Principle of frequency discriminator

Frequency discrimination circuits used to demodulate FM signals can be divided into three categories. The first type is FM-AM-FM conversion type. In this type, a constant amplitude FM wave is first converted into an amplitude modulated FM wave whose amplitude is proportional to the instantaneous frequency of the FM wave through a linear network, and then the amplitude is detected by an amplitude detector. The second type is phase-shifting frequency doubling frequency discrimination type. In this type, FM wave is changed into phase-modulated FM wave by phase-shifting circuit, and its phase change is linear with the instantaneous frequency change of FM wave, and then the phase of phase-modulated FM wave is compared with the original FM wave, and the demodulated signal is taken out by low-pass filter. Because the phase comparator usually consists of multipliers, it is called phase shift multiplication frequency discrimination. The third type is pulse average type. In this type, the FM signal is converted into a unipolar pulse sequence with the same repetition frequency as the instantaneous frequency of the FM signal by a zero-crossing comparator, and then the average value of the pulse sequence is taken out by a low-pass filter, thus recovering the signal proportional to the instantaneous frequency change. Frequency discriminator is a kind of ceramic filter element with phase-shifting frequency discrimination characteristics, which is mainly used in audio intermediate frequency amplification or demodulation circuit of TV or video recorder and frequency discriminator circuit of FM FM radio. It can be divided into two types: balance type and difference type. The former is used for balanced demodulation of synchronous phase detector, and the latter is used for differential demodulation of differential peak frequency detector. Narrow-band JTCV 10.7M series patch frequency discriminator with German key FM audio is applied to FM program detection of various IC, and the frequency is converted into useful audio signal.

The characteristic of FM wave is that the amplitude remains constant, while the instantaneous frequency changes linearly with the size of modulation signal, which represents the information to be transmitted. In analysis or experiment, it is often expressed by low-frequency sine wave. The purpose of frequency discrimination is to detect low-frequency modulation signals from FM waves, that is, to complete frequency-voltage conversion. The circuit that can accomplish this function is called a frequency discriminator.

The phase-modulated wave demodulation circuit takes out the original modulation signal from the phase-modulated wave, that is, the output voltage is proportional to the instantaneous phase offset of the input signal, also known as the phase detector. For the demodulation circuit of FM wave, the original modulation signal is extracted from FM wave, that is, the output voltage is proportional to the instantaneous frequency offset of the input signal, which is also called frequency discriminator.

Phase detection circuits are usually divided into analog circuits and digital circuits. In the integrated circuit system, the commonly used circuits are product phase detection and gate phase detection. The phase discriminator can not only demodulate the phase wave, but also form a phase-shifted frequency discriminator circuit. Especially in the phase-locked loop, it has been widely used as the main component.