I have been looking to design an active elliptic filter for a dac project that I have been working on to speed up audio at a specified multiplier. The dac can speed 44.1 audio all the way up mhz sample rate ranges. I was wondering if anyone can recommend a tool to help design an active elliptic reconstruction filter?
Previously I have attempted to implement a passive elliptic filter stage using the rf-tools elliptic filter calculator but struggled with the very specific inductor and capacitor values required for said filter. Implementing a passive filter multiple times as required for the project would be a complex undertaking so I'm thinking an active stage would be easier to implement and also be more repeatable when needed? If my target sample rate was 2.8224 mhz images will start appearing at around half of the sample rate so I'm guessing a filter utilising a 1.3mhz cut off would be sufficient to suppress the dac images? My maximum theoretical frequency would be around 1.28mhz. If anyone has any pointers I am eager to listen.
Previously I have attempted to implement a passive elliptic filter stage using the rf-tools elliptic filter calculator but struggled with the very specific inductor and capacitor values required for said filter. Implementing a passive filter multiple times as required for the project would be a complex undertaking so I'm thinking an active stage would be easier to implement and also be more repeatable when needed? If my target sample rate was 2.8224 mhz images will start appearing at around half of the sample rate so I'm guessing a filter utilising a 1.3mhz cut off would be sufficient to suppress the dac images? My maximum theoretical frequency would be around 1.28mhz. If anyone has any pointers I am eager to listen.
The audio range is up to 20 kHz regardless of the sampling rate. You don't need different filters for different sampling rates, just one that cuts above 20 kHz.
This project is for a high speed duplication master so I do need to preserve frequencies at the selected audio multiplier. The project requires a separate reconstruction filter for each multiple of 44.1 khz but I am wanting to implement a fixed speed reconstruction filter before moving on to multiple reconstruction filters for different speeds. The software I have been working on speeds up the pcm data depending on the multiplier used. So if I choose x16 speed the maximum frequency of 20khz is now 320khz or 1.28mhz at 64x.
Does the tool you use for the passive filter spit out a list of pole and zero locations? If so, you only need to know how to make an active filter with those pole and zero locations, which is easier than starting from scratch. Besides, there are methods to transform an LC filter into an active filter.
MATLAB, but not free. However, Octave is often cited as a free equivalent.
These can make filters using library functions, return poles and zeroes, plot gain/phase, group delay, step response etc. The results may then be automatically transformed to an active (opamp RC) filter, as MarcelvdG said above.
Python's scipy.signal module has all the stuff for filter design both analog and digital, for free, and portable...
https://docs.scipy.org/doc/scipy/reference/signal.html
I believe they are closely modelled on the Matlab stuff.
Right, I think the OP should be looking at the following if using Python.
ellip(N, rp, rs, Wn[, btype, analog, output, fs]) : Elliptic (Cauer) digital and analog filter design.
ellipap(N, rp, rs): Return (z,p,k) of Nth-order elliptic analog lowpass filter.
ellipord(wp, ws, gpass, gstop[, analog, fs]): Elliptic (Cauer) filter order selection.
ellip(N, rp, rs, Wn[, btype, analog, output, fs]) : Elliptic (Cauer) digital and analog filter design.
ellipap(N, rp, rs): Return (z,p,k) of Nth-order elliptic analog lowpass filter.
ellipord(wp, ws, gpass, gstop[, analog, fs]): Elliptic (Cauer) filter order selection.