Receiver Front-End
Theory of Operation 4-17
300MHz SPECIFIC CIRCUIT DESCRIPTION
8.0 Receiver Front-End
The receiver is able to cover the range from 336 to 390 MHz. It consists of four major blocks: front-
end, mixer, first IF section and IF IC. Antenna signal pre-selection is performed by two varactor
tuned bandpass filters. A double balanced schottky diode mixer converts the signal to the first IF at
45.1 MHz.
Two crystal filters in the first IF section and two ceramic filters in the second IF section provide the
required selectivity. The second IF at 455 kHz is mixed, amplified 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-Amplifier
A two pole pre-selector filter tuned by the varactor diodes D8301 and D8302 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 (D8303) limits any inband
signal to 0 dBm to prevent damage to the pre-amplifier.
The RF pre-amplifier is an SMD device (Q8301) with collector base feedback to stabilize gain,
impedance, and intermodulation. The collector current of approximately 11-16 mA is drawn from the
voltage 9V3 via L8302 and R8302.
A second two pole varactor tuned bandpass filter provides additional filtering to the amplified signal.
The varactor diodes D8304 and D8305 are controlled by the same signal which controls the pre-
selector filter. A following 1 dB pad (R8310, R8314, R8316) stabilizes the output impedance and
intermodulation performance. If the radio is configured for a base station application, R8319 is not
placed, and TP8301 and TP8302 are shorted.
8.2 Mixer and Intermediate Frequency (IF) Section
The signal coming from the front-end is converted to the first IF (45.1 MHz) using a double balanced
schottky diode mixer (D8401). Its ports are matched for incoming RF signal conversion to the 45.1
MHz IF using low side injection. The injection signal (VCO MIXER) coming from the mixer buffer
(Q8881) is filtered by the lowpass consisting of (L8403, L8404, C8401 - C8403) and has a level of
approximately 10 dBm.
The mixer IF output signal (RX IF) from transformer T8401 pin 2 is fed to the first two pole crystal
filter Y5201. The filter output in turn is matched to the following IF amplifier.
The IF amplifier 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 filter Y5202. A dual hot carrier diode (D5201) limits the filter output
voltage swing to reduce overdrive effects at RF input levels above -27 dBm.
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