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I have designed this bandstop filter to a desired specification:

Stopband attenuation = 40db Passband ripple = 2db etc ...

enter image description here

I understand that the stopband ripple can be given by the following:

Stopband ripple = −20log10(δs) 40 = −20log10(δs) δs=0.01

therefore stop-band ripple δs = 0.01. where I am confused is how this value can be converted into dB so that it can be compared to the stop-band ripple seen in the MATLAB simulation (which is in dB). 40dB is the stopband attenuation, not the stopband ripple (or is it?). the simulation is showing a stopband ripple of around 32dB.... is the stopband attenuated and ripple with same ? what am I missing ?

thank you for your time

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  • $\begingroup$ For most filters you specify the Stopband attenuation, and the Passband Ripple - not the ripple in the Stopband. Are you sure you're not getting mixed up between the two? $\endgroup$
    – David
    May 17 '18 at 18:18
  • $\begingroup$ thats exactly what I am explaining, I am confused with regards to the stopband ripple. what is the expected stopband ripple size? $\endgroup$ May 17 '18 at 18:20
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I understand that the stopband ripple can be given by the following:

Stopband ripple = −20log10(δs) 40 = −20log10(δs) δs=0.01

Where does that come from? I have never heard the term "stopband ripple".

Stopband attenuation of 40 dB means that in the stop band any level of -40dB OR LESS is allowed. How much less than -40dB or where it hits -40dB isn't a controlled variable. -50dB, -60dB etc all are fine.

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