A filter is a frequency selection device that allows specific frequency components in a signal to pass through while greatly attenuating other frequency components. By utilizing the frequency selection function of filters, interference noise can be filtered out or spectrum analysis can be performed. In other words, any device or system that can significantly attenuate or suppress other frequency components in a signal through a specific frequency component is called a filter. A filter is a device that filters waves. "Wave" is a very broad physical concept, and in the field of electronic technology, "wave" is narrowly limited to describing the process of the fluctuation of various physical quantities' values over time. This process is converted into a time function of voltage or current through the action of various sensors, which is called the time waveform of various physical quantities, or it is called a signal. Because the independent variable time is a continuous value, it is called a continuous time signal, also commonly referred to as an analog signal.

Filtering is an important concept in signal processing. In DC stabilized power supplies, the function of the filtering circuit is to minimize the AC component in the pulsating DC voltage, retain its DC component, reduce the ripple coefficient of the output voltage, and make the waveform smoother.

main parameter

The main parameters of the filter are:

Center Frequency: The frequency f0 of the passband of the filter, usually taken as f0=(f1+f2)/2, where f1 and f2 are the left and right side frequencies of the bandpass or bandstop filter that decrease by 1dB or 3dB relative to each other. Narrowband filters often calculate the passband bandwidth with the minimum insertion loss point as the center frequency.

Cutoff Frequency: Refers to the frequency point on the right side of the passband of a low-pass filter and the frequency point on the left side of the passband of a high pass filter. Usually defined as a relative loss point of 1dB or 3dB. The reference standard for relative loss is: low-pass based on DC insertion loss, while high pass based on insertion loss at sufficient high pass frequency without parasitic stopbands.

Passband bandwidth: refers to the spectral width that needs to be passed through, BW=(f2-f1). F1 and f2 are based on the insertion loss at the center frequency f0.

Insertion Loss: The attenuation caused by the introduction of a filter to the original signal in the circuit, characterized by loss at the center or cutoff frequency. If full band insertion loss is required, it should be emphasized.

Ripple: refers to the peak value of insertion loss that fluctuates with frequency on the basis of the loss mean curve within a bandwidth range of 1dB or 3dB (cutoff frequency).

Passband Ripple: The variation of insertion loss within the passband with frequency. The in band fluctuation within a 1dB bandwidth is 1dB.

In band standing wave ratio (VSWR): An important indicator to measure whether the signal in the passband of a filter is well matched and transmitted. Ideal match VSWR=1:1, VSWR>1 in case of mismatch. For an actual filter, the bandwidth that satisfies VSWR<1.5:1 is generally smaller than BW3dB, and its proportion to BW3dB is related to the filter order and insertion loss.

Return Loss: The number of decibels (dB) in which the ratio of the input power to the reflected power of the port signal is equal to 20Log10 ρ，ρ Is the voltage reflection coefficient. When the input power is fully absorbed by the port, the return loss is infinite.

Stop band suppression system: an important indicator to measure the performance of filter selection. The higher the indicator, the better the suppression of out of band interference signals. There are usually two ways to propose it: one is how much dB is required to suppress for a given out of band frequency fs, and the calculation method is the attenuation at fs; Another approach is to propose an indicator that characterizes the similarity between the amplitude frequency response of the filter and the ideal rectangle - the rectangle coefficient (KxdB>1), where KxdB=BWxdB/BW3dB, (X can be 40dB, 30dB, 20dB, etc.). The higher the order of the filter, the higher the degree of rectangle - that is, the closer K is to the ideal value of 1, and the greater the difficulty of production.

Delay (Td): Refers to the time required for a signal to pass through a filter, which is numerically the derivative of the diagonal frequency of the transmission phase function, i.e. Td=df/dv.

In band phase linearity: This indicator characterizes the magnitude of phase distortion introduced by the filter to the passband transmitted signal. The filter designed according to the linear phase response function has good phase linearity.

According to the processed signal, there are two types: analog filters and digital filters.

According to the frequency band of the signal passed through, it can be divided into five types: low-pass, high pass, band-pass, bandstop, and all-pass filters.

Low pass filter: It allows low-frequency or DC components in the signal to pass through, suppressing high-frequency components or interference and noise;

High pass filter: It allows high-frequency components in the signal to pass through and suppresses low-frequency or DC components;

Band pass filter: It allows signals in a certain frequency band to pass through, suppressing signals, interference, and noise below or above that frequency band;

Band stop filter: It suppresses signals within a certain frequency band and allows signals outside that frequency band to pass through, also known as notch filter.

All pass filter: An all pass filter refers to a signal whose amplitude does not change throughout the entire frequency band, meaning that the amplitude gain is equal to 1 throughout the entire frequency band. A general all pass filter is used for phase shifting, which means changing the phase of the input signal. Ideally, the phase shift is proportional to the frequency, equivalent to a time delay system.