User Manual Part 2

The power control circuit maintains a constant power output as changes occur in power amplifier

temperature and voltage. It does this by sensing forward power and then varying the drive to Q6

to maintain a constant output power. The drive to Q6 is controlled by varying the voltage applied

to the variable attenuator circuit described in Section 5.2.2.1. In addition, the current applied to

final amplifier Q6 is sensed, and if it becomes excessive, power is cut back to a low level.

The power output level is set in 127 steps by a Digital-to-Analog (D/A) converter formed by shift

the following functions:

• It allows the RF power output level to be adjusted using the PCTune™ software.

• It allows the microcontroller to cut back power if the power amplifier temperature sensed by

thermistor TR1 becomes excessive.

• It allows high and low power levels to be selected by the user or to be fixed by programming

using the PCConfigure™ software.

Forward Power Sense

The forward power signal from the directional coupler is applied to pin 2 of amplifier U2-A. This

is a DC signal that increases proportionally to forward power. The other input to U2-A on pin 3 is

a DC reference voltage from the D/A converter. This signal sets the power output of the

transmitter.

U2-A is a difference amplifier which amplifies the difference between the reference voltage on

pin 3 and the forward power signal on pin 2. The turn-on time of U2-A is controlled by the time

constant of C17 and R28, and negative AC feedback to prevent oscillation is also provided by

C17.

This circuit operates as follows: Assume the output power attempts to increase. The DC voltage

applied to U2-A, pin 2 then increases which causes the output voltage on pin 1 to decrease.

Transistors Q1 and Q2 then turn off slightly which decreases the supply voltage to the variable

attenuator circuit. The input power to Q6 then decreases. The Q10 output power then decreases to

maintain a constant power output. R26 and R24 limit the voltage gain of Q2 and Q1 to

approximately two volts.

Delayed PTT

Transistor Q3 is used to delay power output for a short time after the transmitter is keyed. This

allows the synthesizer and exciter to stabilize so that the transmitter does not transmit off

frequency. The signal which controls Q3 is from microcontroller on the logic board. In the

receive mode this output is low, so Q3 is off. Pin 2 of U2-A is then pulled high by the 8-volt

supply applied through R29 and CR1. This causes the output on pin 1 of U2-A to go low which

shuts off Q2 and Q1. Then when the transmitter is keyed, after a short delay Q3 then turns on and

diode CR1 is reverse-biased. Only the forward power signal is then applied to pin 2 of U2-A.

Over-Current Shutdown

Current to Q6 is monitored by sensing the voltage drop across R33. Pins 3 and 6 of U3 are

connected across this resistor. As current increases, the output voltage on U3 pin 8 increases. This

voltage is applied to Schmitt trigger U2-B. When the voltage on pin 6 rises above the reference

on pin 5, the output on pin 7 goes low. This lowers the voltage applied to U2-A, pin 3 which