CIRCUIT DESCRIPTION

UHF TRANSMITTER DESCRIPTION

3.9.5DIRECTIONAL COUPLER, LOW-PASS FILTER

The transmit signal is fed to a directional coupler formed by adjacent sections of microstrip. The forward component of output power is rectified by CR602 and developed across R607 and fed to the power control circuit. Reverse power is not detected in this transceiver.

From the directional coupler the transmit signal is fed to a low-pass harmonic filter formed by L602- L604 and several capacitors. This filter attenuates harmonic frequencies occurring above the transmit band. R603 dissipates static buildup on the antenna.

The ambient power amplifier temperature is sensed by thermistor R601. The resistance of a thermistor decreases as temperature increases. R601 and R143 on the audio/logic board form a voltage divider, and the voltage across this divider is monitored by an A/D converter input of the microcontroller (pin 63). If the PA temperature increases above limits set in soft- ware, the power is first cut back. Then if it continues to rise, the transmitter is turned off.

3.9.6 POWER CONTROL (U500A/B, Q500-Q503)

Introduction

The power control circuit maintains a constant power output as changes occur in temperature and voltage. It does this by varying the supply voltage to predriver Q509 (unrevised 430-470 MHz) or third amplifier Q511 (all others). This changes the power output of that stage which in turn controls the power output of the transmitter. The power control circuit senses forward power to control power output. The current to final amplifier Q601 is also sensed, but it affects power output only if it becomes excessive. Gradual power shutdown then occurs.

The power output level is set in 127 steps by D/A converter U801 that is controlled by the microcon- troller. This allows power to be adjusted from the front panel using the test mode and also different power levels to be programmed for each system. In addition, it allows the microcontroller to cut back power when power amplifier temperature is excessive as just described.

U500A, Q500/Q502 Operation

The forward power signal from the directional coupler is applied to pin 2 of amplifier U500A. This is a DC signal that increases in proportion to forward power. The other input to U500A is a DC voltage from a D/A converter formed by shift register U801 and several resistors. This stage is similar in design to D/A converter U305 described in Section 3.4.4. Programming of U801 is described in Section 3.7.8. The voltage from this D/A converter sets the reference voltage on pin 3 which sets the power output of the transmitter.

U500A is a difference amplifier which amplifies the difference between the reference voltage on pin 3 and the forward power signal on pin 3. The turn-on time of U500A is controlled by the time constant of C502 and R508. Negative AC feedback to prevent oscillation is also provided by C502. This circuit operates as follows: Assume the output power attempts to increase. The DC voltage applied to U500A, pin 2 then increases which causes the output voltage on pin 1 to decrease. Transistors Q502 and Q500 then turn off slightly which decreases the supply voltage to predriver Q509 (or third amplifier Q511). The output power then decreases to maintain a constant power output. R510 and R513 limit the voltage gain of Q500 and Q503 to approximately two.

Delayed PTT

Transistor Q503 is used to delay power output for a short time after the transmitter is keyed. This allows the synthesizer and the exciter to stabilize so that the transmitter does not transmit off-frequency. The signal which controls Q503 is from the Q2 output (pin 17) of latch U111 on the audio/logic board. In the receive mode this output is low, so Q503 is off. Pin 2 of U500A is then pulled high by the 8-volt supply applied through R505 and CR503. This causes the output on pin 1 of U500A to go low which shuts off power to Q509 (or Q511). Then when the transmitter is keyed, the Q503 control signal goes high after a short delay. Q503 then turns on and diode CR503 is reverse biased. Only the forward power signal is then applied to pin 2 of U500A.

 

February 2001

3-21

Part No. 001-9800-001