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Dec 6 2019

Filter (Signal Processing) Basics in Electronics

Ⅰ. Introduction

In electronics, a filter (signal processing) is a device or process that removes some unwanted components or features from a signal. Filtering is a class of signal processing, the defining feature of filters being the complete or partial suppression of some aspect of the signal. Most often, this means removing some frequencies or frequency bands. However, filters do not exclusively act in the frequency domain; especially in the field of image processing many other targets for filtering exist. As is known to all, electronic filters remove unwanted frequency components from the applied signal, enhance wanted ones, or both.

Filtering Out the Noise (signal processing)

Filtering Out the Noise (signal processing)

Ⅱ. Classifications and Functions

Filters have different effects on signals of different frequencies: the signal is passed through the passband with a small attenuation in the passband; the transition between the passband and the stopband causes the signal to be attenuated to varying degrees; the signal is greatly attenuated within the stopband.

According to the different distributions of the three frequency bands of the filter in the full frequency band, the filters can be divided into the following four basic types: low-pass filters, band-pass filters, high-pass filters, and band-stop filters. In addition, there is another kind of filter, all-pass filter, which can pass signals of various frequencies, after passing, the phase of different frequency signals changes differently. In fact, the all-pass filter is a kind of phase shift device.

Filters can be built in a number of different technologies. Before that, it is necessary to know some basics of it deeply.

Ⅲ. Question and Answer

1. How to select EMI filters?

Some people think that the higher the insertion loss of an EMI filter, the better, and the more stages of the filtering network, the better. In fact, this is not the right way to choose a EMI filter. In addition, the more stages of the filtering network, the more expensive, the larger the size and weight. In practice, the best way to select and evaluate an EMI filter is to install it on a device for testing. As is known to all, the performance of a filter depends largely on the load impedance of the device. It cannot be derived from one data of impedance insertion loss. Because it is a complex function of the filtering element impedance and the equipment impedance, and its magnitude and phase change within the frequency range. What's more, different performance levels of conducted radiation control (FCC, VDE) and sensitivity control required by the filter selection test are performed on the device.

2. Do all filter networks with the same circuit and component values have the same performance?

It could be concluded from the experiments that the performance of all filter networks with the same circuit and component values is not the same. The component values are specified and measured at a certain frequency (usually 1KHz), and the performance of the filter is required throughout the frequency range, not just a frequency value at which the component is measured. Also the structural form of a component and the method of accessing the filter are very important to the performance of the filter.

3. Is the installation way important to the performance of the filter?

Installation and wiring methods have a great impact on the performance of the filter. The EMI  filter is best installed at the power line input of the device. The filter is an obstacle to high-frequency signals, and its function must not be affected by discretely coupling the power input and power output line or any other conductor of the protected equipment.

4. Why design a low-pass EMI filter to increase insertion high-frequency loss?

Key word 1: Common mode interference

Common mode interference generation

It has distributed capacitance between the switching power supply (MOS will be added the heat sink design when the output power is large) and the ground (reference ground of the testing system); and the high-frequency components of the square wave voltage of the switch MOS and output rectifier diode are transmitted to the ground (reference ground) through the distributed capacitance. This forms a loop with the power supply line. In other words, high-frequency component generates the common mode interference by forming a loop with the power line through the distributed capacitance.

Key word 2: Differential mode interference

Differential mode interference generation

The switching tube works in the on-off state: the current flowing through the power line rises linearly when the switching tube is turned on; when the switching tube is turned off, the current suddenly changes to 0. Therefore, it is a high-frequency repetitive triangular wave pulsating current when flowing through the power line, which is rich in high-frequency harmonic components. As the frequency increases, the amplitude of this harmonic component will become smaller and smaller, therefore, the differential mode interference decreases as the frequency increases. 

Note: As the frequency increases, the distributed capacitance between the switching device and ground becomes obvious, at the same time, the common mode interference becomes higher, although the small common mode current will cause large interference.

 EMI Filters

EMI Filters Image

When installing the EMI filter, pay attention to the following points:

1) There must be a good electrical connection between the housing of the EMI filter and the ground. Do not install the filter on an insulating material plate or the components have painted surface, and install it on a metal case. Also avoid using long grounding wires, because this will greatly increase the ground inductance and resistance, which will seriously reduce the common mode rejection performance of the filters. A better way is to use a metal screw and a spring (star row) washer to fix the metal shield shell of the filter to the cabinet at the system power inlet tightly, or use the twisted coppers to connect to the ground.

2) During installation, do not bundle the input and output cables of the filter together. Because this intensifies the electromagnetic coupling between the input and output of the filter, which seriously damages the filter to suppress EMI signals.

3) Do not install the filter inside the equipment shield. Because of this, the EMI signals on the internal circuits and components of the device will be directly coupled to the outside of the device due to the radiation of EMI signals on the terminal leads of the filter, causing the shielding device to lose the suppression of EMI radiation generated by the internal circuits and components.

It is recommended to use the original shielding settings of the device to effectively isolate the input and output ends of the filter to minimize the possible electromagnetic coupling between them.

Generally, the housing of the filter is connected to the frame or cabinet of the equipment to be protected, and the wire-side wires should be kept short and well isolated from the load-side wires. The ideal isolation system is a wall-mounted filter with a line socket.


Ⅳ. Filter Main Parameters

  • Center frequency

The main parameters of the filter: the center frequency of the filter's pass-band f 0, generally f 0 = (f 1 + f 2) / 2, f 1 and f 2 are boundary frequencies of band pass or band stop  filter, which decreased by 1dB or 3dB. In addition, narrowband filters often use the minimum point of insertion loss as the center frequency to calculate the pass-band bandwidth.

  • Cutoff frequency

It refers to the right frequency point of the pass-band of the low-pass filter and the left frequency point of the pass-band of the high-pass filter, and it is usually defined by relative loss points, 1dB or 3dB. The relative reference for the relative loss is: the low-pass is based on the insertion loss at DC, and the high-pass is based on the insertion loss at a high-pass frequency at which no parasitic stop-band occurs.

  • Pass-band bandwidth (BWxdB)

It refers to the width of the spectrum that needs to pass, BWxdB = (f2-f1), where f1 and f2 are based on the insertion loss at the center frequency f0, with the condition of corresponding left and right frequency points at X (dB) are decreased. Generally, the value range of X is BW3dB, BW1dB, and BW0.5dB, which are used to characterize the filter's pass-band bandwidth. In addition, fractional bandwidth = BW3dB / f0 × 100 [%], also commonly used to characterize the pass-band bandwidth of the filter.


  • Insertion loss

It refers to the loss of the original signal in the circuit due to the introduction of the filter, it is characterized by the loss at the center or cutoff frequency. If it is the full-band interpolation loss, it must be emphasized.

Note: When adding a filter at the input end, the impedance of the filter should be mismatched with the impedance of the power supply. The more severe the mismatch, the more ideal the attenuation is, and the better the insertion loss characteristics. That is, if the internal resistance of the noise source is low impedance, the input impedance of the EMI filter connected to it should be high (such as a series inductor with a large amount of inductance); if the internal resistance of the noise source is high impedance, the input impedance of the EMI filter should be low (such as a large parallel capacitor). Due to the imbalance of the line impedance, the two components will convert to each other during transmission, and the situation becomes complicated.


  • Ripple

It refers to the peak-to-peak value of the insertion loss that fluctuates on the basis of the average loss curve with the frequency in the 1dB or 3dB bandwidth (cutoff frequency).


  • Passband riplpe

The amount of change in insertion loss in the passband with frequency. For example, in a 1dB bandwidth, it is 1dB.


  • Passband standing wave ratio (VSWR)

An important indicator for measuring whether the signal in the filter passband is well transferred. Ideal VSWR is 1: 1, when mismatched, VSWR> 1. For an actual filter, the bandwidth that satisfies VSWR <1.5: 1 is generally less than BW3dB, and the proportion when at BW3dB is related to the filter order and insertion loss.


  • Return loss

The decibels (dB) of the ratio of the port's signal input power to the reflected power, and it is also equal to | 20Log10ρ |, where ρ is the voltage reflection coefficient. In addition, when the input power is completely absorbed by the port, the return loss value is infinite.


  • Stop band rejection

It is an important index to measure the performance of filter selection. The higher the index, the better the suppression of out-of-band interference signals. There are usually two formulations: one is how much dB is required to suppress a given out-of-band frequency fs, and the calculation method is the attenuation fs=As-IL; another is to propose a characterizing filter whose amplitude-frequency response is close to the ideal rectangle index of degree-rectangular coefficient (KxdB> 1), KxdB = BWxdB / BW3dB, (x can be 40dB, 30dB, 20dB, etc.). The more the filter order, the higher the rectangularity, in other words, the closer the K is to the ideal value 1, the more difficult it is to make a ideal filter.


  • Delay (Td)

It refers to the time required for the signal to pass through the filter. The value is the derivative of the diagonal frequency of the transmission phase function, that is, Td = df / dv.


  • In-band phase linearity

This indicator characterizes the phase distortion introduced by the filter on the transmission signal in the passband. The filter designed according to the linear phase response function, which has good phase linearity, but its frequency selectivity is very poor. It is only used to pulsed or phase-modulated signal transmission system applications.


  • Order (stage)

For high-pass and low-pass filters, the order is the sum of the number of capacitors and inductors in the filter circuit. For a band-pass filter, the order is the total number of parallel resonators; for a band-stop filter, the order is the total number of series and parallel resonators.


  • Absolute bandwidth / relative bandwidth

 This indicator is usually used for band-pass filters, which characterizes the frequency range of signals that can pass through the filter, and reflects the frequency selection of the filter. Relative bandwidth is the percentage of absolute bandwidth to center frequency.


  • Standing wave

It indicates the impedance matching between the filter port and the required system, and also it indicates how much of the input signal failed to enter the filter and was reflected back to the input.


  • Loss

It represents the energy lost after the signal passes through the filter, that is, the energy consumed by the filter.


  • Passband flatness

The absolute value of the difference between the maximum loss and the minimum loss in the passband of the filter, which characterizes the difference in energy consumption of filters for different frequency signals.


  • Out-of-band rejection

It is the "attenuation" outside the passband frequency range of the filter, which characterizes the filter's ability to select unnecessary frequency signals.


  • Absolute group delay

The time it takes for a signal to pass from the input port to the output port within the passband of the filter.

  • Group delay fluctuation

The difference between the maximum and minimum absolute group delay in the passband of the filter, which characterizes the dispersion characteristics of a filter.


  • Power capacity

It refers the maximum power of the passband signal that can be input to the filter.


  • Phase consistency

The difference in the phase of the transmitted signal between different filters of the same index in the same batch, which characterize the differences (consistency) between batch filters.


  • Amplitude consistency

The difference of transmission signal loss between different filters with the same index in the same batch, which represents the differences (consistency) between batch filters.

 Electronic Filter

Electronic Filter

Ⅴ. Main Characteristic Indexes of Filter

  • Characteristic frequency

① The passband cutoff frequency fp = wp / (2p) is the frequency of the boundary point between the passband and the transition band, at which the signal gain decreases to the specified lower limit.

② Stopband cut-off frequency fr = wr / (2p) is the frequency of the boundary point between the stopband and the transition band, at which the signal attenuation (reciprocal of the gain) decreases to the specified lower limit.

③ The corner frequency fc = wc / (2p) is the frequency when the signal power is attenuated to 1/2 (about 3dB). In many cases, fc is often used as the passband or stopband cutoff frequency.

Natural frequency f0 = w0 / (2p), when there is no loss in the circuit, it refers to the resonance frequency of the filter, and complex circuits often have multiple natural frequencies.


  • Gain and attenuation

The gain of the filter in the passband is not constant.

① For the low-pass filter passband gain Kp, for the ordinary filters, it refers to the gain at w = 0; for the high-pass, it refers to the gain at w → ∞; for the band pass, it refers to the gain at center frequency.

② For the band-stop filter, the stop-band attenuation should be given, and the attenuation is defined as the inverse of the gain.

③ The change amount of the passband gain △ Kp, refers to the maximum change amount of the gain at each point in the passband. If △ Kp is in dB, it means the variation of the gain dB value.


  • Damping coefficient and quality factor

The damping coefficient is a characterization of a filters damping effect on a signal with an angular frequency at w0, and is an indicator of energy loss in the filter.

The reciprocal of the damping coefficient is called quality factor, and is an important indicator of the frequency selection characteristics of the valence bandpass and bandstop filters, Q = w0 / △ w, where △ w in the formula is the 3dB bandwidth of the bandpass or bandstop filter, w0 is the center frequency, and in many cases the center frequency is equal to the natural frequency.


  • Sensitivity

The filtering circuit is composed of many components, and changes of parameter values of each component will affect the performance of the filter. The sensitivity of a certain performance index y of the filter to the change of a certain component parameter x is recorded as Sxy, which is defined as: Sxy = (dy / y) / (dx / x).

This sensitivity is not the same concept with the sensitivity of measuring instruments or circuit systems. The smaller the sensitivity, the stronger the fault tolerance of the circuit, and the higher the stability.


  • Group delay function

When the filter's amplitude-frequency characteristics meet the design requirements, in order to ensure that the output signal distortion does not exceed the allowable range, certain requirements should be put forward for its phase-frequency characteristics ∮(w). In filter design, the closer the group delay function d∮ (w) / dw is to a constant, the smaller the signal phase distortion.

 Low-Pass Electrical Filter

Low-Pass Electrical Filter

Ⅵ. Filter Installation Considerations

Although the on-board filter is not ideal for high-frequency filtering, if properly applied, it can meet the electromagnetic compatibility requirements of most civilian products. To avoid misuse, pay attention to the following rules:


  • Clean ground

If you decide to use the on-board filter, pay attention to leave a "clean ground" at the cable port when wiring, and the filter and connector are installed on it. From the previous discussion, it can be seen that the interference on the ground line of the signal is very serious. If the filter capacitance of the cable is directly connected to this kind of ground, it will cause serious common-mode radiation problems. In order to obtain better filtering effect, a clean ground must be prepared, and the signal ground can only be connected at one point. This circulation point is called a bridge. All signal lines pass through the bridge to reduce the area of the signal loop.


  • Side-by-side setting

The unfiltered parts of the wires in one group, and the filtered parts is another group. Otherwise, the filtering part of one wire will re-contaminate the unfiltered part of the other wire, making the overall filtering parts invalid.

  • Close to the port of the cables

The distance between the wave guide and the panel should be as short as possible. If necessary, use a metal plate to block it to isolate field interference.


  • Lapping of the filter with the chassis

The dry ground where the filter is installed must be reliably overlapped with the metal chassis. If the chassis is not metal, a larger metal plate must set under the circuit board as the filtering ground. The connection between ground and the metal case should ensure a very low RF impedance. If necessary, use electromagnetic liner connection to increase the overlap area and reduce the RF impedance.


  • Short ground wires

The inductance effect of the pins must be considered, especially, partial wiring of the filter and the connection structure between the circuit board and the chassis (metal plate) should be paid special attention.

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    • Mama on 2019-12-26 18:06:39

    Recently, the market for dielectric filters has developed well. Its main advantages are large power capacity and low insertion loss, but there are two major disadvantages: first, the volume is large, (in centimeters), which occupies a lot of space compared to integrated circuits. Second, it is a discrete device and cannot be integrated with the signal processing circuit, and there is signal processing chip that needs to pass through a non-negligible transmission line. So impedance matching must be performed, therefore, the circuit has a complicated structure and signal attenuation somehow.

    • Ebby on 2019-12-28 17:13:31

    If the connection between the filter case and the device case is poor, it is equivalent to a distributed capacitance between them. This will cause the filter to have a large ground impedance at high frequencies, where the distributed capacitance and distributed inductance resonate, the ground impedance tends to infinity.

    • RachelYe on 2020-1-7 17:59:18

    The most commonly used filters are low-pass and band-pass. Low-pass is widely used in image suppression of the mixer section and harmonic suppression of frequency source. Band-pass is widely used in front-end signal selection of receivers, spurious suppression after transmitter power amplifier, and frequency source dispersion suppression.

    • Justin on 2020-1-10 17:14:34

    If the filter is poorly grounded and the grounding impedance becomes large, some interference signals will pass through the filter. In order to solve this, the insulation paint on the chassis should be scraped off to ensure a good electrical connection between the filter housing and the chassis.

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