Receiver

The receiver is a dual-conversion superheterodyne using two intermediate frequencies, 45 MHz and 450 kHz.

The RF signal from the duplexer RX port is applied to the RF amplifier.

Next the signal is filtered with SAW-filter Z321. The filter is followed by a single balanced diode mixer, which has 6 dB conversion loss.

After that mixer signal is filtered with the crystal filter Z350, which has 15 kHz bandwidth. Next the IF signal is amplified by V380. From the amplifier the IF-signal is applied to the second mixer.

The second mixer, the LO buffer transistor, IF amplifier and quadrature detector are all integrated in the circuit N370. The second LO frequency, 44.55 MHz, is third harmonic of the VCTCXO frequency. LO signal is realized with tank circuit C372 and L371. After the mixer the 450kHz IF signal is filtered with ceramic filter Z370. The IF amplifier output signal is phase shifted by resonance circuit. After this the signal is fed to a quadrature detector.

Signal DAF is low pass filtered by R372 and C379. The DAF, RSSI and 2nd IF signal (450 kHz) are fed to the audio/logic unit.

During battery-save mode the voltage VRX is down and all the receiver blocks and the RX synthesizer are powered down.

The first injection frequency for receiver is generated by a digital phase locked loop (PLL). The output frequency of the loop (LO) is obtained from a voltage-controlled oscillator (VCO) G530. The VCO output signal is amplified by RX-LO-buffer and fed to the receiver mixer. The injection level required by the receiver mixer is about +3 dBm. In addition, the signal is fed back to the dualsynthesizer circuit N820.

The overall divisor of the chain is selected according to the desired channel.

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