[sdiy] VCO saw integrator cap questions

[David G. Dixon]

I've got a couple of stupid questions about caps, specifically for a
saw-core VCO integrator.  It is difficult for me to obtain polystyrene caps,
so I would like to go with metallized polypropylene.  I have read that their
leakage properties are even superior to polystyrene.  So here are my
questions:

1) What are the most important properties for a VCO integrator cap, and why
is polystyrene considered to be so superior?  Is metallized polypropylene
just as good?  Better?  What about C0G monolithic ceramics?  They are
supposed to be extremely stable with temperature.  Why don't we use them for
VCOs?

2) (This is probably a really dumb question, but) I bought some metallized
polypropylene caps from my local electronics shop which are little yellow
boxes with 0.2" lead spacing.  However, I see pictures of synth modules on
the web with these massive yellow boxes which appear to have about 0.4" lead
spacing.  Is one superior to the other?  What type of caps are these large
yellow boxes, and why are they so big?

3) I get ceramic caps from my school, and they are these pretty little blue
things, some only about 3 mm wide and maybe 1.5 mm thick, with white
markings (I think they're EPCOS, supplied by Digikey).  They are C0G in
smaller values (up to 2n2), and typically have 0.1" lead spacing, although
some are 0.2".  However, I have never noticed these in any of the pictures
of synth modules I have seen on the web, but I usually see caps which are
much larger.  Is this just coincidence, or should I be avoiding these caps
for some reason?  I use them for coupling, bypassing, and speed-up
applications, but never for timing (integrators, filters, etc).  Could I?
Should I?  They seem to work perfectly well.

After 18 months of synth diy, I must confess that I'm still fairly confused
about caps in general.  People say to use this or that cap for this or that
application, but I'm not really sure why.  I understand that one would want
low leakage for a sample and hold application, but other than that, I'm more
or less in the dark.  Is there a good source of straight dope on the
selection of caps with specific properties for specific applications?  For
example, I read on a previous [sdiy] thread that one should use either
polystyrene or metallized polypropylene for VCOs, but polypropylene for
VCFs.  The author failed to mention why, however.  Does the metallized
variety have some deleterious effect on VCFs?  I've used silvered mica in my
filters, and they seem to work extremely well.