[sdiy] Help with Timing Capacitor types.

[Mike Beauchamp]

There must be as many opinions on what type of capacitor to use for
timing caps as there are types of capacitors! The general consensus I
get is to not use mylar, but to use Polystyrene (as stated in the
guide that has been posted on here several times according to the
Archives). Digikey and a few other places don't seem to have
Polystyrene capacitors at all, let alone .047uF ones. Several VCO
circuits online suggest Silver Mica as an alternative, but I can't
find anything over .01uF on Digikey (and that thing is $6!).

So I went through ever type of capacitor that digikey sells, reading
the datasheets. Out of them all, 2 of them mentioned excellent
temperature stability and recommended for use in oscillator circuits.
I don't know how applicable this is for specific use as timing caps in
VCO's and stuff, but here's what I found:

Polypropylene Film
eg: panasonic ecqp(z) polypropylene film.
Datasheet states: "excellent temperature and frequency characteristics
excellent long termp stability.
recommended: dolby circuits, oscillation circuits"
1% and 2% tolerances.

Polyphenylene Film
eg: panasonic echs(z) pps film capacitor
datasheet says: "wide operating temperature, excellent temperature
coefficient - 100ppm/C
excellent frequency characteristics
recommended: automotive, oscillation circuits."
2% and 5% tolerances I BELIEVE.

Those were the only two types of non SMD caps that mentioned anything
in their datasheet about temperature stability and recommended for use
in oscillation circuits.

I noticed a few webpages recommending Polyester Film as a substitute
for Polystyrene. However, none of their datasheets talk about
temperature stability or recommend use in oscillators.

Polyester Film
eg: panasonic ecqv(l)
"excellent electric characterists in non-inductive construction
recommended: general purpose, noise suprpession in logic circuit"

Based on this research, I'm planning on trying some Polypropylene Film
and Polyphenylene Film capacitors. Just wondering if anyone has any
reasons that I may be overlooking on why not to use these types?

All of these opinions and the odd thing is that the VCO I'm building
lists the timing capacitor as Mylar on the parts-list! Maybe I should
calm down and just use a mylar cap (as it does seem to be working on
my prototype, although I haven't measured how much it drifts over
temperature, etc) since that's waht the design calls for? Maybe the
temperature compensating resistor value is based on the drift that a
mylar capacitor is expected to do and by replacing it with something
else, I'll end up with something less table? Anyways, sorry for
rambling..
-- 
Mike Beauchamp
http://mikebeauchamp.com