Main classification of filters

According to the signal processed, it can be divided into analog filter and digital filter.

According to the frequency band of the signal, it can be divided into low-pass, high pass, band-pass, band stop and all-pass filter.

Low pass filter: it allows the low frequency or DC component of the signal to pass through, and suppresses the high frequency component or interference and noise;

High pass filter: it allows the high-frequency component of the signal to pass through and suppresses the low-frequency or DC component;

Band pass filter: it allows the signal of a certain frequency band to pass through and suppresses the signal, interference and noise lower or higher than that frequency band;

Band stop filter: it suppresses the signal within a certain frequency band and allows the signal beyond the frequency band to pass through, also known as notch filter. [1]

All pass filter: all pass filter means that the amplitude of the signal will not change in the whole frequency band, that is, the amplitude gain in the whole frequency band is equal to 1. Generally, all pass filter is used for phase shifting, that is to say, changing the phase of the input signal. Ideally, the phase shifting is proportional to the frequency, which is equivalent to a time delay system.

According to the components used, it can be divided into passive and active filters.

According to the different position of the filter, it is generally divided into on-board filter and panel filter.

On board filters are installed on the circuit board, such as PLB and JLB series filters. The advantage of this kind of filter is economy, but the disadvantage is that the effect of high frequency filter is not good. The main reasons are as follows

1. There is no isolation between the input and output of the filter, so coupling is easy to occur;

2. The grounding impedance of the filter is not very low, which weakens the effect of high frequency bypass;

3. The connection between the filter and the chassis has two adverse effects One is that the electromagnetic interference in the internal space of the chassis will be directly induced to this section of line, which will flow out of the chassis along the cable, with the help of the cable radiation to make the filter fail; the other is that the external interference will generate radiation with the help of this section of line before being filtered by the filter on the board, or directly couple with the circuit on the circuit board, causing the sensitivity problem;

Filter array board, filter connector and other panel filters are generally directly installed on the metal panel of the shielding chassis. Because it is directly installed on the metal panel, the input and output of the filter are completely isolated and well grounded, and the interference on the cable is filtered out on the chassis port, so the filtering effect is quite ideal.