Receiver Front-End; Figure 2-6:Uhf Receiver Block Diagram; Uhf Receiver - Motorola Professional Radio Service Manual

Antenna
Pin Diode
Antenna
RFJack Switch
2.6.1

Receiver Front-End

The RF signal received by the antenna is applied to a low-pass filter. For UHF, the filter consists of
components L101, L102, C102, C103, and C104. The filtered RF signal is passed through the
antenna switch circuit consisting of two pin diodes (CR101 and CR102) and a pi network (C106,
L104, and C107). The signal is then applied to a varactor tuned filter bandpass.
The UHF bandpass filter consists of components L301, L302, C302, C303, C304, CR301, and
CR302. The filter is electronically tuned by DACRx from the ASFIC (U404) which supplies a control
voltage to the varactor diodes (CR301 and CR302) in the filter as determined by the microprocessor
depending on the carrier frequency. 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 Q301 via C307. After being
amplified by the RF amplifier, the RF signal is further filtered by a second varactor tuned bandpass
filter, consisting of L306, L307, C313, C317, CR304, and CR305.
Both the pre and post-RF amplifier varactor tuned filters have similar responses. The 3 dB bandwidth
of the filter is approximately 50 MHz. This enables the filters to be electronically controlled by using a
single control voltage from DACRx.
The output of the post-RF amplifier filter is connected to the passive double balanced mixer consisting
of components T301, T302, and CR306. Matching of the filter to the mixer is provided by C381. After
mixing with the first local oscillator (LO) signal from the voltage controlled oscillator (VCO) using low
side injection, the RF signal is down-converted to a 45.1 MHz IF signal.
Varactor RF
Tuned Filter Amp
Control Voltage First LO
from ASFIC
from FGU
Recovered Audio
Squelch
RSSI
16.8 MHz
Reference Clock
Figure 2-6: UHF Receiver Block Diagram
Varactor
Mixer
Tuned Filter
AGC
Demodulator
IF IC
SPI Bus
IF
Crystal
Amp
Filter
AGC
Processing
Synthesizer
Second
LO VCO
2-7