FORWARD POWER AMPLIFIER

TO 31R2-2GRC171-2

TM 11-5820-815-14

NAVELEX 0967-LP-544-5010

The level of current flow is a function of the current

of variable capacitor C22, capacitors C24 and C26,

shaping network which allows current to decrease at an

resistor R31, and detector diode CR12. The reflected

exponential rate from about 100 to 0 milliampere as the

power sensor consists of variable capacitor C21,

voltage drop across it varies from 5 to 0 V dc. As the

capacitors C23 and C25, resistor R30, and detector

AGC voltage increases above 2.5 V dc, the output of

diode CR11. Both sensors develop 3.5 to 4 V dc for 20

U1B decreases which, in turn, begins to turn receive

watts of forward or reflected power.

attenuator Q4 off. This increases the voltage drop

across Q4 and reduces the voltage drop across the

4-236. FORWARD POWER AMPLIFIER. Forward

current shaping network. The result of this circuit action

power amplifier U2A (figure FO-24) amplifies the forward

causes the voltage applied to the noninverting input to

power output of directional coupler A7A2 to provide at its

U1B to increase and track the AGC voltage and at the

output approximately 4 volts per 20 watts (2 volts per 5

same time reduces the voltage drop across the current

watts) of forward power. The circuit functions as follows.

shaping network to reduce current flow through CR2

Forward power output from the directional coupler of

(increases series rf attenuation). When the AGC voltage

approximately 2 volts per 20 watts (1 volt per 5 watts) is

reaches about 4.8 V dc (maximum voltage attainable at

applied to the noninverting input of U2A through resistors

the noninverting input to U1B), the output of U1B goes

R34 and temperature-compensation network R32-R33.

negative and turns Q4 off completely. This reduces the

A negative voltage from potentiometer R41 of voltage

current flow through CR2 to about zero. Receive series

divider R44-R43-R41 is also applied to the noninverting

diode CR2 now provides maximum attenuation to the

input of U2A through resistor R39 and temperature-

receive rf signal.

compensation diode CR14.

This negative voltage

provides forward-biased voltage to direction coupler

4-232. When AGC voltage exceeds about 4.8 V dc, the

detector diode CR12 to bias it at the point of conduction

output of shunt diode control amplifier U1A decreases

(approximately 100 microamperes). To compensate for

below +4.2 V dc and begins to forward bias receive

the

negative

bias

voltage,

negative

voltage

shunt diode CR3 to reduce its rf resistance. As AGC

(approximately -0.4 V dc) is developed across resistor

voltage increases to about 8 V dc, the output voltage of

R47 of voltage divider R45-R46-R47 and applied to the

U1A decreases which, in turn, increases the voltage

inverting input of U2A. Potentiometer R41 is adjusted to

across the current shaping network. This allows current

give zero output voltage from U2A when forward power

through CR3 to increase as the AGC voltage increases.

is 0 watt. Variable resistor R49 controls the gain of U2A

As current through CR3 increases, the rf resistance to

and is adjusted to give 4-volt output voltage for 20 watts

ground (through C7) decreases to attenuate the rf signal.

of forward power. Capacitor C32 in conjunction with

When the transmitter is keyed, the rf attenuation voltage

resistor R45 and capacitor C27 in conjunction with

goes to about 11 V dc. This causes maximum signal

resistor R44 decouple the bias voltages applied to the

attenuation (receiver muting) to the receive rf module.

forward power amplifier from the -12-V dc line.

4-233. RF FILTER. Rf filter A7A1 (figure FO-24) is a 4-

4-237. REFLECTED POWER AMPLIFIER. Reflected

pole, tunable bandpass filter. Under control of d/a servo

power amplifier U3 (figure FO-24) amplifies the reflected

amplifier module A1, servo motor B1 positions the filter

power output of directional coupler A7A2 to provide at its

to the center frequency as determined by the radio set

output approximately 4 volts per 5 watts of reflected

control. Position feedback potentiometer R20 feeds

power. The circuit functions the same as forward power

position information back to d/a servo amplifier module

amplifies U2A except variable resistor R53 is adjusted to

A1.

Balance potentiometers R21 and R22 allow

give 4-volt output voltage for 5 watts of reflected power.

adjustment of the voltage across R20 so that the position

Variable resistor R40 is adjusted to give a slight negative

feedback voltage tracks the filter position with respect to

output voltage (about -0.120 V dc) for 0 watt of reflected

center frequency.

power. This negative voltage prevents the pa/antenna

power comparator circuits in power amplifier module A8

4-234. The filter provides a bandwidth of 1.5 MHz from

from tripping at small forward power levels or during

center frequency and maximum insertion loss of about 2

modulation troughs.

dB within this bandwidth. At 7 MHz from center

frequency, the filter provides more than 40 dB of

4-238. PERCENT MODULATION MONITOR. The

attenuation; at 15 MHz from center frequency, the filter

percent modulation monitor (figure FO-24), consisting of

provides more than 70 dB of attenuation; and at 30

amplifier U2B and variable attenuator Q5, processes the

MHz from center frequency, the filter provides more than

forward power output signal from U2A to develop a dc

80 dB of attenuation.

percent modulation output voltage (P4-14) that is

proportional to the percent modulation of the transmitted

4-235. DIRECTIONAL COUPLER. The directional

signal. The forward power output signal is composed of

coupler (A7A2 of figure FO-24) senses forward and

an audio component that represents the modulation

reflected power. The forward power sensor consists

envelope

superimposed

positive

4-42