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
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
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
| February 2001 |
Part No. |