Kenwood NXR-710 Service Manual

Also see for NEXEDGE NXR-710: Instruction manual Instruction manual Instruction manual Instruction manual Instruction manual

NXR-710102-3. Transmitter Main PLL circuit (SUB unit)■ VCOThe TX VCO circuit consists of two VCOs (VCO A X58-508: Q352, VCO B X58-508: Q353). Those VCOs generatesa transmit carrier. VCO A Q352 produces a transmitter fre-quency from 136.000MHz to 154.995MHz. VCO B Q353produces a transmitter frequency from 155.000MHz to174.000MHz.Those VCO oscillation frequencies are determined bytwo systems of voltage control terminals: “CV” and “TXAS-SIST”.The voltage control terminals, “CV” and “TXASSIST”,are controlled by the PLL IC (X58-508: IC300) and MCU(X53-449: IC20) and the output frequency changes continu-ously according to the applied voltage. For the modulationinput terminal, “MO”, the output frequency changes ac-cording to the applied voltage.D355D352D358D359SW Q350TXVCOATX_CVTXASSISTTXVCOBMO80C80CVOVCO_AQ352BufferQ354SW Q351D357D353D360D361VCO_BQ353Fig. 3 Transmitter VCO circuit■ PLL IC (X58-508: IC300)The PLL IC compares the differences in phases of theVCO oscillation frequency and the transmitter PLL referencesignal (16.8MHz), returns the difference to the VCO CV ter-minal and realizes the “Phase Locked Loop” for the returncontrol. This allows the VCO oscillation frequency to accu-rately match (lock) the desired frequency.When the frequency is controlled by the PLL, the fre-quency convergence time increases as the frequency dif-ference increases when the set frequency is changed. Tosupplement this, the MCU is used before control by thePLL IC to bring the VCO oscillation frequency close to thedesired frequency. As a result, the VCO CV voltage does notchange and is always stable at approx. 3.0V.The desired frequency is set for the PLL IC by the MCU(X53-449: IC20) through the 3-line “SPSD0”, “SPSCLK0”,“/TXPCS” serial bus. Whether the PLL IC is locked or not ismonitored by the MCU through the “LDT” signal line. If theVCO is not the desired frequency (unlock), the “LDT” logicis low.Ref 16.8MHzLDTSPSCLK0SPSD0/TXPCSBufferQ300TX VCO/PLLIC300LPFTX_CVBPFFINLoop filterFig. 4 Transmitter Main PLL IC circuit2-4. Modulation level adjustment circuitThe Modulation level adjustment circuit adjusts the mod-ulation waveform balance. This circuit consists of IC804,IC603, IC604, IC605 and IC606.The modulating signal comes from the Control unit(X53-449) through the interface connector (CN800 Pin 14).The modulating signal is produced by the modulation low-pitched tone to the transmitter modulation 16.8MHz PLL cir-cuit and adds the high-pitched modulation to the transmittermain PLL.IC804 is an electronic volume control IC. It has 8 elec-tronic volume control circuits. The modulation level adjust-ment circuit uses 2 electronic volume control circuits inIC804. The 1st electronic volume control circuit adjusts themodulating signal and is fed to IC603 and the 2nd electronicFig. 2 Transmitter reference 16.8MHz PLL circuitTX VCO/PLLMODPLL ICX58-5080-10Ref10MHzor 19.2MHzLDTIC602SPSCLK0SPSD0/16_8PCS AMPQ602BufferQ603AMPIC604AMPIC60316.8MHzLPFLoop filterX600MBOSC16.8MHzCIRCUIT DESCRIPTION