Figure 4-4. Voltage-Controlled Oscillator . Simplified Schematic Diagram

TO 31R2-2GRC171-2

TM 11-5820-815-14

NAVELEX 0967-LP-544-5010

three bands. The only other difference is that the low-

band vco has a low-pass filter (A2A1L3, C8, and C9) at

in conjunction with capacitors A2A1C4 and A2A1C5,

its output to filter second harmonic frequencies which fall

determines the oscillating frequency of the vco. Tuning

below the 400-MHz upper frequency limit of the

voltage from low-pass filter A2A5FL1 applied through

synthesizer. Each vco is a sealed, non repairable,

resistor A2A1R1 and choke A2A1L2 to varactor

nonadjustable unit.

However, to understand circuit

A2AlCR1 causes the frequency of the vco to vary. As the

operation the low-band vco will be discussed.

tuning voltage increases, the capacitance of A2A1CR1

decreases causing the frequency to increase.

adjusting A2A1L1 and A2A1C3, the range of the vco

4-70. Radio control frequencies that fall in the low- band

frequency is set so varactor A2A1CR1 controls the

range cause the low-band decoder to apply a logic 1

frequency from below the lowest frequency (195 MHz) to

voltage through resistors A2A7R18 and A2A1R5 to the

above the highest frequency (250 MHz) of the low band.

base of transistor switch A2A1Q2 turning it on. When on,

The vco output signal is developed across emitter

A2A1Q2 applies a ground to the voltage- controlled

resistor A2A1R4 and fed through coupling capacitor

oscillator circuit exciting A2A1Q1 into oscillation. At the

A2A1C7. resistive pad A2A1R6-R7-R8, low-pass filter

same time A2A1Q2 provides a current path to ground

A2A1C8-L3-C9, and diode A2A4CR2 to the vco output.

from the +12-V dc supply through A2A4R15, A2A4CR2,

Resistor values of A2A1R6, R7, and R8 are test selected

A2A1L3, and resistive pad A2A1R6-R7-R8. This forward

to set the output of the vco to about 800 millivolts which,

biases diode A2A4CR2 to enable the low-band vco

in turn, sets the synthesizer transmit rf and receive

output and at the same time reverse biases diodes

injection output levels at 14 2 dB mW when loaded into

A2A4CR3 and A2A4CR4 to open the midband and high-

50 ohms.

band outputs. In the vco circuit, voltage developed

across resistor A2A1R3 of voltage divider A2A1R2-

4-71.

Decoupling network A2A7R4-C61-C4-L1 de-

A2AlR3 and applied through choke A2A1L4 sets the bias

point of A2A1Q1. The tuned circuit, composed of

couples the three voltage-controlled oscillators from the

capacitor A2A1C3, varactor A2AlCRl, and inductor

+18-V dc supply voltage. This prevents the 3.2- MHz

A2A1L1

tcxo frequency from interfering with the vco frequency.

4-16