RF Output Amplification

11-5820-919-40-1

2-118.

ALC AR3-1.

The ALC amplifier

stage can be retarded by the PA disable

processes the same information as the

switches Q4, Q5.

These two switches are

overshoot control (buffered VFWD and

normally enabled (conducting) by the

VREFL) but the reference voltage is

overshoot control (AR4-2, AR3-2). If

lower.

In addition, the ALC affects

the enabling voltage is reduced, Q4 and

the Xmt RF at the modulator/demodulator

Q5 will decrease conduction and reduce

module and the reaction time is slower.

the rf amplification.

Refer to the ALC paragraph in the

modulator/demodulator

(paragraph

2-90).

2-114.

RF Output Amplification. Next,

As a result of the longer reaction time

the rf is amplified to about 20 watts by

the output stage (T3, Q10, Q11, T4, T5).

(typically 5 milliseconds), the over-

shoot control acts to dampen the initial

This amplification is also done using a

peak of the Xmt RF until the ALC reduces

pushpull configuration, using T3 as an

the Xmt RF at the modulator/demodulator.

input transformer and T4-T5 as output

transformers. This stage is powered by

+31v from the DC-DC converter.

2-119.

Sidetone Enable AR2-1, Q15.

This circuit determines whether the

2-115.

Temperature

Compensation.

Both

power output is adequate for transmit

the driver stage (T1, Q3, Q6, T2) and

communication (about 6 watts).

the output stage (T3, Q10, Q11, T4, T5)

If SO,

audio feedback is provided to the oper-

use temperature-compensating bias cir-

ator.

Again, the buffered VFWD and

cuits; (1) Q1, Q2 and (2) Q7, Q8, and

VREFL voltages are compared against

Q9, respectively.

These circuits reduce

reference values. The difference is

the tendency for the gain of the push-

that the output of AR2-1 passes through

pull amplifiers to increase as the tem-

switch Q15.

Q15 is enabled by both the

perature

rises.

Both bias circuits use

Keyline signal (grid) from the modulator/

the +6.5v transmit for input voltage.

demodulator and an adequate power level.

The activated state (Q15 conducting) of

2-116. Overpower Detector AR4-1. The

the Sidetone Enable is +1.4v or less.

overpower detector receives Xmt RF input

The Tune In Progress signal provides

via isolation transformer T6.

Any large

Sidetone Enable by passing ground

excess in the power reflecting back at

through diode CR24 during the automatic

the power amplifier will cause a voltage

tuning process. When Tune In Progress

to be applied to the overshoot control

is present, the operator hears a 1-kHz

and ALC circuits.

tone.

2-117.

Overshoot Control AR4-2, AR3-2.

The overshoot control acts on informa-

2-120.

Tune Check Enable AR2-2, Q16.

tion regarding antenna match in the form

If, while transmitting, the antenna

of buffered VFWD and VREFL inputs from

should become detuned, the Tune Check

the harmonic filter, or input from the

Enable will be applied to the antenna

overpower detector AR4-1.

VFWD is a

tuner.

Buffered VFWD and VREFL are

measure of the energy going to the

sampled and compared against a reference

antenna and VREFL is a measure of the

value at AR2-2.

If the vswr is worse

energy that is reflecting back at the

than 1.6:1, the Tune Check Enable will

harmonic

filter.

VFWD and VREFL are

cause retuning the next time PTT is

summed, amplified, and compared against

pressed. Q16 ensures that the radio

a reference value. If the power reflec-

is in transmit mode and the Keyline is

tion condition is momentarily high, the

grounded.

In the active state, this

overpower detector will cease to apply

line is about +6v. Tune In Progress

an enabling voltage to the PA disable

will prevent a false Tune Check Enable

switch. The rf amplification will be

from being generated during the tuning

reduced and power will be reduced to a

process.

safe level.

2-47