A filter is a filter circuit consisting of capacitors, inductors, and resistors. The filter can effectively filter out the frequency point of a particular frequency in the power cord or frequencies other than that frequency point to obtain a power signal for a specific frequency, or to eliminate the power signal after a specific frequency.
A filter is a frequency selection device that allows a specific frequency component in a signal to pass through and greatly attenuated other frequency components. Using this frequency selection effect of the filter, interference noise can be filtered out or spectrum analysis can be performed. In other words, any device or system that allows a particular frequency component in a signal to pass through and greatly attenuated or inhibits other frequency components is called a filter. A filter is a device that filters waves. "Wave" is a very broad physical concept, in the field of electronic technology, "wave" is narrowly limited to the specific description of the various physical quantities of the value of the process of fluctuations over time. The process is converted into a time function of voltage or current, called a time waveform of various physical quantities, or a signal, by the role of various sensors. Because the argument time is taken continuously, it is called a continuous time signal and is habitually referred to as an analog signal.
Filtering is an important concept in signal processing, the role of the filter circuit in the DC regulatory power supply is to minimize the ac component of the pulsation of the DC voltage, retain its DC component, so that the output voltage ripple coefficient is reduced, the waveform becomes relatively smooth.
Key parameters of the filter:
1、Center Frequency: The frequency f0 of the filter passband, generally taking f0 s (f1 plus f2)/2, f1, f2 for the band pass or band resistance filter left and right relative drop 1dB or 3dB edge frequency points. Narrowband filters often calculate passband bandwidth at a frequency centered on the smallest point of interpolation.
2、Cutoff Frequency: Refers to the right frequency point of the pass band of a low-pass filter and the right band of a high-pass filter. Typically defined as a relative loss point of 1dB or 3dB. The reference reference for relative loss is: low pass is based on DC interpolation loss, while Qualcomm is based on sufficient high pass band frequency where parasitic band does not appear.
3、Passband bandwidth: refers to the spectrum width that needs to be passed, BW=(f2-f1). f1, f2 is based on the insertion loss at the center frequency f0.
4、Insertion Loss: Due to the attenuation of the original signal in the circuit due to the introduction of the filter, the loss is indicated at the center or cut-off frequency, as highlighted if full-band interpolation loss is required.
5、Ripple: The peak of a 1dB or 3dB bandwidth (cut-off frequency) range in which the insertion loss fluctuates with the frequency on the basis of the loss mean curve.
6、In-band fluctuations (Passband Ripple): The amount of in-band insertion loss varies with frequency. The in-band fluctuation within the 1dB bandwidth is 1dB.
7、In-band resident wave ratio (VSWR): An important measure of whether the signal in the filter passband is well matched for transmission. Ideal match VSWR is 1:1, VSWR is greater than 1 when mismatched. For an actual filter, the bandwidth that meets VSWR less than 1.5:1 is generally less than BW3dB, and its proportion to BW3dB is related to filter order and interpolation.
8、Return Loss: The number of decabytes (dB) of the ratio of port signal input power to reflected power, which is also equal to 20Log10ρ,ρ is the voltage reflection factor. The echo loss is infinite when the input power is fully absorbed by the port.
9、Band suppression: an important measure of filter selection performance. The higher the indicator, the better the suppression of the out-of-band interference signal. There are usually two types of deductions: one is to require how much dB is suppressed for a given out-of-band frequency fs, the other is to suggest an indicator of how close the filter amplitude response is to the ideal rectangle - the rectangular coefficient (KxdB greater than 1), KxdB,BWxdB/BW3dB, (X can be 40dB, 30dB, 20dB, etc.). The higher the filter order, the higher the rectangle size - that is, the closer K is to the ideal value 1, the more difficult it will be to make.
10、Delay (Td): refers to the time it takes for a signal to pass through the filter, numerically an indicator of the vernal frequency of the transmission phase function, i.e. Td=df/dv.
11、In-band phase linearity: This indicator shows the phase distortion size introduced by the filter to the transmission signal in the passband. Filters designed by linear phase response functions have good phase linearity.