Kenwood TK-790 Service Manual

Also see for FleetSync TK-790: Specifications Instruction manual Specifications Instruction manual

28TK-790/(B)/H(B)5. VCO/PLL CircuitThe VCO of TK-790 consists of two VCO circuits whichone oscillates the transmit signal with Q306 and the otherdoes the first local receive signal with Q305.Each oscillators are switched by turning the source linefor Q307, Q308 and Q309 on and off. The signal output ofthe VCO is amplified by the buffer amplifier Q310 and di-vided into two signals which one is amplified by Q312 andthe other is done by Q311. The signal from Q312 passesthrough D306 which is transmit/receive switch, is amplifiedby Q201 and Q204, and is output from the CN203.The signal from Q311 passes through the low-pass filterand is applied to IC301 which is PLL frequency synthesizerwith VCXO ; X301. The VCXO of which the frequency stabil-ity is within 2.0ppm (temperature range of –30 to +60°C)generates 16.8MHz. The PLL-IC consists of three modulusprescaler, fractional divider, reference divider, digital phasecomparator with charge pump output.This PLL-IC is Fractional-N type synthesizer and performsis 40 or 50 or 60kHz reference signal which is eighth of thechannel step (5 or 6.25 or 7.5kHz). The input signal from thepins 5 and 8 of the PLL-IC is divided down to the 40 or 50 or60kHz and compared at digital phase comparator.The pulsed output signal of the digital phase comparatoris applied to the charge pump and transformed into DC sig-nal. The DC signal from the pin 14 of the PLL-IC passesthrough the active low-pass filter (loop filter), is applied tothe VCO and controls to keep the frequency of the VCO.The serial data (DT, CP, EP) from the microprocessorIC516 is input to the PLL-IC. And PLL lock condition is al-ways monitored by the pin 28 (UL) of IC516.CIRCUIT DESCRIPTIONFig. 83. Squelch CircuitThe output from IC101 enters FM IC again, then passedthrough a band-pass filter. The noise component outputfrom IC101 is amplified by Q109 and rectified by D103 toproduce a DC voltage corresponding to the nose level.The DC voltage is sent to the analog port of the CPU(IC516/pin 27). And IC101 output a DC voltage (RSSI) corre-sponding to the input of the IF amplifier. The CPU reads theRSSI signal via pin 26. IC101 determines whether to outputsounds from the speaker by comparing the input voltage ofpin 27 with the present value.Only during scan, the RSSI DC voltage is used togetherwith the noise wave detection pin's DC voltage (pin 27).SQ voltageANT input levelSQ closeSQ openPresetvalueRSSI voltageANT input levelPreset valueFig. 7 Squelch and RSSI voltage vs ANT input level4. RX BPF (L110~112)The signals are then fed into band-pass filter that usesvaractor diode tuning to reject unwanted signal compo-nents, and is fed to the 1st mixer.8CLTX VCOPLL-ICIC516μ-comRX VCOBUFFAMP AMPAMPSWSWSWLPFMO8CLSTRQ306Q307SW Q308Q309Q305IC301DTCPULQ3115CX301MB MO8RQ310 Q312 D306TXRXEPTX-RX UNITCONTROL UNIT8C 8T8CLVCXO 5C5C9CLQ303,304Fig. 9 VCO/PLL circuit