Kenwood NX-720 Service Manual

Also see for NEXEDGE NX-720: Installation guide Instruction manual Technical details Service manual Instruction manual

Contents

NX-720(G)/72011CIRCUIT DESCRIPTIONFig.1 Frequency configuration1. OverviewThe NX-720 is a VHF Mobile transceiver designed tooperate in the frequency range of 136 to 174MHz. The unitconsists of a receiver, transmitter, phase-locked loop (PLL)frequency synthesizer, baseband parts, power supply, andcontrol circuits.2. Frequency ConfigurationThe receiver is a double-conversion super-heterodyneusing a first intermediate frequency (IF) of 49.95MHz and asecond IF of 450kHz. Incoming signals from the antenna aremixed with the local signal from the PLL circuit to producethe first IF of 49.95MHz. This is then mixed with the 50.4MHzsecond local oscillator output to produce the 450kHz secondIF. The transmit signal frequency is generated by the PLLVCO, and modulated by the signal from the DSP. It is thenamplifi ed and fed to the antenna.ANTSW RFAMP1stMIXAFPATCXOMICAMPTXAMPPOWERAMPCF 450kHzMCF49.95MHzIF SYSTEMPLL/VCO16.8MHzANTSPMICTX/RX: 136~174MHzX350.4MHzBaseband Circuit185.95~223.95MHz136~174MHz3. Receiver System3-1. RF circuitAn incoming RF signal from the antenna terminal ispassed through the antenna switch (D110, 111 and 107)and then the bandpass filter (L215, 216). The bandpass filteris adjusted by a variable capacitor. The input voltage to thevariable capacitor is regulated by the voltage output from theD/A converter (IC712). The signal is amplified by an RF am-plifier (Q202), and passed through the bandpass filter (L207,209, 210 and 220). The resulting signal is applied to the firstmixer (Q201) where it is mixed with the first local oscillatorsignal output from the frequency synthesizer to produce thefi rst IF (49.95MHz).3-2. IF circuitThe first IF signal is passed through a four-pole mono-lithic crystal filter (XF1) to reject the adjacent channel signal.The fi ltered fi rst IF signal is amplifi ed by the fi rst IF amplifi er(Q305) and then applied to the IF system IC (IC303). TheIF system IC provides a second mixer, AGC amplifier , andRSSI (Received Signal Strength Indicator).The second mixer mixes the first IF signal with the50.4MHz of the second local oscillator output and producesthe second IF signal of 450kHz.The second IF signal is passed through the ceramic filter(CF1) to reject the adjacent channel signal. The filtered sec-ond IF signal is amplifi ed by the AGC amplifi er.The signal from the AGC amplifier is input to an AD con-verter in ASIC (IC507) through the ceramic fi lter (CF2).ANTBPFQ202RF AMPQ305IF AMPQ2011st MIX XF1MCFD107,D110,D111ANTSWTCXOIC303MIX,AGC1st localOSC (VCO/PLL) CF1LPFL116,L117 L215,L216BPFL207,L209L210,L22016.8MHzQ303x32nd OSCX1BPFCF2IC 507ASICIC 711(1/2)IC 502DSPIC 711(2/2)VOLIC 712 IC 714SPFig.2 Receiver System