Samsung SCH-1500 series Service Manual

Also see for SCH-1500 series: User guide

6-6 Samsung Electronics6-3 Receiver (For AMPS)LOW NOISE AMPLIFIER (LNA)The low noise amplifier featuring 1.6dB NoiseFigure and 16dB gain amplifier a weak signalreceiver from the base station to obtain theoptimum signal level.DOWN CONVERTER (MIXER)First local signal is applied to this down converter.The down converter transfers the signal amplifiedat the LNA into 85.38 MHz IF signal. 85.38MHz IFsignal is made by subtracting 881 ±12.5 MHz RFsignal from 966 ± 12.5 MHz first local signal. TheLAN is U404 and down converter is U412.LOCAL BUFFER AMPLIFIERBuffer (U411) amplifies signal to be applied to thelocal input of the down converter (U412) when aphase is locked between U660 and U640.IF AUTOMATIC AGIN CONTROLLER (AGC)AMP85.38MHz IF signal is applied to IF AGC amplifier.AGC output level is applied to BBA (Base BandAnalog). The AGC amp (U403) keep the signal at aconstant level by controlling the gain. Dynamicrange is 90dB, up gain +45dB, and down gain-45dB.RF BAND PASS FILTER (BPF)The RF BPF (F403) passes only a specific frequency(881.49±12.5MHz) from the signal received from themobile station. The band width is 25MHz.IF SAW BAND PASS FILTERIF SAW BPF (F404) is used for AMPS system having30kHz channel spacing and ±15kHz bandwidth.The filter also climinates the spurious signals at themixer.VOLTAGE CONTROLLED OSCILLATORThe VCO (U660) generates the signal having966MHz center frequency and ±12.5MHz deviationwith voltage control. PLL IC (U640) controls thissignal.PHASE LOCKED LOOP (PLL)Input reference frequency is generated at VCTCXO(U680) and the divider signal is generated at VCO.PLL compares the two signals and generates thedesired signal with a preprogrammed counterwhich controls voltage.VOLTAGE CONTROLLED TEMPERATURECOMPENSATED CRYSTAL ASCILLATORIt provides 19.68MHz reference frequency to PLL(U640) and BBA (U600). A correct frequency turingis made by the voltage control.DUPLEXERDuplexer (F405) controls to transmit through theantenna only the signals within acceptable Txfrequency range (836 ±12.5MHz). It also matchesLNA (U404) input in receiving part and PA out intransmitter part with the antenna.Circuit Description