Radio Transmitter T-1054(P)/GRC144(V) Modulator 1A8 Circuit Functioning

TM 11-5820-695-35

set the bias for emitter follower A2D. The output A2-D

ulator differential amplifier. Diodes CR14 and CR15

is routed from load resistor R7 through coupling

comprise a limiter, which limits the voltage amplitude at

capacitor C4 to alarm monitor 1A4, and to the meter

the input to CR12 and CR13.

selector switch on meter panel assembly 1A15A8

through attenuator assembly 1A3. With the meter

Resistor R78 and capacitor C69 are used to set the ac

selector switch in the TRAFFIC position, the meter

ground point. Resistors R68 and R69 are diode load

indicates the presence of a baseband signal at the

resistors. The circuit is arranged so that the FREQ

modulator input.

CONT lead at P1-8 is positive with respect to the FREQ

CONT RETURN lead at P11 when the modulator

frequency is high. A metering signal (CENTER FREQ

NOTE

METERING) is also applied to the meter selector switch

through metering resistors R71, and R72.

Amplifiers A1 and A2 are integrated

circuits. Al is type A702 and A2 is

(3) The supply voltage (+15 volts) is

type CA3018.

These circuits are

supplied from an external source to P113. Inductors IA,

illustrated in figure 5-24('11).

L19, 121, and L22, and capacitors C&3, C66, and C67

provide power supply isolation. The + 16 volts is

(e) Differential amplifier A2A and A2B is

applied to J6 through R67 for monitoring.

driven by the low level dc correction signal from

electronic frequency control 1A7. Resistors R10, R11,

2-35.

Radio Transmitter T-1054(P)/GRC144(V)

R12, and R13 provide bias for the base circuits and

Modulator 1A8 Circuit Functioning (fig. 5-26)

R16, R17, and R18 set the emitter voltages. C9 through

Radio transmitter modulator 1A8 is comprised of

C13 are bypass capacitors. The correction signal is

modulator circuits and frequency control circuits. The

applied between the bases of A2A and A2B through

modulator circuits produce a 160 MHz subcarrier which

isolating resistors R20 and R21,. With no driving signal

is frequency modulated by the baseband input signal

present, both transistors will conduct. Since they are

(pcm/ow). The frequency control circuits produce a 70

identically biased, their collector currents will be

MHz signal (sample of modulator frequency) which is

essentially equal. Therefore the output of A2A (junction

applied to electronic frequency control 1A7 where it is

of RP4 and R16) will be positive with respect to the

compared with a precise, locally generated, 70 MHz

output of A2B (collector of A2B). This results in a

signal. A dc signal, proportional to the modulator

reverse bias being impressed (through isolating resistor

frequency error, is generated in electronic frequency

R77 and rf choke L3, IA) across varactor diodes CR1

control 1A7 and applied to the modulator circuits for

and CR2 in the 160 MHz voltage controlled oscillator.

center frequency correction (para 2-34).

When the correction signal at A2A goes positive with

respect to A2B, A2A will conduct more and A2B will

Modator Circuits. The modulator circuits

conduct less. With increased current through collector

consist of a baseband amplifier, differential amplifier,

resistor R14, the output voltage of A2A will decrease.

160 MHz voltage controlled oscillator, and limiter-

Similarly, the output (collector) voltage of A2B will

amplifier stages.

increase because of the reduced voltage drop across

R19. Thus, the voltage difference between these

(1)

The pcm/ow input at J1, is applied to

outputs decreases resulting in a decrease in the bias

operational amplifier Al through coupling capacitor C28

voltage applied to the varactor diodes in the 160 MHz

from parallel terminating resistors R1 and R2. Variable

voltage controlled oscillator. It will also be seen that

resistor R2, is a front panel control which is used to

when A2B input signal is positive with respect to A2A,

adjust the baseband level into the vco, thereby setting

the bias applied to the 160 MHz voltage controlled

the system deviation. Resistors R4 and R6 establish

oscillator will increase. EFC DISABLE switch S1 is

bias and C3 provides bypass. Resistors R6, R8, R29,

connected across the error signal input from electronic

and capacitors C5 and C6 are compensation elements

frequency control 1A7. With this switch closed, the error

which set the gain and shape the amplifier response.

signal is disabled, and A2A and A2B conduct under

The supply voltage to this stage is BY % passed by C8.

internal bias conditions. The EFC DISABLE switch is

The output at pin 7 of Al is applied from load resistor

used

R67 to the tank circuit of the 150 MHz vco, through

coupling capacitor C7. Emitter follower A2D, also

connected to the input at J11 through coupling capacitor

C1, samples the baseband input. Resistors R3 and R9

2-72