4 . 1 F M T U N E R Front End
The FM front end of SX-3800 includes a 3 ganged tuning capacitor, a dual-gate MOS FET- equipped L-stage RF amplifier, and a modified Clapp circuit local oscillator. This oscillator is a voltage controled oscillator employing a vari-cap
(variablecapacitancediode). When the quartz-lock system (refer to "Quartz-locksystem") is not in
operation, a constant voltage is applied to the diode.
lF Amplifier and Detector
These employ 3 ICs and 3 dual-elementceramic filten. The IC (HA1201) of the first 2 stagecon- stitutes a single-stagedifferential amplifier current- limiting limiter. The IC (PA3007-A) in the third stage,an improvement on the former IF systemIC (PA3001-A), includes an IF limiter amplifier, quadrature detector, meter drive, and other cir- cuits. Performance in terms of distortion, S/N ratio, delay characteristics,and other parameters, shows a marked improvement in comparison to the PA3001-4.
Multiplex Decoder
The recently developed multiplex decoder IC (PA4006-A) combinesMPX decodingwith muting functions in a single IC, thereby handling the functions of the more conventional MPX IC (PA1-001-A)and AF MUTING IC (PA1002-A).
Distortion ratings and SIN ratio have been further improved by incorporating a chopper type MPX decoder. The chopper type switching circuit (see Fig. 4-1) operates by switching the signal either to ground or to the through circuit, thereby eliminating the generation of unwanted noise or distortion. Furthermore, since the PA4006-A fea- tures DC direct-coupled switching with the detec-
tor, there is no deterioration in separationat the low frequency end.
Besidesthe decoder and muting circuits, the PA4006-A alsoincorporatesthe pilot signalcancel- ler, stereo auto selector, VCO killer circuit, ivluT amplifier, and MUT control circuit.
Compositesignal
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InOutOutput
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Switchins,is".rc-1q/ t h L
Fig . 4 - 1 Choppertype switchingcircuit
ouartz-Lock system
The quartz-locksystemfeaturedin the SX-3800 stereo receiver is a frequency servocontrol system employinga crystalresonator.Any displacementin the intermediatefrequency (iF) is detected asa DC voltage by the discriminator (equipped with a crystal resonator), resulting in the local oscillator frequency being corrected and subsequently locked. This extremely stable frequency servo- control systemthus ensuresthat tuned frequencies remain tuned securelyfor aslong asrequired.
When the IF signalappearsat pin no.l? of the IF system IC (PA300?-A), it is amplified and applied to crystal detector (see Fig. 4'2)which consistsof diodesconnectedin parallelin a series resonancecircuit equipped with a crystal resona- tor. The resonancefrequency is the sameasthe IF frequency(10.?MHz), which meansthe impedance at this time will be minimal, resulting in the output beingreducedto a minimum level.If the input fre- quency increases,the reactanceof the capacitance
