IF IC (U5201). Motorola GM950 403-470 MHz, GM950, GM950 136-174 MHz, GM950 66-88MHz

Transmitter Power Amplifier (PA) 5-25W

8.3

IF IC (U5201)

The first IF signal from the crystal filters feeds the IF IC (U5201) at pin 6. Within the IF IC the

45.1MHz first IF signal mixes with the second local oscillator (LO) at 44.645MHz to the second IF at

455 kHz. The second LO uses the external crystal Y5211. The second IF signal is amplified and filtered by two external ceramic filters (FL5201, FL5202). Back in the IF IC the signal is demodulated in a phase-lock detector and fed from IF IC pin 28 to the audio processing circuit ASFIC U0201 located in the controller section (line DET AUDIO).

The squelch circuit of the IF IC is not used. Instead the squelch circuit inside the audio processing

IC ASFIC (U0201) determines the squelch performance and sets the squelch threshold. The detector output signal from IF IC (U5201) pin 28 (DET_AUDIO) is fed to the ASFIC pin H7.At IF IC pin 11 an RSSI signal is available with a dynamic range of 70 dB.

The RSSI signal is interpreted by the microprocessor (U0101 pin 44) and in addition after buffering by op-amp U0202-3 available at accessory connector J0400-15.

9.0

Transmitter Power Amplifier (PA) 5-25W

The radio’s 5-25 W PA is a four stage amplifier used to amplify the output from the exciter to the radio transmit level. It consists of the following four stages in the line-up. The first (Q5510) is a bipolar stage that is controlled via the PA control line (line PWR CNTL). It is followed by another bipolar stage (Q5520), a MOS FET stage (Q5530) and a final bipolar stage (Q5536).

Devices Q5510 and Q5520 are surface mounted. Bipolar Transistor Q5536 and MOS FET Q5530 are directly attached to the heat sink.

9.1

Power Controlled Stage

The first stage (Q5510) amplifies the RF signal from the VCO (line EXCITER PA) and controls the output power of the PA. The output power of the transistor Q5510 is proportional to its collector current which is adjusted by a voltage controlled current source consisting of Q5612, Q5611 and

Q5621. The whole stage operates off the K9V1 source which is 9.1V in transmit mode and nearly 0V in receive mode.

The collector current of Q5510 causes a voltage drop across the resistors R5623 and R5624.

Transistor Q5612 adjusts the voltage drop across R5621 controlled through the PA control line

(PWR CNTL). The current source Q5621 adjusts the collector current of Q5510 by modifying its base voltage via (R5502, L5501) until the voltage drop across R5623 and R5624 plus V

BE

(0.6V) equals the voltage drop across R5621 plus V

BE

(0.6V) of Q5611. If the voltage of PWR CNTL is raised, the base voltage of Q5612 will also rise causing more current to flow to the collector of

Q5612 and a higher voltage drop across R5621. This in turn results in more current driven into the base of Q5510 by Q5621 so that the collector current of Q5510 is increased. The collector current settles when the voltage over the series configuration of R5623 and R5624 plus V

BE

(0.6V) of

Q5621 equals the voltage over R5621 plus V

BE

(0.6V) of Q5611.

By controlling the output power of Q5510 and in turn the input power of the following stages the ALC loop is able to regulate the output power of the transmitter. Q5611 is used for temperature compensation of the PA output power.

In receive mode the PA control line (PWR CNTL) is at ground level and switches off the collector current of Q5612 which in turn switches off the current source transistor Q5621 and the RF transistor Q5510.

5A.3-20 Theory of Operation