Reciever (For Models With Pcb 8415533H02) - Motorola GP344 Detailed Service Manual

1-6
4.0 Receiver (for models with PCB 8415533H02)
Antenna
Pin Diode
Antenna
Switch
RF Jack
4.1 Receiver Front-End
The RF signal is received by the antenna and applied to a low-pass filter. For VHF, the filter consists
of L3531, L3532, C3532 to C3536. The filtered RF signal is passed through the antenna switch. The
antenna switch circuit consists of two PIN diodes(D3521 and D3551) and a pi network (C3531, L3551
and C3550).The signal is then applied to a varactor tuned bandpass filter. The VHF bandpass filter
comprises of L3301, L3303, C3301 to C3304 and D3301. The bandpass filter is tuned by applying a
control voltage to the varactor diode (D3301) in the filter.
The bandpass filter is electronically tuned by the DACRx from IC404 which is controlled by the
microprocessor. Depending on the carrier frequency, the DACRx will supply the tuned voltage to the
varactor diodes in the filter. Wideband operation of the filter is achieved by shifting the bandpass filter
across the band.
The output of the bandpass filter is coupled to the RF amplifier transistor Q3302 via C3306. After
being amplified by the RF amplifier, the RF signal is further filtered by a second varactor tuned
bandpass filter, consisting of L3305, L3306, C3311 to C3314 and D3302.
Both the pre and post-RF amplifier varactor tuned filters have similar responses. The 3 dB bandwidth
of the filter is about 12 MHz. This enables the filters to be electronically controlled by using a single
control voltage which is DACRx .
Varactor
RF Amp
Tuned Filter
Control Voltage
from ASFIC
Demodulator
Recovered Audio
RSSI
BW SEL
Figure 4-1 VHF Receiver Block Diagram
Varactor
Tuned Filter
First LO
from VCO
Limiter
455kHz
Filter
455kHz
Filter
THEORY OF OPERATION
Crystal
Mixer
Filter
U3220
455kHz
Filter
Second LO
IF Amp