Using VCO to achieve direct frequency modulation of varactor diode 3

Phase locked frequency stabilization technology - For direct frequency modulation circuits with variable capacitance diodes, there are many factors that can cause changes in the oscillation frequency due to the modulator being composed of a common LC self-excited oscillator and parallel variable capacitance diodes. These factors include the nonlinearity of the variable capacitance diode, changes in power supply voltage, changes in load, changes in environmental conditions such as temperature, aging of circuit components, mechanical vibration, etc. In order to eliminate these factors that cause instability of the center frequency, in addition to paying attention to the design of the circuit and structure, automatic phase control circuits should also be used to stabilize the center frequency within the specified range. ---Figure 3 is a structural diagram of a typical phase-locked frequency stabilization circuit. It consists of four parts: voltage controlled oscillator, phase detector, reference crystal oscillator, and frequency divider. The amplified modulation signal is added to a voltage controlled oscillator for frequency modulation.diode  The modulated high-frequency signal is sent to the subsequent amplification circuit and the other part is sent to a frequency divider for division. The square wave signal output by the frequency divider is sent to the phase detector and compared with the reference signal obtained by dividing the frequency of the reference crystal oscillator to achieve phase locking. diode The output signal of the phase detector is sent to the voltage controlled oscillator through a loop filter to control the oscillation frequency of the voltage controlled oscillator, thereby achieving the goal of stable frequency. ---Due to the frequency modulation, the instantaneous frequency of the output signal of the voltage controlled oscillator always deviates from its reference value, and the function of the loop is to suppress this frequency deviation, which creates a contradiction. To solve this contradiction, the frequency spectrum of the modulated signal should be placed outside the loop passband, which means a low-pass filter needs to be added between the phase detector and the voltage controlled oscillator to filter it out. The loop only works on the part that causes instability of the average center frequency of the voltage controlled oscillator, that is, the modulated signal varies within a small frequency offset range near the center frequency. diode Main circuit working principle - This circuit is actually a low-power frequency modulation transmitter, and its modulation part adopts variable capacitance diode direct frequency modulation technology. Its main function is to achieve frequency modulation excitation sources with intervals of 100kHz in the 87-108mhz frequency band; The input frequency modulation signal is audio (30hz~15khz), and it is required to achieve a maximum frequency offset of 75kHz. The block diagram is shown in Figure 4. ---Below is a brief analysis of the working principles of each main component.diode