Classification and performance of filters

According to transmission characteristics, radio frequency filters are divided into low-pass (LP), high-pass (HP), band-pass (BP), band-stop (BS) and all-pass (AP). They allow the required frequency signals to pass and suppress unwanted frequency signals.
According to the mathematical prototype, there are Butterworth, Chebyshev, Ellipse, Bessel and other filters.
Under the premise of the same order, the pass band of the Butterworth filter is the flattest, and the stop band drops slowly;
the pass band of the Chebyshev filter is rippled and the stop band drops faster;
the pass band of the elliptic filter is rippled and the stop band drops the fastest;
The pass band of the Bessel is rippled, stop band drops slowly, the amplitude-frequency characteristics are the worst, but it has the best linear phase characteristics.
According to the type of resonator, there are LC filters, microstrip line filters, waveguide filters, dielectric filters and coaxial cavity filters.
The quality factor reflects the selectivity of the band-pass filter; the VSWR reflects the degree of matching between the filter and other components in the system; the insertion loss reflects the attenuation of signal power.
Generally, it is hoped that the passband attenuation is the smallest and the stopband attenuation is the largest; In-band fluctuation reflects the fluctuation degree of the signal amplitude in the passband; out-band rejection reflects the attenuation of the filtered signal; group delay reflects the delay time of the signal through the filter.
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