Slope frequency discriminator

　　A frequency discriminator that utilizes the amplitude frequency characteristics of linear networks to transform frequency modulated waves into amplitude modulated frequency modulated waves. The principle circuit diagram of this frequency discriminator is shown in Figure 1 (a). The transistor VT and LC circuit form a tuned amplifier, but the resonant frequency f0 of the circuit and the center frequency fc of the frequency modulated wave are detuned, as shown in Figure 1 (b). Therefore, when the instantaneous frequency of the frequency modulated wave changes, its amplitude also changes accordingly, and the output of the LC circuit is an amplitude modulated frequency modulated wave. By using an envelope detector composed of parallel resistors VD and RC diodes, the required original modulation signal can be detected to achieve frequency detection. The circuit of the slope frequency discriminator is simple, but the amplitude frequency characteristics of the circuit are not linear, resulting in a linear difference in the frequency discrimination characteristics. In addition, this type of frequency discriminator does not have the ability to suppress parasitic amplitude modulation interference. Usually, before frequency discrimination, the modulation wave must be limited to maintain its amplitude unchanged in order to reduce the impact of parasitic amplitude modulation.

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