1-10 (No.RA028)2.4 CIRCUIT DESCRIPTION2.4.1 Frequency ConfigurationThe receiver utilizes double conversion. The first IF is 49.95MHz and the second IF is 450kHz. The first local oscillator signal is sup-plied from the PLL circuit.The PLL circuit in the transmitter generates the necessary frequencies. Figure 1 shows the frequencies.Fig.1 Frequency configuration2.4.2 Receiver SystemThe receiver is double conversion superheterodyne.The frequency configuration is shown in Figure 1.Fig.2 Receiver system2.4.2.1 Front-end RF AmplifierAn incoming signal from the antenna is applied to an RF amplifier (Q506) after passing through a transmit/receive switch circuit (D305,D306, D309 and D310) , BPF (L512, L513, and varactor diodes : D504, D505).After the signal is amplified (Q506), the signal is filtered by a BPF (L507, L508 and varactor diodes : D502, D503) to eliminate unwant-ed signals before it is passed to the first mixer.The voltage of these diodes are controlled by the TVC according to the channel frequency. (See Figure 2)2.4.2.2 First MixerThe signal from the RF amplifier is heterodyned with the first local oscillator signal from the PLL frequency synthesizer circuit at thefirst mixer (Q503) to create a 49.95MHz first intermediate frequency (1st IF) signal. The first IF signal is then fed through one pair ofmonolithic crystal filter (MCF : XF500) to further remove spurious signals.2.4.2.3 IF Amplifier CircuitThe first IF signal is amplified by Q501, and the enters IC501 (FM processing IC). The signal is heterodyned again with a second localoscillator signal within IC501 to create a 450kHz second IF signal. The second IF signal is fed through a 450 kHz filter in IC501 tofurther eliminate unwanted signals before it is amplified and FM detected in IC501.RFAMP1stMIXAFPATCXOMICAMPx3TriplerRFAMPPOWERAMPMCF49.95MHzIF SYSTEMPLL/VCO16.8MHz50.4MHzANTANTSWRXTXSPMICIC503DC OP AMPQ500TriplerX1TCXOIC501FM IC system IC709Baseband IC1st localOSC (VCO/PLL)TVTVCANTSWD305,D306D309,D310L512,L513D504,D505BPFQ506RF AMPL507,L508D502,D503BPFQ503MIXXF500MCFQ501IF AMP
