“Frequency modulation is the use of modulated signals to control the frequency change of the carrier. Direct frequency modulation is to use the modulation signal to directly control the frequency of the carrier oscillator, so that it changes linearly according to the law of the modulation signal.
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1. Generation of FM signal
There are two main methods of frequency modulation: direct frequency modulation and indirect frequency modulation.
1) Direct frequency modulation method
Frequency modulation is the use of modulated signals to control the frequency change of the carrier. Direct frequency modulation is to use the modulation signal to directly control the frequency of the carrier oscillator, so that it changes linearly according to the law of the modulation signal.
Figure 1 FM modulator
As can be seen from the above figure, the oscillator whose FM modulation can be controlled by an external voltage is called a voltage-controlled oscillator (VCO).Each voltage-controlled oscillator itself is an FM modulator, because its oscillation frequency is proportional to the input control voltage, that is
If the modulation signal is used as the control voltage signal, FM waves can be generated.
If the controlled oscillator is an LC oscillator, you only need to control a certain reactance element (L or C) of the oscillating loop so that its parameters change with the modulation signal. At present, the commonly used reactance element is a varactor Diode. Using varactor Diodes to achieve direct frequency modulation has become one of the most widely used frequency modulation circuits due to its simple circuit and good performance.
In the direct frequency modulation method, the oscillator and the modulator are combined into one. The main feature of this method is that a larger frequency deviation can be obtained under the requirement of realizing linear frequency modulation. The main disadvantage is that the frequency stability is not high, so it is often necessary to use an automatic frequency control system to stabilize the center frequency.
2) Indirect frequency modulation method
The indirect frequency modulation method (abbreviated as the indirect method) is to integrate the modulated signal, and then phase-modulate the carrier to generate an NBFM signal, and then obtain the WBFM signal through an n-times frequency multiplier. The principle block diagram is shown in the figure below. This method of generating WBFM is called the Armstrong method or indirect method.
Figure 2 Indirect method to produce WBFM
The function of the frequency multiplier in the picture is to increase the frequency multiplication index to obtain wideband frequency modulation WBFM. The frequency multiplier can be realized with a non-linear device, and then a band-pass filter is used to filter out unwanted frequency components. The advantage of the indirect method is high frequency stability, but the disadvantage is that it requires multiple frequency multiplication and mixing, so the circuit is more complicated.
2. Demodulation of FM signal
The demodulation of FM signal is also divided into coherent demodulation and non-coherent demodulation. Coherent demodulation is only applicable to NBFM signals, while non-coherent demodulation is applicable to both NBFM and WBFM signals.
1) Incoherent demodulation
The general expression of FM signal is:
That is to say, the demodulation of the FM signal is to generate an output voltage that has a linear relationship with the frequency of the input FM signal. The device that completes this frequency-voltage conversion relationship is a frequency detector, referred to as a frequency discriminator.
There are many types of frequency discriminators. Figure 3 below depicts a block diagram of the principle of non-coherent demodulation with amplitude frequency discriminators. In the figure, the differentiator and the envelope detector form a frequency discriminator with approximately ideal frequency discrimination characteristics.The function of the differentiator is to change the FM wave with a constant amplitude into an FM wave whose amplitude and frequency change with the modulation signal, namely
The envelope detector detects the amplitude change and filters out the DC, and then demodulates the output after passing through the low-pass filter, namely
Figure 3 Block diagram of the amplitude discriminator
The function of the limiter in Figure 3 is to eliminate the amplitude fluctuations of the FM wave caused by noise and other reasons in the channel. The band-pass filter (BPF) allows the FM signal to pass smoothly, while filtering out-of-band noise and high-order harmonic components. .
There are many types of frequency discriminators. In addition to the above-mentioned amplitude discriminators, there are also phase discriminators, proportional discriminators, quadrature frequency discriminators, slope frequency discriminators, frequency negative feedback demodulators, and phase-locked loops. (PLL) frequency discriminator, etc.
2) Coherent demodulation
Since the NBFM signal can be decomposed into the sum of the in-phase component and the quadrature component, coherent demodulation in linear modulation can be used for demodulation.
It can be seen that coherent demodulation can restore the original modulated signal. This modulation method is the same as coherent demodulation in linear modulation, requiring the local carrier to be synchronized with the modulated carrier, otherwise it will distort the demodulated signal.
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