Yaesu FT-857 Technical Supplement

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Contents

Yaesu FT-857 Technical Supplement Manual pdf 9 page image

9Circuit DescriptionReceive Signal CircuitryRF StagesSignals between 0.1 and 56 MHz received at the antennaterminal pass through a low-pass filter, selected accord-ing to the receiving frequency, then pass to another low-pass filter, to remove unwanted out-of-band signals. Thefiltered receiving signal passes, through one of the fol-lowing circuits, to the 1st Mixer Q1128 (SPM5001): (1)an attenuator network (–10dB) which consists of resistorsR1101, R1108, R1109; (2) a “through” circuit enabled viadiodes D1011 and D1012 (both DAP236U); or (3) RFamplifier Q1025 (2SC5374).Received 430 MHz signals, after passing through a high-pass filter composed of L3058, L3069, C3250, C3251,C3253, C3258, and C3298, are passed through low-passfilter composed of L3043, L3044, C3209, C3211, C3214,C3215, C3246, and C3255, and through a directional cou-pler, to the UHF T/R switch circuit composed of diodeswitch D3019/D3022 (both UM9957F), D3023, andD3039 (both HSU277). Then the signals are fed to the1st Mixer Q1128 via the RF-AMP Q1026 (2SK2685).Received 145 MHz signals, after passing through a high-pass filter composed of L3055, L3056, L3067, C3248,C3249, C3252, and C3254, are passed through a low-passfilter composed of L3040, L3041, C3204, C3205, C3210,C3213, and C3216, and a directional coupler, to the VHFT/R switch circuit, composed of diode switch D3018/D3021 (both UM9957F). Then the signals are fed to 1stMixer Q1128 via the RF-AMP, Q1024 (BB304CDW).Received 76-108 MHz signals, after passing through ahigh-pass filter composed of L3055, L3056, L3067,C3248, C3249, C3252, and C3254, are passed throughlow-pass filter composed of L3040, L3041, C3204, C3205,C3210, C3213, and C3216, and a directional coupler, tothe T/R switch circuit, composed of diode switch D3018/D3021 (both UM9957F). Then it is fed to the Wide-FMIF IC Q1058 (CXA1611N) on the MAIN Unit.1st Mixer Circuit/1st IF CircuitThe 1st mixer on the MAIN Unit consists of quad MOSFET Q1128 (SPN5001), where the receiving signal ismixed with the 1st local signal (68.430-538.330 MHz) fromthe PLL Unit. The resulting output signal (68.33 MHz)passes through monolithic crystal filter (MCF) XF1001(MF68Q, BW: ±6.0 kHz) to obtain the 1st IF signalwith acenter frequency of 68.33 MHz. The IF signal passesthrough the 1st IF amplifier Q1073 (BB305CEW) to the2nd Mixer, Q1083 and Q1084 (both 2SK302Y).2nd Mixer Circuit/2nd IF CircuitThe 2nd Mixer consists of FETs Q1083 and Q1084 (both2SK302Y) on the MAIN Unit, where the 1st IF signal ismixed with the 2nd local signal (67.875 MHz). The re-sulting output signal (455 kHz) is applied to the 2nd IFfilter which is matched to the receiving mode: eitherCF1004, CF1005 or an optional mechanical filter.Noise Blanker CircuitA portion of the 2nd IF signal is amplified by Noise BlankerAmplifiers Q1075 and Q1079 (both BB305CEW) on theMAIN Unit, and then rectified by D1064 (1SS372). Thisoutput is applied to the Noise Blanker Controllers, Q1093(2SC4154E) and Q1099 (2SA1602A), which a yieldBlanking signal according to the timing of the incomingnoise pulses. Then Blanking signal controls the NoiseBlanker Gate D1066 (BAS316), to slice out the impulsenoise from the signal.AGC CircuitThe AGC circuit consists of D1061 (1SS372), transistorQ1090 (2SC4154E), and associated parts on the MAINUnit. Output from the AGC circuit is fed back to the IFAGC circuit that controls the gain of the IF amplifier FETs.FM IF Circuit/FM Demodulator CircuitOn FM, the 2nd IF signal passes through the buffer ampli-fier Q1094 (2SC4154E) and 2nd IF filters (CF1002 andCF1003) to the FM IF IC Q1080 (TA31135FN) whichcontains a mixer, limiter amplifier, filter amplifier, squelchtrigger, and demodulator. The demodulated audio signalat Q1080 passes through a low-pass filter (R1339 andC1282) and a de-emphasis circuit (R1303 and C1345),then proceeds to the Audio Amplifier Circuit.The squelch circuit selectively amplifies the noise com-ponent of the demodulator output using the filter ampli-fier inside the FM IF IC and an active band-pass filterconsisting of an externally attached resistor and capacitor.Signal detection is performed by D1057 (DA221).SSB/CW Demodulator CircuitThe 2nd IF SSB/CW signal passes through buffer ampli-fiers Q1088 and Q1081 (both BB305CEW) to the SSBbalanced demodulator Q1071 (SA602AD) which pro-duces audio by applying the carrier signal from the CAR-DDS IC Q1062 (AD9835BRU). The demodulated audiosignal is stripped of high-frequency components by anactive low-pass filter, op-amp IC Q1120 (NJM2902V),then is applied to the Audio Amplifier Circuit.AM Demodulator CircuitThe 2nd IF AM signal passes through buffer amplifiersQ1088 and Q1081 (both BB305CEW) to the AM demodu-lator D1055 (BAS316), yielding demodulated audio sig-nal which is applied to the Audio Amplifier Circuit.Audio Amplifier CircuitThe demodulated audio signal is passed through AF pre-amplifier Q1119 (NJM2902V) and electronic volume con-trol IC Q1087 (M62364EP) to the AF Amplifier IC Q1105(TDA2003H) which drives the internal or external speakerto a maximum output of approximately 2.5 Watts.