Motorola GM1200E Service Manual page 56

Transmitter Power Amplifier (PA) 5-25W
A second two pole varactor tuned bandpass filter provides additional filtering to the amplified signal.
The dual varactor diode D3304 is controlled by the same signal which controls the pre-selector filter.
If the radio is configured for a base station application, R3318 is not placed and TP3301 and TP3302
are shorted.
11.2 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 (D3331). Its ports are matched for incoming VHF signal conversion to the
45.1MHz IF using high side injection. The injection signal (VCO MIXER) coming from the mixer
buffer (Q3770) is filtered by the lowpass consisting of (L3333, L3334, C3331 - C3333) and has a
level of approximately 10 dBm.
The mixer IF output signal (RX IF) from transformer T3301 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.
11.3 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.
12.0 Transmitter Power Amplifier (PA) 5-25W
The radio's 5-25 W PA is a three stage amplifier used to amplify the output from the exciter to the
radio transmit level. It consists of the following stages in the line-up. The first (Q3511) is a bipolar
stage that is controlled via the PA control line (line PWR CNTL). It is followed by a MOS FET stage
(Q3521) and a final bipolar stage (Q3531). Devices Q3511 and Q3521 are surface mounted. Bipolar
Transistor Q3531 is directly attached to the heat sink.
3.1-26 Introduction/Theory of Operation