Using VCO to achieve direct frequency modulation of varactor diode 4

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. VCO circuit - The VCO circuit is a critical circuit component for implementing frequency modulation and carrier generation, as shown in Figure 5. ---Four identical performance super abrupt change junction variable capacitance diodes (MV209) adopt a complex series parallel form,diode  exchanging the complexity of the circuit for performance improvement, and using partial access method to connect the resonant circuit, that is, the variable capacitance diode (C) is connected in series with the coupling capacitor (c1) with smaller capacity, and then connected in parallel with a capacitor (c2) to form the total capacitance of the circuit. This approach will not change the frequency modulation characteristics of the varactor diode. Although it will reduce the maximum frequency offset of the frequency modulation circuit to a certain extent, it can improve the problem of center frequency drift caused by temperature changes in the junction capacitance of the varactor diode. At the same time, by adjusting the size of the coupling capacitor c1, it can ensure that the varactor diode operates in the linear region and control the frequency offset size. On the premise of ensuring maximum frequency offset, try to eliminate nonlinear distortion and reduce phase noise of the output signal as much as possible. ---Select bipolar transistor 2sc3356 with low corner frequency and low noise index in the starting circuit to improve the spectral quality of the near end of the VCO circuit spectrum. Add a first stage emitter follower after the starting circuit to reduce the impact of the load circuit on the starting circuit and achieve good performance. After passing through the transmitter follower, the modulated signal is divided into two paths. One path is fed back to the fin port of MC145170 to form a phase-locked loop, and the other path is sent to the amplification circuit at the back end to meet the output power requirements of the system.diode