Modulator Circuits

TM 11-5820-695-35

(3) The frequency of 150 MHz VO) Q103

the same.

The paragraphs below highlight the

is determined by resonant circuit C114, L103 and the

differences.

capacity of varactor diodes CR101 and CR102.

Varactor diodes CR101 and CR102 are always operated

Modulator Circuits.

reverse biased. They exhibit an inverse square law

capacitance-reverse bias characteristic, that is, the

(1)

The data/orderwire signal at J1 INPlT

capacitance varies approximately as the inverse of the

is coupled through C157 to switch BANDSET S102.

square root of the reverse bias voltage. By varying the

Switch S102 either couples the signal straight through or

bias voltage, or baseband input signal, the resonant

through unity gain inverter amplifier Q11. At the output

frequency of the tank is made to vary. With only the

side of S102, the signal is connected to switch

bias voltage from the differential amplifier applied, the

BANDSET S101 and to emitter follower stage Q10. The

tank resonant frequency will determine the oscillator

output of Q10 is coupled through C4 and routed through

frequency.

If the bias is increased the diode

P1-8 to attenuator assembly 1A3. Switch S101 couples

capacitance will decrease, causing the tank to resonate

the signal straight through for C-band position and

at the higher frequency. Similarly, a decrease in bias

through 10 dB attenuator R101/R102 for K-band

will cause decrease in the resonant frequency. Since

position. The signal at the output side of S101 is

this bias voltage represents the center frequency error,

applied to the input of the baseband amplifier through

as detected in electronic frequency control 1A7, the

the network R148, R149, R103 and C140. The input

voltage controlled oscillator center frequency follows the

level is set at R149 to provide a modulation index of 20

frequency of the reference oscillator in electronic

MHz/volt. The output of the 3-stage baseband amplifier

frequency control 1A7. The center frequency is initially

is coupled into the VDC stage Q103 through the

set during alignment by means of front panel variable

baseband frequency response control network C109,

capacitor C114. If a baseband signal is applied at the

R113, R117, L102. The center frequency of the VGC is

junction of CR1 and CR2, the frequency will be made to

controlled by a dc signal from differential amplifier

vary about the center frequency; thereby producing a

A101, and the VGO is frequency modulated by the

frequency modulated 150 MHz output. Since electronic

signal from the baseband amplifier.

frequency control 1A7 senses the average frequency, it

only corrects for center frequency drift and has no effect

(2) Differential amplifier A101 is driven

on the frequency modulation of the carrier. The output

by the frequency correction signal from frequency

across load resistor R120 is applied to buffer stage

control 1A7 (P1-6, -7). A reference voltage derived

Q109 through C116 and the buffer output is coupled

from +15 vdc through R115, R116, C152 and CR105 is

through C119 to the input of 3-stage limiter amplifier

applied through R150 at A101-3. AFC disable switch S1

Q104, Q105 and Q106. Diodes CR103 and CRl04 limit

in the input circuit is normally open and it is used to set

the output level to 0 dBm +3 dB.

the output level of A101. With S1 closed, R116 is

adjusted to provide an output level of +5 vdc at A101-6.

(4) The three stages of the limiter

The AFC signal from 1A7 is applied to differential

amplifier operate at saturation level and provide an

amplifier A101 through R20 and R144. With an AFC

amplitude limited output level of +28 dBmn at the

signal applied (S1 open), the output at A101-6 varies

collector of Q106. The middle stage, Q105, has a

above or below +5 V' according to the AFC error

network to provide for flat frequency response across

voltage. The output signal at A101-6 is applied through

the bandwidth. This network is comprised of the

decoupling network R153, R118, C111, C113 and L102

collector

to the cathodes of varactor diodes CR101 and CR102.

Change 6 2-74.1