Using VCO to achieve direct frequency modulation of varactor diode 1

Abstract: This article introduces a phase-locked loop circuit that uses VCO to achieve frequency modulation, and provides key technologies such as direct frequency modulation and phase-locked using varactor diodes, design of loop filters, and experimental results. This circuit not only has low phase noise, high stable carrier, and minimal nonlinear distortion, but also has ideal audio modulation frequency response. Keywords: FM; Variable capacitance diode; Automatic phase control  diode

---FM radio has the advantages of strong anti-interference performance and clear sound, and has achieved rapid development. The frequency band of FM radio stations is usually around 200-250kHz, with a bandwidth tens of times wider than that of AM radio stations, making it easy to transmit high fidelity stereo signals. Due to the limitation of the bandwidth of amplitude modulated waves, there is a contradiction between passband width and interference in the receiver, diode so the frequency of audio signals is limited to the range of 30-8000Hz. During frequency modulation, the frequency range of the audio signal can be expanded to 30-15000 Hz, making the frequency spectrum of the audio signal more abundant and greatly improving the sound quality. ---Many small and medium power frequency modulation transmitters use varactor diode direct frequency modulation technology, which directly modulates the frequency on the LC oscillation loop operating at the transmission carrier frequency, diode and uses crystal oscillators and phase-locked loops to stabilize the center frequency. Compared to intermediate frequency modulation and frequency doubling methods, this method has a simpler circuit, better performance, fewer side waves, and is easier to maintain, making it a more advanced frequency modulation scheme.diode