CIRCUIT DESCRIPTION

800/900 MHz RECEIVER DESCRIPTION

RF chokes L502 and L503, ferrite bead EP502, and several capacitors isolate the power control circuit from RF signals.

A50-ohm, 3 dB pad formed by R529-R531 provides an output impedance of 50 ohms at the J602. Impedance matching between Q507 and this pad is provided by two sections of microstrip, C524, and C527. Power output at J602 is typically up to 250 milliwatts.

3.12.2POWER AMPLIFIER MODULE (U600), FINAL (Q651)

Power amplifier module U600 on the PA board has approximately 19 dB of gain. Pins 2, 3, and 4 of U600 are the supply voltage inputs to three separate gain stages. The supply voltage on pin 2 (VS1) is switched by Q600 and limited to 12 volts by CR601 and R600. Switch Q600 is controlled by the same signal used to control 8V transmit switch Q505/Q504 (see Section 3.12.1).

The supply voltage applied to pins 3 and 4 (VS2/ VS3) is the unswitched battery from the power jack fed through R601. Therefore, power is applied to these pins of U800 even when transceiver power is turned off. The power control circuit senses transmitter current by monitoring the voltage drop across R601.

The output signal on U600, pin 5 is then applied to Q651 (30 watt models only). With lower power models, Q651 is not used. Amplifier Q651 provides approximately 5 dB of gain. The output impedance on U600, pin 5 is 50 ohms, and it is matched to Q651 by a section of microstrip, C651, C652, and C653. Class C bias of Q651 is provided by L651. The unswitched battery supply applied to Q651 is isolated from RF by L602, EP601, and several capacitors. Impedance matching is provided on the output by C654-C656, C614, and two sections of microstrip.

3.12.3 ANTENNA SWITCH

The antenna switch circuit consists of Q602, CR602, CR604, several other components, and also a quarter-wave 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 U801. This is the same signal that controls transmit 8-volt supply switch Q505/Q504. 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 Q601 through R610, R609, CR604, L604, CR602/R602, and L602. In addition, 8 volts is applied through R200 to CR200 located at the input of the receiver on the RF board.

CR200, CR602 and CR604 are PIN diodes like CR901 described in Section 3.10.2. When a PIN diode is forward biased, it presents a very low impedance. Therefore, in the transmit mode the transmit signal has a low-impedance path to the antenna through CR602 and coupling capacitor C621.

The signal is blocked from the receiver by two grounded quarter-wave lines. A discrete quarter-wave line is formed by C635, C622, and L604 is effectively AC grounded on the receiver end by CR604 and C633. When one end of a quarter-wave line is grounded, the other end presents a high impedance to the quarter- wave frequency. Therefore, this line presents a high impedance into the receiver for the transmit signal. Further receiver isolation is provided by another quarter-wave line in the receiver. This line is grounded through C202 by CR200.

In the receive mode, all three PIN diodes are reverse biased. Therefore, CR602 presents a high impedance into the transmitter for the receive signal, and since the two quarter-wave lines into the receiver are no longer grounded, there is a low-impedance path into the receiver. L604 presents a low impedance because it is no longer grounded by CR604, and the quarter-wave line presents a low impedance because it is no longer grounded by CR200. Resistors R602 and R608 improve the isolation provided by CR602 and CR604 when they reverse biased (receive mode).

3.12.4DIRECTIONAL COUPLER, LOW- PASS FILTER

A directional coupler is formed by adjacent sections of microstrip near C621. The forward component of output power is rectified by CR603 and devel-

 

February 2001

3-30

Part No. 001-9800-001