[sdiy] Designing 4-pole filters with identical 2-pole stages - why not?

[Mattias Rickardsson]

On 21 December 2015 at 23:54, Neil Johnson <neil.johnson71 at gmail.com> wrote:
>
>> I've been designing 4-pole Butterworth filters using 2-pole Sallen-Key sections.
>> The usual way to do this is to set the Q of the first stage to 0.541, and the Q of the second to 1.307. Multiplying one by the other gives an overall Q of 0.707, which is our Butterworth response.
>
> Hmm.... in my experience the "usual way" is to specify what you want,
> then grab a filter cookbook like Williams and Taylor or the venerable
> Zverev, read out the coefficients from the appropriate table,
> frequency and impedance scale, and there's your answer.  Not much to
> it.
>
> Or am I missing something?

If designing 4-pole Butterworth filters using 2-pole Sallen-Key
sections is to "specify what you want" and you then use tables or
other design methods to end up with the answer Q=0.541 and Q=1.307,
and you do this several times, wouldn't it be appropriate to call it
"the usual way"? :-)

/mr  -  the usual whey smoothie breakfast is kicking in ;-)