m88-service-manual-

M88CIRCUIT DESCRIPTIONFrequency configurationThe receiver utilizes double conversion.The first IF is 10.695MHz and the second IF is 455kHz.Thefirst local oscillator signal is supplied from the RF module.The RF module in the transmitter generates the necessary frequencies.Fig.1 shows the frequencies.D26.565-27.405MH21'ST MIXMCF45州RFAFANT SWAMPIF SYSTEMAMP10.695MH210.240MH2POWERSUPPILESAM MODAM MODAMMODRXRFRFMICRF MODULEAMPAMPAMPFMTXFig.1 Frequency configurationReceiverThe frequency configuration of the receiver is shown in Fig.2.Front-end RF amplifierAn incoming signal from the antenna is applied to an RF amplifier(Q103)after passing through atransmit/receive switch circuit (D100,D1009,D110 and D108 are off).After the signal is filtered through aband pass filter(L103,L104 and L105)to eliminate unwanted signals before it is passed to the first mixer.■First MixerThe signal from the RF amplifier is heterodyned with the first local oscillator signal from the RF moduleat the first mixer(Q104)to create a 10.695MHz first intermediate frequency (1st IF)signal.The first IFsignal is filtered through a band pass filter(L106,L107 and L108)and then fed through the monolithiccrystal filter(MCFs:CB100)to further remove spurious signals.The band-pass filters are tuned to a desired frequency by varicaps (D112,D113,D114).A tuningvoltage corresponding to the desired signal is applied to each varicap through the DC amplifier of theU106A to tune to the receive frequency.■IF amplifierThe first IF signal go into second mixer Q113,second mixer mix first IF and 10.24MHZ second IF outputY100.The signal is heterodyned again with a second local oscillator signal.The second IF signal is thenfed through a 455kHz ceramic filter(CF100)to further eliminate unwanted signals.The signal is amplifiedby Q114 and Q105,and then the second IF signal enter U101 (FM processing IC)in FM mode or changed