[sdiy] Fixed filter bank questions
David G Dixon
dixon at mail.ubc.ca
Sat May 11 18:12:14 CEST 2019
As I will be building this by hand, and I have no plans to make more than
one or two (although I'm always open to Wigglers sending me PMs and asking
me to make something), I will be hand-binning the capacitors.
I don't trust my DVM to measure capacitance -- it seems to add a constant
value to each reading. The way I measure caps is that I have built a
relaxation oscillator (actually a triple unit) with precise resistor values
and with sockets for plugging in caps. I measure the frequency and then
calculate the true cap value in Excel, and separate the caps into 1% bins
(contained in separate little Lee's Electronics baggies, of which I have at
least 1000). In the case of this FFB, the actual value of the caps isn't
terribly important -- I built that oscillator thingy for the
phase-displacement network I designed for my frequency shifter project, and
it worked great (and allowed me to eliminate 12 multiturn trimmers from the
board and a whole lot of tuning from the build). In that case, I only used
1, 10 and 100n caps (four of each value) so the binning took a few minutes.
So, I'll probably buy 100 of each E6 value from Digikey, and when they
arrive I'll sit down and just measure them all and bin them. It'll take a
couple hours. I'll listen to music and think about circuits while I do it,
so no big deal.
As to the original question:
> > Yves did it a different way. He used the same three resistor values in
> > every filter (R2 = 47k, R1A = 22k, and R1B = 1.8k), and used capacitors
> > in parallel to find two capacitors which add up to the correct capacitor
> > value for each frequency. This means that his resistor choices are very
> > convenient, but each dual filter requires 8 capacitors at two different
> > sizes, rather than just 4 capacitors of a single size. He has
> > himself to the six most common standard capacitor mantissas (10, 15, 22,
> > 33, 47, 68) in selecting these sizes. The frequency errors he gets
> > (ignoring capacitor tolerances) are as high as 3.7%, with an average of
> > 1.4%.
I think that this decision was done by recognizing that resistors are
much tighter speced than capacitors, so combining two capacitors of
values gives you a simple way of reducing the effective tolerance of the RC.
Unless you plan on producing in volume (where indeed you'd need to consider
the extra cost of having to have two dozen more feeders on the pick and
machine and inventory of oddball values you might not be using up soon in
other projects), I don't really see much of a difference between your idea
the way Yves is doing it. If you actually measure the capacitors before you
order the resistors to match those values, your's might even be preferrable.
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