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2-14

Figure 2-11: VHF Receiver Block Diagram

2.10.1 Receiver Front-End

The RF signal is received by the antenna and applied to a low-pass filter consisting of L3531, L3532,

C3532 to C3563. 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 RF signal is then applied to a varactor tuned bandpass filter which consists of L3301, L3303,

C3301 to C3304, and D3301. The 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 IC 404 which is controlled by the

microprocessor. Depending on the carrier frequency, the DACRx supplies the tuning 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 3dB bandwidth

of the filter is about 12MHz. This enables the filters to be electronically controlled by using a single

control voltage which is DACRx.

The output of the post-RF amplifier filter is connected to the passive double balanced mixer which

consists of T3301, T3302, and CR3301. Matching of the filter to the mixer is provided by C3317,

C3318, and L3308. After mixing with the first LO signal from the voltage controlled oscillator (VCO)

using high side injection, the RF signal is down-converted to the 45.1MHz IF signal.

Demodulator

Synthesizer

Crystal

Filter

Mixer

Amp

Varactor

Tuned Filter

Varactor

Tuned Filter

Antenna

AGC

Control Voltage

from ASFIC

First LO

from FGU

Recovered Audio

Squelch

RSSI

SPI Bus

16.8 MHz

Reference Clock

Second

LO VCO

RFJack

Pin Diode

Antenna

Switch