Mixer And Intermediate Frequency (If) Section; If Ic (U5201); Transmitter Power Amplifier (Pa) 5-25W; Power Controlled Stage - Motorola GM1200E Service Manual

8.2

Mixer and Intermediate Frequency (IF) Section

The signal coming from the front-end is converted to the first IF (45.1 MHz) using a double balanced
schottky diode mixer (D5401). Its ports are matched for incoming RF signal conversion to the
45.1MHz IF using low side injection. The injection signal (VCO MIXER) coming from the mixer buffer
(Q5771) is filtered by the lowpass consisting of (L5403, L5404, C5401 - C5403) and has a level of
approximately 10 dBm.
The mixer IF output signal (RX IF) from transformer T5401 pin 2 is fed to the first two pole crystal
filter Y5201. The filter output in turn is matched to the following IF amplifier.
The IF amplifier Q5201 is actively biased by a collector base feedback (R5201, R5202) to a current
drain of approximately 5 mA drawn from the voltage 5V STAB. Its output impedance is matched to
the second two pole crystal filter Y5202. A dual hot carrier diode (D5201) limits the filter output
voltage swing to reduce overdrive effects at RF input levels above -27 dBm.
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.
Introduction/Theory of Operation
Transmitter Power Amplifier (PA) 5-25W
3.1-19