IIR filter

GNU Radio
1

Does anyone have a working example of the IIR filter block in GNU
Radio Companion that I can see, and the filter designer app, table, or
equation which generated the coefficients?

The GRC documentation states “fftaps and fbtaps must have equal
numbers of taps”, however biquad IIRs traditionally have three B
coefficients (feed forward) and 2 A coefficients (feed backward). This
conflicts with the ‘Direct Form 1’ shown here:

Is b0 already implied to be set to 1 automatically, with the
expectation that I would normalize all values to b0?

Or, do I need to specify b0, then for the A coeffs put a zero as a
placefiller?

Negating A coeffs doesn’t appear fix the issue in either case.

Also, I noticed that if I set an unequal number of coefficients (in
violation of the docs) there still is no error. (it still doesn’t work
though)

2

Does anyone have a working example of the IIR filter block in GNU
Radio Companion that I can see, and the filter designer app, table, or
equation which generated the coefficients?

Could you try grc files attached in
Re: [Discuss-gnuradio] GSoC: Filter Design tool update.
I think those were tested with gnuradio-3.6.1.

You can either use gr_filter_design tool (new version) or scipy
functions (scipy.signal.iirdesign/scipy.signal.iirfilter) for designing
IIR filters.

Negating A coeffs doesn’t appear fix the issue in either case.

To know more about the negation of “a coeffs” please follow this thread:
IIR Filter taps - GNU Radio - Ruby-Forum.

-Sreeraj

3

Thanks for the example links, they were very helpful. There were two
things I was doing wrong: First I didn’t use square brackets for the
sets, and to use the biquad calculator featured on earlevel.com, the B
coefficients needed to start with a 1.0 filler and the rest negated
like this:

Feed-forward taps: [a0,a1,a2]
Feedback taps: [1,-b1,-b2]

The linked .grc files work for me. (GRP 4.6.1) The filters therein
behave as expected.

Where is the new version?
https://github.com/gnuradio/gnuradio/blob/next/gr-filter/python/design/filter_design.py
This one turns my cursor into a crosshair and does nothing else until I kill it.

Try launching gr_filter_design in apps
(https://github.com/gnuradio/gnuradio/tree/next/gr-filter/apps/)

The link you sent mentioned a new version but I haven’t found it;
googling “gr_filter_design” doesn’t bring it up. I just see the
FIR-only versions.

Default “gr_filter_design” in “next” branch supports IIR filter design.
Also have a look at examples folder (gr_filtdes_*).

-Sreeraj