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
Predriver Q509 is an RF power MOSFET. The gate is biased at approximately
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
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
Further receiver isolation in the transmit mode is provided by a grounded
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
| February 2001 |
Part No. |