Two FM modulation circuit diagrams to share
FM, namely: frequency modulation, frequency modulation, refers to the modulation method in which the frequency of the high-frequency carrier changes with the modulating signal. The amplitude of FM remains unchanged. As the receiving distance increases, the amplitude decreases due to propagation attenuation, and the degree of FM signal frequency change Determined by the modulation signal. As shown below.
The generation of FM frequency modulation signal mostly uses the variable capacitance characteristic of the junction capacitance of a varactor diode or a triode to change the capacitance of the resonant circuit and achieve the purpose of frequency modulation.
The frequency of the FM signal is related to the amplitude of the modulation signal.
For example, a sine wave is used as the modulation signal. When the amplitude of the sine wave is zero, the actual frequency of the FM signal is the frequency of the carrier frequency, that is, the center frequency. When the amplitude increases in the positive direction, the actual frequency increases on the basis of the center frequency, and vice versa.
The greater the amplitude of the modulation signal, the greater the deviation of the actual frequency of the FM signal from the center frequency. This is how FM transmits information.
FM methods can be divided into direct FM and indirect FM.
1, direct frequency modulation
Direct FM is the most commonly used FM method. It uses the modulation signal to directly control the high-frequency oscillator to change the tuning loop parameters, thereby changing the output frequency of the oscillator. As shown in the figure below, it is the principle block diagram of direct FM.
The following figure shows a frequency modulation circuit implemented with a varactor diode.
Frequency modulation circuit realized with varactor diode
The principle is that a capacitor three-point oscillator composed of a transistor VT, etc., the oscillation frequency is determined by the L and C2 resonant circuit. The modulation signal voltage is loaded on the varactor diode VD through R1, so that the PN junction capacitance of VD changes accordingly.
Since the VD is connected in parallel to the LC resonance circuit, the change of the junction capacitance will change the parameters of the LC resonance circuit, so that the oscillation frequency changes with the amplitude of the modulation signal. The final FM signal is output from the L tap through C4.
The following figure is a frequency modulation circuit realized with a triode.
Frequency modulation circuit realized with triode
The principle is that a capacitor three-point oscillator composed of a transistor VT, etc., the oscillation frequency is determined by the L1, C3 resonant circuit and the junction capacitance of VT. The modulation signal loads the base of the transistor VT through C1, so that the junction capacitance of VT changes accordingly. Thereby changing the parameters of the LC resonant tank, so that the oscillation frequency changes with the amplitude of the modulation signal. The final FM signal is output from the VT collector through C6.
2, indirect frequency modulation
Indirect frequency modulation is to integrate the modulation signal with an integrating circuit. The phase modulator is phased by the integral signal in yoga. The output from the phase modulator is the frequency modulation signal. As shown in the figure below, it is the principle block diagram of indirect frequency modulation.
Our other product:
