[sdiy] Fwd: Shruthi 4PM (was re something else...)

[cheater00 .]

Roman,

On Fri, Nov 29, 2013 at 11:10 AM, Roman Sowa <modular at go2.pl> wrote:
> For example in notch filter the notch comes from summing 2 signals at
> certain frequency shifted by exactly 180 degrees. So in theory this one
> frequency (Dirac spike in spectrum) should attenuate to zero. If there are
> mismatches between stages of the filter in amplitude, this will never go to
> 0, because at the frequency when there is 180 degrees difference the
> amplitudes are not the same, and at the spot when they are, relational shift
> is not 180 degrees.
> Even worse if it's 4-pole filter. Then you have to match 4 amplitudes and
> corner frequencies.
> A few simple calculation can give you exact attenuation of notch frequency
> in dB if you asume 1% deviation in parameters. Why not make a simulation of
> ideal filter with altered values?
>
> Let's go further - 0.1% resistors are not a big deal nowadays, I buy them
> for about 0.25EUR, but precision capacitors is a whole different story. OTOH
> I never needed them so far, so I may be wrong. Also you can always add a
> trimmer cap.
>
> Roman

If the capacitor is wrong, the frequency at which the filter is tau/2
out of phase will shift, however the amplitude at that point will not
change. That is, the amplitude at the new point is the same as the
amplitude would have been at the old point. So it's not an issue for
two-pole filters, while it might be an issue for higher order filters.
Selecting capacitors might be a chore, especially if you need to later
send them out to manufacturing. However, you could put say 8
capacitors in place of one, with an unpopulated solder bridge which
would put one of them in circuit. Then when the boards come back from
manufacturing, you hook up a test harness which characterizes that
pole. If separate poles are on separate boards you can then select
them out of a database. If they're all on the same board, it'll tell
you which set of capacitors will be the closest. My point is that if
you're manufacturing with pick & place, it'll likely be cheaper to
have more capacitors than to select them yourself, send them in, and
make sure the contractor knows how to place them by using elaborate
packaging and documentation.

Cheers,
D.