CIRCUIT DESCRIPTION

UHF TRANSMITTER DESCRIPTION
3.9TRANSMITTER CIRCUIT DESCRIPTION (UHF MODELS)

NOTE: A transmitter block diagram is in Figure 3-4.

3.9.1 FIRST AND SECOND AMPLIFIERS

Revised 430-470 MHz and all 470-512 MHz Models

The input signal to the exciter is the transmit frequency from buffer amplifier Q801 in the synthe- sizer. It is at a level of approximately 0 dBm and is applied to first amplifier Q509. Impedance matching on the input of Q509 is provided by C540, C552, L503, C550, C551, and L504. Biasing is provided by R531 and R535, and C526/C527 and C521/C523 decouple RF signals. Impedance matching on the output is provided by L501, C541, C555, and C556. Second amplifier Q510 is similar in design to Q509. These stages together produce about 20 dB of gain.

Power to Q509 is switched on in the transmit mode by Q506 and Q507. This switch is controlled by the microcontroller through the Q7 output (pin 11) of shift register U801. This output is high in the transmit mode and low in the receive mode. This signal also controls the antenna switch circuit on the PA board described in Section 3.9.4.

This transmit 8V supply is not delayed which allows Q509 and the transmitter frequency to stabilize before power is produced. The delayed PTT signal is applied to the RF board on J201, pin 2. This signal controls the power control circuit described in Section

3.9.6.The emitters of Q505 and Q507 are grounded through Q508. That transistor is turned off when the logic is in an undetermined state such as during Flash programming. This ensures that the transmitter is turned off during these times.

Unrevised 430-470 MHz Models

The input signal to the exciter is the transmit frequency from buffer amplifier Q801 in the synthe- sizer. It is at a level of approximately 0 dBm and is applied to first amplifier Q506. Impedance matching on the input of Q506 is provided by C529, C537, L505, and C530. The input level is set by R528 and R530. Biasing is provided by R520 and R525, and C525/C526 and C520/C521 decouple RF signals.

Impedance matching on the output is provided by L502, C570, C531, L506, and C532. Second amplifier Q507 is similar in design to Q506. These stages together produce about 20 dB of gain. The 8-volt supply to Q506 and Q507 is switched by Q504 similar to the revised 430-470 MHz version just described.

3.9.2 THIRD AMP AND PREDRIVER

Revised 430-470 MHz and all 470-512 MHz Models

From Q510 the signal is fed to third amplifier Q511 which is a power MOSFET that provides approximately 10 dB of gain. Impedance matching with Q510 is provided by L502, C542, C557. L505, and C543. Resistors R532 and R534 lower the Q of the input matching circuit which improves stability. The gate of Q511 is biased by R527 and R528. Decoupling of RF signals is provided by C528/C529 and C516/C517.

Supply voltage to Q511 is from the power control circuit described in Section 3.9.6. This circuit varies the supply voltage to change the power output of Q511 in order to maintain constant transmitter power output.

Impedance matching with between Q511 and predriver Q512 is provided by several capacitors and sections of microstrip. Microstrip is a form of transmission line with distributed series inductance and shunt capacitance. The characteristic impedance is determined by the width of the microstrip and the PC board material and thickness (distance from ground plane).

Class C biasing of Q512 is provided by L506 and ferrite bead EP2. Several more capacitors and sections of microstrip on the output of Q512 provide matching with the 50-ohm input impedance of the power ampli- fier. This stage provides a gain of approximately 10 dB, resulting in a power input to the PA board of up to approximately 8 watts.

Unrevised 430-470 MHz Models

From Q507 the signal is fed to third amplifier Q508 which provides approximately 10 dB of gain. Impedance matching with Q507 is provided by L503, C571, C533, C538, L504, L507, C579, and C539. Biasing is provided by R5542, R555, and R532.

 

February 2001

3-19

Part No. 001-9800-001