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Description Of Frequency Determining And Stabilizing Circuit Introduction The frequencies for transmitter and receiver local frequencies are all derived a signal 4.5 [MHz] crystal by means of a phase locked loop. The first local oscillator frequencies are 16.27 [MHz] (CH.1) to 16.71 [MHz] (CH.40). The second local frequency is fixed at 10.240 [MHz] to generate second IF 455 [KHz]. During transmit, the VCO of the PLL operates 13.4825[MHZ] (CH.1) to 13.7025 [MHz] (CH.40). The VCO frequency goes to the doublers circuit Q301, L301, L302 which doublers the frequency to generate 26.965 [MHz] (CH.1) to 27.405 [MHz] (CH.40). Q503 Q504 Q301 T o Transmitter VCO Doubler 13.4825[MHz] (CH.1) 26.965 [MHz] (CH.1) ~ 13.7025 [MHz] (CH.40) ~ 27.405 [MHz] (CH.40) The VCO operating frequency for the receiver is 16.27 [MHz] (CH.1) to 16.71 [MHz] (CH.40) as the first local oscillator, injected through the buffer amp Q506 into the first FET balanced mixer Q107, Q108 Descriptions Of Each Block 1. Introduction The synthesizer is implemented with the following Components : PLL IC (IC6) X-TAL (X401) VCO, Varicap Diode (D503,504) IC101 is a MCU that includes LCD driver and all of control circuit. The PLL IC (IC6) of PLL block, The Q503, L404, C511, C512, C513, C507,Varicap diode D503 and D504 are clap oscillator circuit to operate as a VCO. The Q505 is a switching transistor to turn on or turn off the tuning capacitor in the VCO oscillator tank circuit for transmitter or receiver. The Q506 works as a buffer amp for RX local frequencies (16 [MHz]) and TX carrier generating frequencies (13 [MHz]). 2. Reference Frequency The crystal, X401(4.5 [MHz]) and other components at pin1 and 16 of IC6 can make a reference frequency oscillator with internal amplifier. 2-1. VCO The Q503 and surrounding parts are consisting a clap oscillator work as a VCO of IC6 with appropriate control voltage on D503 and D504, the can be oscillate over the required range of 13.4825[MHz] to 16.71 [MHz]. 2-2. Programmable Divider And PLL Data Control The MCU(IC101) and PLL IC (IC6) communicate by serial data for PLL IC data setting. The programmable inputs for each channels set by MCU (IC101). Each input signal to control the PLL IC is done with the provided key input pins 20 and 21 of IC101. 2-3. Phase Detector And VCO Control The phase detector is a digital phase comparator which compares the phase of the reference signal with programmable divider output square waves and develops a series of pulses whose DC level depends on the phase error of each signal. The phase detector pulse output is fed to an active low pass filter and RC low pass filter