Samsung SCH-1900 Service Manual

Also see for SCH-1900: User guide Owner's manual

Samsung SCH-1900 Service Manual Manual pdf 14 page image

Circuit Description4-4 Samsung Electronics4-2 Receiver SectionLOW NOISE AMPLIFIER (LNA)The low noise amplifier featuring 1.6dB NoiseFigure and 19dB gain amplifies a weak signalreceived 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 210.38 MHz IF signal. 210.38MHzIF signal is made by subtracting 1960°æ30MHz RFsignal from 1749.62°æ30MHz first local signal. TheLNA is Q303 and down converter is U302.IF AMPThe IF amp (Q310) amplifies the output signal ofthe down converter (U302).RF IF AUTOMATIC GAIN CONTROLLER(AGC) AMP210.38 MHz IF signal is applied to IF AGCamp,the AGC output level is applied to BBA (BaseBand Asic). The AGC amp (U303) keeps the signalat a constant level by controlling the gain.Dynamic range is 90dB, up gain +45dB, and downgain -45dB.RF BAND PASS FILTER (BPF)The RF BPF(F302) accepts only a specificfrequency (1960°æ30MHz) from the signalreceived from the mobile station. The bandwidthis 60MHz.IF SAW BAND PASS FILTER FOR CDMAIF SAW BPF (F303) is used for CDMA systemhaving 1.23 MHz wide band and °æ630kHz bandwidth. The filter also eliminates the image productgenerated at the mixer.BUFFERBuffer (Q311) amplifies the signal from the VCO(U305) to be applied to the local input of the downconverter (U302) when a phase is locked betweenU304 and U305.The VCO (U305) generates the signal having1750MHz center frequency and °æ30MHzdeviation with the voltage control. PLL IC(U304)controls this signal.PHASE LOCKED LOOP (PLL)Input reference frequency is generated atVCTCXO (U306) and the local signal is generatedat VCO. PLL compares the two signals and allowsthe VCO controlled by voltage to generate thedesired signal with pre-programmed counter.VOLTAGE CONTROLLED TEMPERATURECOMPENSATED CRYSTAL OSCILLATORIt provides 19.68MHz reference frequency to PLL(U304) and BBA (U401). A correct frequencytuning is made by the voltage control.DUPLEXERDuplexer (F301) controls to transmit through theantenna only the signals within acceptable Txfrequency range (1880°æ30MHz) and to receivethrough the antenna only the signals withinacceptable Rx frequency range (1960°æ30MHz). Italso matches LNA(Q303) input in receiving partand PA output in transmitter part with theantenna.