[sdiy] Ladder filters and gain drop, that old chestnut

Andrew Simper andy at cytomic.com
Mon Aug 31 07:04:12 CEST 2015


On 31 August 2015 at 12:05,  <mskala at ansuz.sooke.bc.ca> wrote:
> On Mon, 31 Aug 2015, Andrew Simper wrote:
>> And a point to note is the half power point is not -3 dB exactly, as
>> dB isn't exactly related to powers of 2:
>>
>> 20*log(10, sqrt(1/2)) ~= -3.0102
>>
>> where ~= means approximately equal to.
>
> Are your filters built with components accurate enough, and
> temperature-stable enough, for a difference of 0.01dB in the response
> curves to be meaningful?
> --
> Matthew Skala
> mskala at ansuz.sooke.bc.ca                 People before principles.
> http://ansuz.sooke.bc.ca/

When working out the equations it is much easier to use exact values
rather than dB, the solutions come out much neater!

If you are interested in actual filter shapes you are best off working
out the exact equations and then you can do monte-carlo analysis by
randomising the component values and working out a realistic
difference from the ideal values, and this can help choose filter
topology and component tolerances that fit with your requirements.

Cheers,

Andy


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