Receiver Section - Motorola GP2000 Service Manual

Theory of Operation
3.6.2 Antenna Switching
Switching of the antenna between the transmitter and the receiver is accomplished by the antenna
transmit/receive switch consisting of diodes Q409 and Q410. In the transmit mode, switched TX B+
is applied through R419 and RF choke L417, hard forward biasing the two diodes on Q410, thus
permiting the ßow of RF power from the output of the low-pass Þlter. L414 and C445 isolate the
receiver circuit from the transmitter power input.
3.6.3 Power Control
Output power is controlled by the CPU (Q101), the D/A converter (Q121), the dual Op-Amp (Q419),
which is used as a differential ampliÞer and comparator. Current is sensed by the voltage drop
across R445. When the radio is in high power mode, this voltage is compared to the
D/A converter voltage at 5-watt. When the radio is in low power mode, this voltage is compared to
the D/A converter voltage at 1-watt. The power output is then reduced or increased by varying the
applied voltage to the gate of the power ampliÞer Q411 and Q412.
3.6.4 Transmitter Audio Circuits
The transmitter audio circuits consist of the audio processing circuits, the CTCSS circuits and the
DCS circuits.
3.6.4.1 Audio Processing
Transmit speech audio is provided by either the internal electric microphone M101 or the external
microphone. The audio is pre-emphasized by 6dB per octave by C615 and R655, and then its signal
is ampliÞed. The microphone audio is directed to ampliÞer Q602B, Q602A and low-pass Þlter
Q602C and Q602D. Q602A has limiting function. The modulation adjustment is done by Pot RV601,
and directed to a four-pole active low-pass Þlter. The resulting signal is then limited, and is directed
to a low pass Þlter (18dB per octave roll-off above 3KHz). The audio is then directed through the
25KHz/12.5KHz channel spacing SW Q601 to varactor diode Q329 in transmit VCO. By varying the
voltage on the varactor diode Q329 at an audio rate, VCO output is frequency-modulated.
3.6.4.2 CTCSS Tone Encoder / Digital Code Squelch (DCS) Encoder
CTCSS signals and DCS signals are synthesized by CPU Q101 and appear as pulse waveform on
I/O line Pin 7. This I/O line is connected to a digital-to-analog converter network (consisting of C136,
C173, R150, R177 and R182), which produces a pseudo-sine wave at its output. The waveform is
smoothed by low pass Þlters Q610B to produce an acceptable sine wave output. The CTCSS tone
signal is adjusted to the proper level by RV602.
3.7

Receiver Section

3.7.1 Receiver Front End
In the receive mode, the RF signal enters thorough the antenna, then through the low-pass Þlter
C439-C445, and L414-L416. The diodes Q409 and Q410 are biased off, so that the output of the
low-pass Þlter is coupled (C437, C438, L413 and R418) to the Þrst band-pass Þlter C401-C406,
C408-C410, L401, L402, PCB_L1, Q401, R401, and to the Front End RF overload protection diode
pair Q403. The signal from the band-pass Þlter is directed to the input of the RF ampliÞer Q404. The
output of the RF ampliÞer is sent to three stages of band-pass Þlters, consisting of C415-C429,
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