[sdiy] Sallen-Key Bessel Low Pass - behaviour and RC values

Terry Shultz thx1138 at earthlink.net
Thu Feb 6 17:59:39 CET 2014


Hi All,

If you could tell me the parameters of the Filter design in question
I could build you the Filter on my Momentum Data Systems QED 2000 tools and send back
a set of files for you to try out on your MICRO/DSP UNIT.

WHAT PROCESSOR? 

LowPass from your note

Is this an analog design or digital or both?

WHAT FREQUENCIES are we exploring , cutoff, bit width 16, 24, or 32bit?

any details would be handy to enable your filter design 

regards,

Terry
On Feb 6, 2014, at 6:03 AM, Scott Gravenhorst <music.maker at gte.net> wrote:

> 
> Justin Owen <juzowen at gmail.com> wrote:
>> Hello,
>> 
>> I've been asked to help set up an anti-alias (input) and 
>> reconstruction (output) filter for a micro-based project a friend 
>> is working on. I'll be using a Bessel type Sallen-Key Low Pass 
>> config for both - but I had some initial questions. 
>> 
>> In a general sense, my understanding is that the real world 
>> differences in behaviour/performance between Bessel and 
>> Butterworth in a SallenKey config are mainly down to Q and 
>> cutoff? 
>> 
>> ...and the differences in component selection is that for a 
>> Butterworth topology there is a 2:1 ratio between the values of 
>> C1 and C2 (or C2 and C ATCMB...) and for a Bessel C1 = C2. 
>> 
>> Is that right?
>> 
>> Secondly, there seems to be two schools of thought on whether to 
>> set the filter cutoff frequency at Nyquist (which I believe is 
>> currently 96KHz) or around the highest frequency the DAC will 
>> output (in our case I believe this is about 16KHz). 
> 
> I will assume this is an audio project.  As such, I don't believe we need much
> more bandwidth than that of human hearing.  As such, a cut off of 96 kHz seems a
> bit extreme.  I would put cutoff somewhere between 16 kHz and 20 kHz.
> 
>> Any thoughts on this from those who've been round this block before?
>> 
>> Lastly - I hope you guys don't me asking this on list but I'd be 
>> curious to hear from Tom W. and Olivier G. about their RC 
>> selections of 12K/10nF (plus some gain?) and 39K/100pF - and 
>> anyone else on there fave or suggested RC values. 
>> 
>> Thanks!
>> 
>> - J
>> 
>> 
>> 
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> 
> -- ScottG
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