Digital filters are used in discrete systems; analog filters are used in continuous time systems, and in discrete time systems such as SC (switched capacitor) filters.
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A digital filter consists of an algorithm or device consisting of a digital multiplier, an adder, and a delay unit. The function of the digital filter is to perform arithmetic processing on the digital code of the input discrete signal to achieve the purpose of changing the spectrum of the signal. Digital filters can be implemented in computer software or in large-scale integrated digital hardware.
Analog filters are active and passive, and active filters are primarily op amps, or spanning to op amps, and resistors and capacitors. Passive filters are mainly composed of R, L, and C. Analog filters have problems with voltage drift, temperature drift, and noise, and digital filters do not have these problems, resulting in high stability and accuracy.
From the point of view of implementation, analog filters are generally built with analog devices such as capacitors and inductors. Digital filters can be implemented by software or digital chips. It is very troublesome to replace the capacitor and the inductor when the analog filter parameters are changed. When the digital filter parameters are changed, sometimes it is only necessary to modify the coefficients (such as when the software is implemented).
From the technical point of view, it is very difficult to achieve an analog filter to achieve -60dB, and the digital filter can easily reach this index.
The bridge between communication analog and digital filters I think is the sampling theorem. Generally, the analog signal x(t) is sampled (such as A/D conversion) to obtain a digital signal x(n), and then these digital signals are passed through a digital filter, and the filter outputs a digital signal y(n). y(n) then performs a D/A converter to get y(t). From x(t) to y(t) can be understood as analog filtering.
1. Digital filters are less sensitive to the outside world and have higher reliability. 2. Digital filters enable functions such as accurate linear phase and multi-rate processing that are not possible with analog filters. 3. Digital filters can achieve arbitrary precision signal processing as long as the word length is increased. 4. Digital filters are more flexible and can store signals at the same time. 5, of course, the frequency domain width of the digitally processed signal is limited by the sampling rate
The biggest difference between analog and digital filters is that the digital filter is flipped about the Fs/2 frequency, which is symmetrical; the analog filter is not.
Therefore, a large amount of interpolation filtering is selected in the DAC, and the image frequency is placed at a very distant frequency point, and then the analog frequency filter such as a sound meter is used to filter the image frequency in the RF section. So digital analog filters are indispensable.
1. The analog filter and the digital filter are expressed differently:
The mode filter is represented by H(S), and the number filter is represented by H(Z). The mode filter is mainly based on the approximation of the amplitude-frequency characteristic, and the digital filter can achieve phase matching.
2. There are two types of digital filters: IIR, FIR, and IIR mainly include two kinds of impulse response invariance and bilinear transformation, while FIR can realize linear phase, window method, frequency sampling method, and optimization method. FFT to achieve.
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