1 Classification of filters and working principle of filter

The filter mainly plays the role of frequency selection in the circuit. It can suppress (or attenuate) useless frequency signals (interference frequency signals) and allow useful frequency signals to pass. The working principle of filter is mainly based on its differentiated response to different frequency signals. Engineers often use filters in circuit applications, mainly for suppressing interference signals and processing and transmitting useful data. Usually, filters have frequency response characteristics. The frequency characteristics of filters have amplitude-frequency response and phase response. The ideal filter should have infinite amplitude attenuation in the stopband range, while the amplitude-frequency response and linear phase response in the passband range are zero attenuation. The amplitude-frequency response of the filter is divided into passband and stopband. The passband refers to the frequency range of the signal that can pass, the stopband refers to the frequency range of the attenuated signal or the blocked signal, and the cutoff frequency refers to the boundary between the stopband and the passband.

Filters can be divided into: active filters, passive filters, crystal filters, mechanical filters, phase-locked loop filters, switched capacitor filters, ceramic filters, etc.

According to the signal processing method, filters can be divided into: analog filters and digital filters.

According to the frequency selection function, there are: high-pass filter, low-pass filter, stopband filter, passband filter, comb filter. Low-pass filter and high-pass filter are the two most common types of filters. Other filters can be composed of these two filters, that is, low-pass filter and high-pass filter can form a band-pass filter after being connected in series, and low-pass filter and high-pass filter can form a band-stop filter after being connected in parallel.

According to the standard classification of the “best approximation characteristics” of passband filtering, it can be divided into: Butterworth filter (maximum flat type), Chebyshev filter (equiripple type), Bessel filter (linear phase shift type).

In addition, there are some special filters, such as special filters that meet certain frequency response characteristics and phase shift characteristics, for example, linear phase shift filters, time delay filters, network filters in audio, and surface acoustic wave filters in TVs.

2 Working principle of filter

In actual engineering, most filters play a frequency selection role, so we focus on the working principle of filter frequency selection. The circuit principle is shown in Figure 1.

Figure 1 Circuit Schematic Diagram

High-pass filter: A high-pass filter is a combination of capacitors, inductors, resistors, etc. that allows signal components above a certain frequency to pass through, while greatly suppressing signal components below the frequency. A high-pass filter is a circuit that allows mid- and high-frequency signals to pass through but does not allow low-frequency signals to pass through. It is widely used in power amplifier circuits. For example, in order to enhance the mid-range and treble components in the circuit to drive the speaker, a high-pass filter is used to filter out the bass components in the audio signal. Usually, high-pass filters and low-pass filters are used at the same time in the circuit, and their function is to transmit the required sound frequency signal. For example, the schematic diagram of the passive high-pass filter circuit (see Figure 2)

Figure 2 Schematic diagram of high-pass filter

Bandpass filter: refers to the transmission of signals in a specific frequency band, while attenuating all signals outside this frequency band, that is, the in-band signal passes through and the out-of-band signal is filtered out. The working principle of filter here is that only signals in a specific frequency range are allowed to pass through, and the rest of the frequencies are effectively suppressed. The main parameters of the bandpass filter are high and low cutoff frequencies (FH and fL), bandwidth (BW), center frequency fC, and center frequency gain, such as the active analog bandpass filter circuit schematic (see Figure 3).

Figure 3 Active analog bandpass filter

Bandstop filter: The stopband filter is the opposite of the bandpass filter. It refers to a filter that attenuates certain frequency components to the minimum and can pass most frequency components. The input voltage is applied to the low-pass filter and the high-pass filter at the same time, and then the output voltages of the two circuits are summed to obtain a bandstop filter. The cutoff frequency of the low-pass filter should be less than the cutoff frequency of the high-pass filter. Practical circuits often use passive low-pass filters and high-pass filters in parallel to form passive band-stop filter circuits, such as the principle of the stop-band filter circuit (see Figure 4)

Figure 4 Schematic diagram of the stop-band filter circuit

Comb filter: As the name suggests, its characteristic curve is like a comb, which only allows signals in certain frequency ranges to pass through, and there are many passbands and stopbands arranged at a certain frequency and with the same interval. On this basis, the working principle of filter has evolved into periodically transmitting or suppressing signal frequencies. Comb filters are more commonly used in modern television technology, and their function is to process the picture quality of television. Comb filters are generally composed of bandpass filters, adders, subtractors, and delays.

Low-pass filter: Low-pass filter is a technology for anti-electromagnetic interference, which has a strong suppression effect on differential and common mode noise. Common EMI suppression filters are mostly low-pass filters. However, electromagnetic interference is transmitted by conduction and radiation. Radiative coupling is suppressed by shielding technology, and EMI suppression filtering technology is used for conductive coupling. EMI suppression filters are used to meet the requirements of electromagnetic compatibility. Since good filtering directly suppresses the flow of disturbance energy on the wire, EMI suppression filtering is a technical measure to suppress electromagnetic disturbance in the conduction mode, and it can also play a certain role in suppressing radiation interference.

For low-pass filters, they are mainly composed of inductors and capacitors. Since inductors have parasitic capacitance and capacitors have parasitic inductance, they will cause efficiency loss, causing leakage magnetic field (commonly known as sling) phenomenon in low-pass filters when they are actually used, especially at high frequencies. Therefore, for low-pass filters, in order to ensure effective filtering within the frequency band, that is, effective filtering in the highest frequency band, it is recommended to install filters separately in each frequency band in the circuit instead of using a single low-pass filter, that is, multi-stage filtering.

The basic circuit composition of a low-pass filter is a parallel capacitor, a series inductor, and a series-parallel resistor. According to the working principle of filter, the parallel capacitor is equivalent to the bypass capacitor, which can suppress high-frequency noise; the series inductor dissipates the heat generated by the high-frequency noise, thereby limiting the passage of noise, and the series-parallel resistor can change the input and output impedance of the circuit network, and can also absorb interference signals.