Hi there, I have a question that has left me rather confused and one which I can't find a way to figure out from the lecture notes either.

A 4th order hihg-pass filter with a cut-off frequency of 1000 Hz is required. What is the maximum frequency which gives an attenuation of at least 96dB (ie. -96dB) below the pass band?

I've found the inverse of the 96dB, which comes to be a very large number (3,981,071,706), so this gives me the gain again..but I don't know what to do next?

Any pointers would be great, thanks.

 

I thought an ideal Butterworth filter gives an attenuation of 20 dB/decade for each "order" (?), so that would let you calculate the rolloff of the gain for a fourth order filter going down in frequency below the cutoff frequency.

 

Ah yes I forgot about that. I know what to do now thanks

 

Remember to account for the attenuation at the corner frequency being 3 dB below that of the passband.

 

Remember to account for the attenuation at the corner frequency being 3 dB below that of the passband.

I suspect the question means "96dB below maximum passband gain", which makes the 3dB attenuation at the corner frequency irrelevant.