Generic Filter

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Generic Filter

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The Generic Filter can be used to design a filter of a specific order, type and characteristic shape.

Filter Characteristics

The filter can have a Bessel, Butterworth or Chebychev characteristic shape. Each characteristic has its own advantages and disadvantages; it depends on the purpose of the filter what to choose here:

 

A Bessel filter is maximal flat in both the pass band and the stop band. It is used when the phase response should be nearly linear throughout the pass band, i.e. the group delay is almost constant throughout the pass band. This preserves the wave shape of filtered signals in the pass band. A disadvantage is the low roll-off steepness. Often a high filter order is required to obtain the desired roll-off.

 

A Butterworth filter is maximally flat in the pass band and monotonic overall. As a consequence of this smoothness, the roll-off steepness is rather low. As with Bessel filters, a rather high filter order is required to obtain the desired roll-off.

 

A Chebychev 1 filter has a ripple in the pass band and is monotonic in the stop band. This filter rolls off faster than Bessel, Butterworth and Chebychev 2 filters at the same filter order. A disadvantage is the greater deviation and discontinuous phase response in the pass band.

 

A Chebychev 2 filter is monotonic in the pass band and has a ripple in the stop band. This filter rolls off faster than Bessel and Butterworth filters at the same filter order. Not as fast as a Chebychev 1 filter, but it is free of a pass band ripple. A disadvantage is the ripple and discontinuous phase response in the stop band.

Filter Type

A low pass filter will filter out high frequencies and let low frequencies pass the filter.
A high pass filter does the opposite; it will filter out low frequencies and let high frequencies pass the filter.
A band pass filter will filter out low and high frequencies and let middle frequencies pass the filter.
A band stop filter does the opposite; it will filter out the middle frequencies and let low and high frequencies pass the filter.

Filter Order

The filter order specifies how steep the filter will roll off at a specified filter frequency. The higher the order, the steeper the roll off will be. Note that the filter order also has a large influence on the phase response of the filter.

Frequency f1

For a low pass filter f1 is the frequency in Hz where the roll off starts. For a high pass filter it is the frequency where the roll off ends. For a band pass or band stop filter f1 is the start frequency of the band.

Frequency f2

For a band pass or band stop filter f2 is the stop frequency in Hz of the band.  It is disabled for low pass and high pass filters.

Max. Ripple R

Only available when a Chebychev 1 filter is selected. R specifies the maximum allowed ripple in dB for the filter in the pass band.

Stop Band R

Only available when a Chebychev 2 filter is selected. R specifies the distance of the stop band to the unity gain axis in dB.  This is the amount of attenuation in dB of the filter in the stop band (the absolute value is used).