Filter Basics (2)
Filter specifications
The main parameters of the filter: insertion loss, return loss, passband bandwidth, in-band ripple, ripple, in-band VSWR, center frequency, cut-off frequency, delay, in-band phase linearity.
1. Insertion Loss:
Due to the attenuation of the original signal in the circuit caused by the introduction of the filter, the loss is characterized by the loss at the center or cut-off frequency, and the full-band interpolation loss needs to be emphasized. Insertion loss is the main index parameter to measure the filter, but the general filter manufacturer cannot obtain the source impedance and load impedance of the actual application environment, so the insertion loss of the filter is tested in the impedance environment of 50Ω (refer to CISPR17) during design and factory testing. Common mode and differential mode are tested as follows:
The filter insertion loss can also be simulated as the ratio of the signal voltage converted from the source to the load to the signal voltage converted from the source to the load when the filter is added:
Filter insertion loss simulation curve
2. Return Loss:
Also known as reflection loss, it is a parameter that indicates the reflection performance of the signal, and the return loss indicates that the part of the incident power is reflected back to the signal source. The return loss is mainly caused by the impedance mismatch in the link, and the relationship between the return loss and the reflection coefficient is as follows:
The greater the return loss, the better, to reduce the effect of the reflected signal on the system, and the return loss is infinity when the input power is fully absorbed.
3. Impedance characteristics:
Select the filter based on the source impedance and the load impedance:
The filter should meet the requirements of the working frequency of the load circuit and the frequency to be suppressed. At the required frequency, the impedance of the filter must match the impedance of the interferer and the load impedance to which it is connected, and if the interferer source is a high impedance, the output impedance of the filter should be low impedance; If the interferer is low impedance, the output impedance of the filter should be high impedance.
4. Other parameters:
Latency (TD):
Refers to the time it takes for the signal to pass through the filter, which is numerically the derivative of the diagonal frequency of the transmission phase function, i.e., TD=DF/DV.
Ripple:
Refers to the peak-to-peak value of the insertion loss fluctuating with frequency on the basis of the average loss curve within the 3dB bandwidth (cut-off frequency).
Pass Band Ripple:
The amount of change in insertion loss within the passband with frequency. The in-band fluctuation within the 1dB bandwidth is 1dB.
In-Band Standing Wave Ratio (VSWR):
An important metric to measure whether the signal within the passband of a filter is well matched for transmission. Ideal match VSWR=1:1, mismatch <1. Where the incident wave and the reflected wave are in the same phase, the voltage amplitude is added to the maximum voltage amplitude VMAX, forming the wave belly. At the opposite phase of the incident wave and the reflected wave, the voltage amplitude is reduced to the minimum voltage amplitude VMIN, forming a wave node. The amplitude value of the other points is between the belly of the wave and the node, and this resultant wave is called a standing wave, and the VSWR ratio is the ratio of the voltage amplitude VMAX at the belly of the standing wave to the voltage amplitude VMIN at the node.
In-band phase linearity:
This metric characterizes the magnitude of the phase distortion introduced by the filter to the transmitted signal in the passband. The filter designed according to the linear phase response function has good phase linearity.
Cutoff frequency:
It refers to the frequency on the right side of the passband of the low-pass filter and the left-hand side of the passband of the high-pass filter, which is usually defined as a relative loss point of 1dB or 3dB. The relative loss is based on the insertion loss at DC for low-pass and for high-pass at sufficient high-frequency band frequencies where there is no parasitic stop band.
Center Frequency:
The frequency of the filter passband is f0, generally f0=(f1+f2)/2, and f1 and f2 are the side frequencies of the bandpass or bandstop filter that are relative to each other by 1dB or 3dB relative to the right. Narrowband filters often calculate the passband bandwidth at the center frequency of the minimum insertion loss.
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Filter installation requirements
1. Requirements for the installation position of the filter:
When a disturbance source affects multiple sensitive devices, the filter is placed close to the disturbance source. Conversely, when there is only one sensitive device and there are multiple sources of nuisance, the filter is placed close to the sensitive device. The advantage of placing the filter close to the disturbance source makes the interference confined to the vicinity of the disturbance source, cuts off the interference path of the disturbance source, and suppresses the emission of the disturbance source.
The filters on the PCB should be installed at the interface, and when multiple filters need to be installed, they should be installed in parallel to prevent coupling before and after the filtering of different signal lines; When there is only one interference source at the signal interface, the filter should be installed as close to the disturbance source as possible.
2. Filter output input wiring requirements:
The input and output lines of the filter must be shielded and isolated, and the input and output wirings will be coupled to each other when they are close to each other, reducing the filtering effect of the filter.
Other signal wiring should avoid being placed close to the interfering signal wiring that needs to increase the filter, so as to avoid coupling the interfering signal source to other signal wiring, and reduce the filtering effect of the filter.
The filtered signal cloth should avoid being close to the strong interference signal wiring, so as to avoid the secondary pollution of the signal line after filtering and reduce the filtering effect of the filter.
3. Grounding requirements of the filter:
The filter is grounded through a fine ground wire, and the high-frequency filtering effect is very poor, so that the grounding shell of the filter is well overlapped with the metal structure plane to improve the grounding effect.
The grounding point of the filter should be static (the ground plane without pollution by strong interference sources), the grounding wire should be as short and thick as possible to reduce the parasitic inductance of the ground, and the grounding point should be selected as the point with the smallest area of the loop of the backflow interference source.