Kenwood NX-3720HG Service Manual page 18

2.4.5.2 VCO
• NX-3720H (K), NX-3720 (E)
There is one VCO contain TX VCO and RX VCO.
The VCO (Q105) generates the carrier for the transmitter and the receiver. VCO oscillation frequency range is 272 to 348MHz. The
transmit frequency range is same as receiver frequency range.
The local signal frequency range is also 272 to 348MHz.
The oscillation frequency is divided into two by IC 302 and it becomes the use frequency.
The VCO oscillation frequency is determined by voltage control terminals "CP" and "ASSIST".
The voltage control terminal "CP" is controlled by PLL (IC302). The voltage control terminal "ASSIST" is controlled by the control
voltage from D/A converter (IC411).
For the modulation input terminal, "VCO_MOD" of PLL (IC302), the output frequency changes according to the applied voltage. This
is used to modulate the VCO output.
• NX-3820H (K)
There is one VCO contain TX VCO and RX VCO.
The VCO (Q105) generates the carrier for the transmitter and the receiver. VCO oscillation frequency range is 300 to 346.7MHz.
The transmit frequency range is same as receiver frequency range.
The local signal frequency range is 900 to 1040MHz. The local signal is three times the number of VCO oscillation frequency range.
The output frequency is divided into two by IC 302 and it becomes the use frequency.
The VCO oscillation frequency is determined by voltage control terminals "CP" and "ASSIST".
The voltage control terminal "CP" is controlled by PLL (IC302). The voltage control terminal "ASSIST" is controlled by the control
voltage from D/A converter (IC411).
For the modulation input terminal, "VCO_MOD" of PLL (IC302), the output frequency changes according to the applied voltage. This
is used to modulate the VCO output.
• NX-3820H (K2), NX-3820 (E)
There is one VCO contain TX VCO and RX VCO.
The VCO (Q105) generates the carrier for the transmitter and the receiver. VCO oscillation frequency range is 266.7 to 313.3MHz.
The transmit frequency range is same as receiver frequency range.
The local signal frequency range is 800 to 940MHz. The local signal is three times the number of VCO oscillation frequency range.
The output frequency is divided into two by IC 302 and it becomes the use frequency.
The VCO oscillation frequency is determined by voltage control terminals "CP" and "ASSIST".
The voltage control terminal "CP" is controlled by PLL (IC302). The voltage control terminal "ASSIST" is controlled by the control
voltage from D/A converter (IC411).
For the modulation input terminal, "VCO_MOD" of PLL (IC302), the output frequency changes according to the applied voltage. This
is used to modulate the VCO output.
2.4.5.3 PLL IC
PLL (IC302) compares the difference in phases of the VCO oscillation signal and the TCXO(X401) reference frequency. And it returns
the difference voltage to the VCO CP terminal and realizes the "Phase Locked Loop".
This allows the VCO oscillation frequency to accurately match (lock) the desired frequency.
When the frequency is controlled by the PLL, the frequency convergence time increases as the frequency difference increases when
the set frequency is changed. To supplement this, The ASSIST Voltage from D/A converter (IC411) is used before control by the PLL
(IC302) to bring the VCO oscillation frequency close to the desired frequency. As a result, the VCO CP voltage does not change and
is always stable at approx. 2.5V.
The desired frequency is set for the PLL (IC302) by the MPU/DSP (IC404) through the 3-line "/DCR_CS", "DCR_DO", "BCLK" serial
bus for PLL. IC404 monitors through the "PLD", whether the PLL (IC302) is locked or not. If the VCO does not lock to desired frequency
(unlock), the "PLD" logic is low.
2.4.6 Control Circuit
The control circuit consists of MPU/DSP (IC404) and its peripheral circuits. IC404 mainly performs the following;
(1) Switching between transmission and reception by PTT signal input.
(2) Reading system, zone, frequency, and program data from the memory circuit.
(3) Sending frequency program data to the PLL.
(4) Controlling the audio mute circuit by decode data input.
2.4.6.1 MPU
The MPU/DSP (IC404) is 32bit RISC processor and Fixed/Floating-Point VLIW DSP, equipped with peripheral function.
This MPU operates at 288MHz (MAX) clock and 3.2V /1.8V/1.2V DC. Controls the flash memory, Mobile DDR, the receive circuit, the
transmitter circuit, the control circuit, and the display circuit and transfers data to or from an external device.
2.4.6.2 Memory Circuit
Memory circuit consists of the MPU/DSP (IC404) and the Mobile DDR (IC402), the flash memory (IC401). The flash memory has
capacity of 512Mbit that contains the transceiver control program for the MPU and stores the data. It also stores the data for transceiver
channels and operating parameter that are written by the FPU. This program can be easily written from external devices. The Mobile
DDR has capacity of 512Mbit. The MPU copies the program to the Mobile DDR from Flash memory. And MPU is used as a work area
Mobile DDR.
1-18 (No.RA054<Rev.001>)