Receiver Front-End
Theory of Operation 4-17
UHF SPECIFIC CIRCUIT DESCRIPTION
8.0 Receiver Front-End
The receiver is able to cover the UHF range from 403 to 470 MHz. It consists of four major blocks:
front-end, mixer, Þrst IF section and IF IC. Antenna signal pre-selection is performed by two varactor
tuned bandpass Þlters. A double balanced shottky diode mixer converts the signal to the Þrst IF at
45.1 MHz.
Two crystal Þlters in the Þrst IF section and two ceramic Þlters in the second IF section provide the
required selectivity. The second IF at 455 kHz is mixed, ampliÞed and demodulated in the IF IC. The
processing of the demodulated audio signal is performed by an audio processing IC located in the
controller section.
8.1 Front-End Band-Pass Filter & Pre-AmpliÞer
A two pole pre-selector Þlter tuned by the varactor diodes D5301 and D5302 pre-selects the
incoming signal (PA RX) from the antenna switch to reduce spurious effects to following stages. The
tuning voltage (FE CNTL VLTG) ranging from 2 volts to 8 volts is controlled by a Digital to Analog
(D/A) converter (U0731-11) in the controller section. A dual hot carrier diode (D5303) limits any
inband signal to 0 dBm to prevent damage to the pre-ampliÞer.
The RF pre-ampliÞer is an SMD device (Q5301) with collector base feedback to stabilize gain,
impedance, and intermodulation. The collector current of approximately 11-16 mA is drawn from the
voltage 9V3.
A second two pole varactor tuned bandpass Þlter provides additional Þltering to the ampliÞed signal.
The varactor diodes D5304 and D5305 are controlled by the same signal which controls the pre-
selector Þlter. A following 3 dB pad (R5310, R5314, R5316) stabilizes the output impedance and
intermodulation performance.
If the UHF radio is conÞgured for a base station application, R5319 is not placed and TP5301 and
TP5302 are shorted.
8.2 Mixer and Intermediate Frequency (IF) Section
The signal coming from the front-end is converted to the Þrst IF (45.1 MHz) using a double balanced
schottky diode mixer (D5401). Its ports are matched for incoming UHF signal conversion to the
45.1MHz IF using low side injection. The injection signal (VCO MIXER) coming from the mixer buffer
(Q5771) is Þltered by the lowpass consisting of (L5403, L5404, C5401 - C5403) and has a level of
approximately 10 dBm.
The mixer IF output signal (RX IF) from transformer T5401 pin 2 is fed to the Þrst two pole crystal
Þlter Y5201. The Þlter output in turn is matched to the following IF ampliÞer.
The IF ampliÞer 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 Þlter Y5202. A dual hot carrier diode (D5201) limits the Þlter output
voltage swing to reduce overdrive effects at RF input levels above -27 dBm.
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