied

center frequency.

to the diode forward conduction cannot start until the

(3) The 150 MHz voltage controlled

junction barrier potential is exceeded.

In silicon

oscillator consists of transistor Q11, bias resistors R22,

junctions this potential is approximately 0.65 volt.

R23, R24, R26, coupling capacitors C16, C18, bypass

During the positive half cycle of the signal, CR3 is

capacitors C14, C17, C19, C20 and tank circuit L2, C16,

forward biased while CR4 is reversed biased. When the

CR1, CR2, R73. Varactor diodes CR11 and CR2 are

instantaneous signal level reaches approximately 0.05

always operated reverse biased.

They exhibit an

volt, OCR conducts bypassing higher levels through

inverse square law capacitance reverse bias

bypass capacitor C24 to ground. During the negative

characteristic, that is, the capacitance varies

half cycle, the bias conditions of CR3 and CR4 are

approximately as the inverse of the square root of the

reversed. At an instantaneous )signal level of -0.65 volt,

reverse bias voltage. By varying the bias voltage, or

CR4 conducts, bypassing higher amplitude (more

baseband input signal, the resonant frequency of the

negative) voltages to ground. Thus, the output of

tank is made to vary. With only the bias voltage from

limiter-amplifier stage Q3 is a frequency modulated sine

the differential amplifier applied, the tank resonant

wave, but clipped at a peak amplitude of _+0.65 volts.

frequency will determine the oscillator frequency. If the

The output of Q6 is coupled through capacitor C25 to

bias is increased, the diode capacitance will decrease,

amplifier Q4, which in turn is coupled through capacitor

causing the tank to resonate at the higher frequency.

C27 to amplifier Q8. Transistor Q4 and Q8 are

Similarly, a decrease in bias will cause a decrease in the

conventional, dc biased voltage amplifiers. Biasing is

resonant frequency. Since this bias voltage represents

by means of resistors ,R85, R36, R37, and R54, R05

the center frequency error, as detected in electronic

and R56. C29 and C48 are emitter bypass capacitors.

frequency control 11A7, the voltage controlled oscillator

At each amplifier output (Q4 and QS) is a low pass

center frequency follows the frequency of the reference

matching section consisting of L23, C28 and L16, C47.

oscillator in electronic frequency control 1A7. The

The low pass matching sections suppress the harmonics

center frequency is initially set during alignment by

generated by the clipping action in the limiter. Q4 is

means of front panel variable capacitor C15. If a

decoupled from the 28v supply by L22, ?R72, FLM, and

baseband signal is applied at the junction of CR1' and

L5. L17, FF16, and L15 provide isolation for Q8.

CR2, the frequency will be made to vary about the

Capacitor C49 couples the output of Q8 to power

center frequency; thereby producing a frequency

amplifier Q9, which operates class C. The operating

modulated 160 MHz output. Since electronic frequency

point is set by bias resistors R69 and R70. Resistor R9

control 1A7 senses the average frequency, it only

is variable and permits bias adjustment. L18, R71, and

corrects for center frequency drift and has no effect on

FL8 block signal currents from the bias circuit. The Q9

the frequency modulation of the carrier. The output

collector output is tuned by L19 and C01 and coupled by

across load resistor R25 is applied to emitter follower

C52 to low pass filter L20 and C63.

The filter

Q2 which prevents loading of the 150 MHz voltage

suppresses harmonics produced in the amplifier. This

controlled oscillator stage. The collector of this stage is

stage is decoupled from the 28v supply by FL?, C50,

bypassed by C21 and its output is developed across

054, and L21. The filter output (frequency modulated

R27. The output of Q2 is applied through coupling

150 MHz signal) is applied to transmitter frequency

capacitor C22 and resistor R28 to the base of limiter-

mixer stage 1A9 via connector P1 (1'50 MHz OUTPUT)

amplifier stage QS.

and attenuator/matching network R63, R64, R65 and to

the meter selector switch on meter panel assembly

(4) Limiter-amplifier Q3 amplifies the

11A15A8 through a metering circuit and pin P8-9

input signal. The operating point of Q3 is set by biasing

(OUTPUT LEVEL METERING). The metering circuit

resistors R31, P,2, and R34 and load resistor R33. The

consists of coupling capacitor C60, load resistors R'59

emitter is bypassed by capacitor C26 and provides

and R61 diode detector CR10, and metering resistors

power supply isolation. Diode limiter CR3, CR4, and

R62 and R66. Resistor R66 is variable and is used to

C24 is connected to the Q3 collector output. Its purpose

adjust the meter deflection. Capacitor C61 bypasses

is to prevent amplitude variations (amplitude

ripple. The 150 MHz output of low pass filter L20 and

modulation) from appearing at the modulator output.

C53 is also applied to transformer mixer T3 in

CR3 and CR4 are silicon diodes and the limiting action

is achieved by utilizing a semiconductor junction

2-73