CIRCUIT DESCRIPTION

UHF TRANSMITTER DESCRIPTION

Impedance matching between Q508 and Q509 is provided by L501, several capacitors, and two sections of microstrip (microstrip is described in the preceding section). Resistor R523 lowers the Q of L501 to make it less frequency selective. Q508 is powered by the switched battery supply. AC signals are decoupled from this supply by C517-C519 and ferrite bead EP500.

Predriver Q509 is an RF power MOSFET. The gate is biased at approximately one-half the drain voltage by R519 and R522. Capacitors C512-C516, C522, and C523 provide decoupling of AC signals. R524 and R527 lower the Q of the input matching circuit which improves stability.

Supply voltage to Q509 is from the power control circuit described in Section 3.9.6. This circuit varies the supply voltage to change the power output of Q509 in order to maintain constant transmitter power output. RF choke L500, ferrite bead EP501, and several capacitors isolate the power control circuit from RF signals. Several capacitors and sections of microstrip on the drain of Q509 provide an output impedance of 50 ohms to the power amplifier board. This stage provides a gain of approximately 10 dB, resulting in a power input to the PA board of up to approximately 8 watts.

3.9.3 DRIVER (Q600), FINAL (Q601)

Driver Q600 on the power amplifier board is used with high power (40 watt) versions only. 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