Receiver Front-End
4-24 Theory of Operation
VHF SPECIFIC CIRCUIT DESCRIPTION
11.0 Receiver Front-End
The receiver is able to cover the VHF range from 136 to 174 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 schottky 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.
11.1 Front-End Band-Pass Filter and Pre-AmpliÞer
A two pole pre-selector Þlter tuned by the dual varactor diode D3301 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 (D3303) limits any inband signal to
0dBm to prevent damage to the pre-ampliÞer.
The RF pre-ampliÞer is an SMD device (Q3301) with collector base feedback to stabilize gain,
impedance, and intermodulation. The collector current of approximately 11-16 mA, drawn from the
voltage 9V3, is controlled by a current source composed of Q3302, R3302, R3300, and R3311 -
R3313. In transmit mode the high K9V1 signal fed through diode D3300 switches off the current
source and in turn the pre-ampliÞer. In receive mode K9V1 must be low to switch on the current
source. A 3 dB pad (R3306 - R3308 and R3316 - R3318) stabilizes the output impedance and
intermodulation performance.
A second two pole varactor tuned bandpass Þlter provides additional Þltering to the ampliÞed signal.
The dual varactor diode D3304 is controlled by the same signal which controls the pre-selector Þlter.
If the VHF radio is conÞgured for a base station application, R3318 is not placed and TP3301 and
TP3302 are shorted.
11.2 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 (D3331). Its ports are matched for incoming VHF signal conversion to the
45.1MHz IF using high side injection. The injection signal (VCO MIXER) coming from the mixer
buffer (Q3770) is Þltered by the lowpass consisting of (L3333, L3334, C3331 - C3333) and has a
level of approximately 10 dBm.
The mixer IF output signal (RX IF) from transformer T3301 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. The 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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