Kenwood TS-950S Service Manual

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TS-950S/SDCIRCUIT DESCRIPTION8) Mixer ~Combines the 36.891 kHz signal with the 491.89kHz signal to produce the 455 kHz output. O5 is aninput buffer; Q6, an output buffer.Since this mixer is used to combine square waves,it generates many harmonics of 491.891 kHz, but itdoes have the desirable characteristic of producingion and noise than IC type mixers.less distor+15V36.691 KHz|26CBE R90OuTBXS.022 12K 455KHzEaMViet1K+ Description of digital signal processingThe DSP-10 converts the analog signal to a digitasignal to impiement modulation and filtering by nu-meric means rather than using conventional RC cir-cuits and analog ICs.Since the modulation and filtering performed by theDSP-10 do not suffer from the inaccuracies of conven-tional analog processing methods is possible to pro-vide ideal properties. tbThe DSP-10 uses a sampling frequency of 49.189kHz for the A/D and D/A converters. When it is prefer-able to use a lower sampling frequency to reduce theprocessing time and improve performance, the sam-pling frequency is reduced by 1/2 or 1/4.1} SSB modulationOverview of processingSeveral different methods are available for digitallyprocessing an SSB signal, they include the direct modu-lation method, the Weaver method, and the Hartleymethod. The DSP-10 uses the direct modulationmethod.There are two generally accepted direct modulationmethods: the filter method which removes the unnec-essary sideband thru the use of an analog filter; or themethod that suppresses the unnecessary sideband bygenerating a copy of the incoming audio signa! that hashad its phase shifted by 90 degrees thru the use of aphase shift network (PSN) and adding it with the origi-ReoFig. 50 Mixer491.891KHz47nal signal and carrier. Since the second method re-sults in an SSB signal that has been obtained thru theuse of phase shifts the use of a filter with steep cutoffcharacteristics, such as the ones used in the filtermethod, is not required. Therefore a higher sidebandsuppression ratio can be obtained from the low-fre-quency range using this broadband phase shifter. Thismethod is far superior to the filter method in obtaininga wide frequency response. In the past this methodhas not been used much because it has been difficultto obtain a PSN (Phase Shift Network) with good char-acteristics due to variations in parts tolerances, circuitstability, and errors in circuit adjustment.The DSP-10 uses the PSN method to generate SSBwith good characteristics thru the use of an accurate,stable phase shifter obtained by digital signa! process-ing.The modulated signal from the A/D converter isapplied to a LPF to limit the bandwidth. The signal isthen split applied to the Phase shift network where thephase of the two signals is shifted by 90 degrees. Theresulting signals, with a phase difference of 90 de-grees, are mixed with carrier signals that are also 90degrees out of phase with each other. The two result-ing signals are then subtractively mixed to produce theSSB signal. The carrier of the SSB modulated wave issuppressed thru the use of a digitally controlled combtype filter and then exits the D/A converter.49.189 KHzbs ag. iadkHz aw 24.594KHz a -|LPFPhase + Comb type+ of =Band ripple Sin WerPhaseLPFdata ROMshiftercharacteristics dataj90*LPF cut-off CosWetData ROMPSNdata ROMPSNdata ROMHPF cut-offPhaseshifterFig. 51 SSB modulation block diagram59