Transmitter, Radio T-1054(P) A/GRC-144(V) Modulator 1A8 Circuit Functioning

TM 11-5820-695-35

applied to the center tap of T3 secondary. The voltage

the frequency control circuits through attenuator network

appearing across the transformer secondary is,

R60, R68, and R74.

therefore, the 220 MHz oscillator frequency

superimposed on the 1'50 MHz modulator frequency.

Frequency Control Circuits. The frequency

When terminal 3 of T3 is positive with respect to

control circuits consist of a 100 MHz crystal controlled

terminal 5, both diodes, CR7 and CR8 conduct, and the

oscillator, a frequency doubler, a mixer, and two 70 MHz

composite signal appears at the top of C38. With the

amplifier stages.

reverse polarity, both diodes are reverse biased and no

signal flows. As a result, only the portion of the

(1) The 110 MHz crystal oscillator

composite signal above the zero axis appears at the

consists of transistor Q5, crystal Y1, tank circuit L6,

C38, L10 junction. The envelope of this clipped wave is

C31, C32, and C33, bias resistors R40, R4J1, and R42,

the 70 MHz difference frequency. A low pass filter

load resistors R76, bypass capacitor C30, coupling

comprised of C38, C39 and L10 extracts the 70 MHz

capacitor C34, and isolating network FL3, L9, C37. The

envelope.

crystal operates at series resonance in the fifth

overtone.

When voltage is applied, the starting

(4) The 70 MHz output from the mixer

transient causes oscillations to start. The crystal signal

applied to 70 MHz amplifier Q6 through coupling

at Q5 emitter is amplified and appears at the collector of

capacitor C40. This stage contains bias resistors R44,

Q5. The tank circuit follows the oscillation, and feeds a

R45, R47; collector load resistor R46, and emitter

regenerative signal to the crystal oscillator. By this

bypass capacitor C42. Isolation is provided by FL4, L11

process, the oscillator signal builds up until the transistor

and 42. The stage is coupled to the second amplifier

gain is reduced to unity, at which point a stable level is

stage Q7 through coupling capacitor C41. The second

sustained. Variable capacitor C33 provides a means of

stage is similar to the first. Dc bias is provided by R48,

varying the frequency by a small incremental value. To

R49, R50 and R51. However, in the emitter circuit only

illustrate, assume the tank is tuned to a frequency above

R51 is rf bypassed by C43 and R50 provides negative

that of the crystal. This will cause the tank to appear

feedback for stabilization. The collector output is taken

slightly capacitive to the collector signal. As a result,

off at tap 2 of transformer T4. The stage is decoupled

the signal fed back to the crystal leads the crystal

from the 115 volt supply by F115 and L13. The 70 MHz

oscillating signal.

For this condition, the crystal

output signal is applied to cable W2 through coupling

frequency will pull to a higher frequency, in an attempt

capacitor C44. Cable W2 is connected to P2 (FIRE

to restore a zero phase condition. This will result in a

CONTROL INTERCONN) connector which supplies the

reduced phase difference between the collector signal

70 MHz reference signal to electronic frequency control

and the tank, and consequently a smaller lead angle in

1A7. A metering circuit is also connected to the output

the crystal feedback signal As a result, the crystal will

of Q7, at the load side of 044. The metering circuit

assume a. frequency of oscillation above its normal

produces a dc signal proportional to the 70 MHz level,

frequency at which the phase is essentially zero. The

which is fed to the meter selector switch on meter panel

crystal frequency shift is very small compared to that of

assembly 1A15A8. Diode detector circuit CR9 and R.52

the tank, because the crystal Q is very high and the Q of

operates in an identical manner to the 1,50 MHz

the tank is relatively low.

oscillator output metering circuit previously described.

Ripple is filtered at the output by D14 and C46. Variable

(2) The 1,10 MHz crystal oscillator signal

resistor R53 is used to adjust the CONT LEVEL

at the collector of Q5 is applied to the primary of T2 in

METERING output.

the frequency doubler. This circuit is, in effect, a full

wave rectifier, in which CR5 and CR6 alternately pass a

2-35.1. Transmitter, Radio T-1054(P)

half cycle of the input signal. The fundamental of this

composite output is twice the input frequency The output

A/GRC-144(V) Modulator 1A8

also contains significant higher order harmonics, which

Circuit Functioning

are blocked by the output tuned circuit made up of C35,

(fig. 5-26.1)

L8, and the inductance contributed by T3 The 220 MHz

Modulator 1A8 in Radio Transmitter T-i054(P)A/MC-

output of the frequency doubler is added to the mixer

144(v) is basically the same as that for modulator 1A8

circuit.

in Radio Transmitter T-1054(P)/CM-144(v). That is the

same circuit functions are performed in both modulators

(3) The 220 MHz doubler output is

and signal routing is

connected to the primary of transformer T3 in the mixer,

while the sampled 1'50 MHz from the modulator is

2-74

Change 6