Motorola cm200 Commercial Series, Service Manual

Page: 162 / 352

2-2 THEORY OF OPERATION
2.2 Receiver Back End
The IF signal from the crystal filter enters the IF amplifier which provides 20 dB of gain and feeds
the IF IC at pin 1. The first IF signal at 44.85 MHz mixes with the second local oscillator (LO) at
44.395 MHz to produce the second IF at 455 kHz. The second LO uses the external crystal Y301.
The second IF signal is amplified and filtered by two external ceramic filters (FL303/FL302 for
12.5 kHz channel spacing and FL304/FL301 for 25 kHz channel spacing). The IF IC demodulates
the signal by means of a quadrature detector and feeds the detected audio (via pin 7) to the audio
processing circuits. At IF IC pin 5, an RSSI signal is available with a dynamic range of 70 dB.
3.0 UHF Transmitter Power Amplifier (438-470 MHz)
The radio’s 40W PA is a three-stage amplifier used to amplify the output from the VCOBIC to the
radio transmit level. All three stages utilize LDMOS technology. The gain of the first stage (U101) is
adjustable and controlled by pin 7 of U103-2 via U103-3. It is followed by an LDMOS stage Q105
and LDMOS final stage Q100.
Figure 2-2 UHF Transmitter Block Diagram
Devices U101, Q105 and Q100 are surface mounted. Q100 is screwed down to the chassis to
ensure good thermal contact. This scheme also ensures sufficient thermal contact between driver
and chassis.
3.1 First Power Controller Stage
The first stage (U101) is a 24 dB gain integrated circuit containing two LDMOS FET amplifier
stages. It amplifies the RF signal from the VCO (TX_INJ). The output power of stage U101 is
controlled by a DC voltage applied to pin 1 from the op-amp U103-3, pin 8. The control voltage
simultaneously varies the bias of two FET stages within U101. This biasing point determines the
overall gain of U101 and therefore its output drive level to Q105, which in turn controls the output
Loop
Pin Diode
Antenna
Switch
RF Jack
Antenna
Harmonic
Filter
Coupler PA-Final
Stage
From VCO
Controlled
Stage
Bias
Temperature
Sense
SPI BUS
ASFIC_CMP
PA
PWR
SET
PA
Driver
Controller
U103-2
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