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

[Roman Sowa]

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

W dniu 2013-11-29 01:09, Tom Wiltshire pisze:
> Nowadays I build all my circuits with 1% resistors as standard, and I
> expect most of you do too, so I'd think of  "requires 1% resistors"
> as "needs decent parts", and probably not as "very sensitive". Still,
> Oberheim have given us a guideline there - 1% or better. The
> repeatability of your CVs and VCAs is going to have to manage that. I
> think it could be an application where a multiplying DAC would be
> ideal - repeatable CV and highly linear VCA in one. In fact, Oberheim
> themselves used one for the FM on the Xpander for exactly that
> reason.
>
> I hope these comments help you to do rather than put you off. I think
> it's a worthwhile idea, but it needs some work to make it usable.
>
> Regards, Tom
>