Read: The Working Principle OF Three Common Low-Pass, High-Pass And Band-Pass Filters

What IS Filter

A filter is a device that filters waves. It is a circuit that allows signals in a certain frequency band to pass through while preventing signals outside this frequency band from passing through.

Filters are mainly low-pass filters, high-pass filters and bandpass filters, according to the working principle of the circuit can be divided into passive and active filters two categories. Today, Xiaobian mainly on the low-pass, high-pass and bandpass three kinds of filters to do the following simple introduction,

 I hope that electronics lovers friends after reading a little harvest.

Low-Pass Filter

The inductor prevents the passage of high frequency signals and allows the passage of low frequency signals, whereas the capacitor has the opposite characteristic. The signal can pass through an inductive filter, or a filter connected to the ground by a capacitor, 

the attenuation of the low-frequency signal is less than that of the high-frequency signal, which is called a low-pass filter.

The principle of low-pass filter is very simple, it is the use of capacitors through high frequency to block low frequency, inductors through low frequency to block high frequency principle. For the high frequency that needs to be cut off, the method of absorbing inductance and blocking is used to prevent it from passing;

 For the low frequency that needs to be released, the characteristics of high capacitance resistance and low inductance resistance are used to let it pass.

The simplest low-pass filter consists of resistive and capacitive elements, as shown below. The function of the low-pass filter is to let below the transition frequency f. The low-band signal passes through and will be above the transition frequency f. The signal is removed.

This low-pass filter works like this: When the frequency in the input signal Vin is lower than the transition frequency f. When the signal is added to the circuit, the low-frequency signal is output by R because of the large tolerance and no shunt effect of C. 

When the frequency in Vin is higher than the transition frequency f. Because the capacitive reactance of C is very small, the high-frequency signal through R is shunt from C to the ground without output, so as to achieve the purpose of low-pass. The RC low-pass filter has a break frequency f. It is determined by:

In addition to this RC circuit, the low-pass filter can also be in the form of LC circuit.

High-Pass Filter

The simplest high-pass filter is the "first-order high-pass filter", its characteristics are generally expressed by the first-order linear differential equation, its left side is exactly the same as the first-order low-pass filter, only the right side is the derivative of the excitation source rather than the excitation source itself. 

When a lower frequency passes through the system, there is little or no output, while when a higher frequency passes through the system, there will be less attenuation.

In fact, for extremely high frequencies, the capacitor is equivalent to a "short circuit", and these frequencies can basically obtain output at both ends of the resistance. In other words, this system is suitable for high frequencies and low frequencies have a greater blocking effect, is a simplest "high-pass filter", 

as shown in the figure below.

The circuit works like this: When the frequency is below f. When the signal is input to this filter, it is blocked due to the high tolerance of C1, and the output decreases, and the lower the frequency, the smaller the output. When the frequency is above f. 

When the signal is input to this filter, because the C1 capacitive reactance is very small, it has no attenuation effect on the signal, so the filter has the effect of letting the high-frequency signal through and blocking the low-frequency signal. The break frequency of this circuit is f. It is determined by:

In addition to components, high-pass filters can also be composed of LC.

Bandpass Filter

A bandpass filter is a circuit that allows only a certain frequency to pass through while effectively suppressing signals of the remaining frequencies. Because of its signal selectivity, it is widely used in electronic design. For example, the RLC oscillator loop is an analog bandpass filter. 

A band-pass filter is a filter that can pass frequency components in a certain frequency range, but attenuates frequency components in other ranges to very low levels, as opposed to the concept of a band-stop filter.

An ideal bandpass filter should have a completely flat passband with no amplification or attenuation within the passband. In fact, there is no ideal bandpass filter. The filter is not able to completely attenuate all frequencies outside the desired frequency range, especially 

if there is a attenuated but not isolated range outside the desired passband. This is often referred to as the phenomenon of the filter's rolling down, and is represented by the number of dB of the attenuation amplitude per ten times the frequency.

Generally, the design of the filter tries to ensure that the roll-down range is as narrow as possible, so that the performance of the filter is closer to the design. However, as the rolling range becomes smaller and smaller, the passband becomes no longer flat and begins to appear "ripples". 

This phenomenon is particularly pronounced at the edge of the passband, an effect known as the Gibbs phenomenon.

The above is a brief introduction to the three common filters, in fact, there are many kinds of filters, here we will not introduce one, later we have the opportunity to do a detailed description of other filters.