Assessments - 3, GPA: 3.7
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It is matched to the 50 ohms by several capacitors and sections of microstrip on the input. Class C self bias is provided by L608 and ferrite bead EP603. From Q600 the signal is fed to final amplifier Q601 which is similar in design to Q600. Each stage has a gain of approximately 5 dB, resulting in an output power from Q601 of approximately 55 watts. The supply voltage to these stages is the unswitched battery supply. Therefore, power is applied even when transceiver power is turned off. Two RF chokes, a ferrite bead, and several capacitors isolate this supply from RF signals. Current to final amplifier Q601 flows through R600, and the power control circuit monitors transmitter current by sensing the voltage drop across it. 3.9.4 ANTENNA SWITCH The antenna switch circuit consists of Q602, CR601, CR603, several other components, and also a section of microstrip and CR200 on the RF board. This circuit switches the antenna to the receiver in the receive mode and the transmitter in the transmit mode. Switching transistor Q602 is controlled by the transmit signal from the Q7 output (pin 11) of shift register U807. This is the same signal that controls transmit 8-volt supply switch Q506/Q507. This signal is high in the transmit mode and low in the receive mode. Therefore, Q602 turns on in the transmit mode and current flows from the collector of final amplifier Q601 through L601, CR601/R602, L606, R608/ CR603, R609, and R610. Diodes CR601 and CR603 are PIN diodes like those in the receiver front end (see Section 3.8.1). When a PIN diode is forward biased, it presents a very low impedance. Therefore, the transmit signal has a low-impedance path through CR601 to the directional coupler and C614. With CR603 also forward biased, it effectively connects L606 to AC ground through C652. A parallel resonant circuit is then formed by L606 and C643 which presents a high impedance into the receiver for the transmit signal. Further receiver isolation in the transmit mode is provided by a grounded quarter-wave line. This quarter-wave line is formed by the section of micro- strip connected to C650/C651 and another section on the RF board. The receiver end of this quarter-wave line is AC grounded by PIN diode CR200 on the RF board. This diode is forward biased in the transmit mode by the 8-volt transmit supply applied through R200. When one end of a quarter-wave line is grounded, the other end presents a high impedance to the quarter-wave frequency (the transmit frequency band in this case). C650 and C651 on the PA board provide impedance matching. In the receive mode, all three PIN diodes are reverse biased. Therefore, CR601 presents a high impedance into the transmitter for the receive signal, L606 presents a low impedance because it is no longer resonant, and the quarter-wave line presents a low impedance because it is no longer grounded by CR200. February 2001 3-20 Part No. 001-9800-001 CIRCUIT DESCRIPTION UHF TRANSMITTER DESCRIPTION 3.9.5 DIRECTIONAL 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 L602L604 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 software, 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 microcontroller. 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 Operat...
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