CIRCUIT DESCRIPTION

800/900 MHz TRANSMITTER DESCRIPTION

oped across R606 and then 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 C624- C627 and several sections of microstrip. This filter attenuates harmonic frequencies occurring above the transmit band. R607 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. This thermistor forms a voltage divider with R143 on the audio/ logic board, and the voltage across this divider is monitored by an A/D converter input of the microcon- troller (pin 63). If the PA temperature increases above limits set in software, the power is first cut back. Then if it continues to rise, the transmitter is turned off.

3.12.5POWER 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 driver Q507. This changes the power output of that stage which in turn controls the power output of the transmitter.

The primary signal sensed to control power

output is forward power from the directional coupler. 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 the microcontroller through D/A converter U801. 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 ampli-

fier temperature or transceiver voltage is excessive as just described in Section 3.12.4.

U500A, Q500/Q502 Operation

U500A is a differential amplifier which amplifies the difference between the reference voltage on pin 3 and the forward power signal on pin 2. The forward power signal from the directional coupler is a DC signal that increases in proportion to forward power. The reference input is a DC voltage from the D/A converter formed by shift register U801 and several resistors. This stage is similar to U305 described in Section 3.4.4, and programming of U801 is described in Section 3.10.9. This reference voltage effectively sets the power output of the transmitter.

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 driver Q507. 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, Q506 in the exciter, and the first stage of PA module U600 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 which turns Q503 off. U500A, pin 2, is then pulled high by 8 volts applied through R505 and CR503. The output of U500A on pin 1 is then low which shuts off power to Q507. Then when the transmitter is keyed, Q503 turns on after a short delay and diode CR503 is no longer forward biased. Only the forward power signal is then applied to pin 2 of U500A.

 

February 2001

3-31

Part No. 001-9800-001