Roy Lewallen wrote in message ...
Paul Burridge wrote:

Yes, ceramics are *hopeless* for tuned circuits; I wouldn't trust the
black tipped ones, either. You can't beat silver mica but they're a
bit hard to find and expensive.

I strongly disagree with this. I've successfully used ceramic capacitors
many times for both high and low Q tuned circuits from HF to UHF. Years
ago, I found that NPO ceramics were decidedly superior to silver micas
for temperature stability, so I use them exclusively for VFO tank circuits.

....

In agreement and support of what Roy wrote, I'd toss out some
additional notes:

o I've seen (been the vicitm of?) silvered micas that exhibit random
tiny jumps in capacitance, which is a really bad thing in oscillators.

o You can get X7R dielectric SMT caps in SMT up to at least 0.1uF in
0603 size, for low voltage ratings. See manufacturers' data sheets
for the largest currently available values. I suspect reliability
suffers if you try to use ones with too high a C*V/unit volume,
though.

o As someone recently pointed out to me (Sphero, I think it was), you
can get C0G up to 0.1uF in SMT--I think he said in 1206 size.

o It's possible to get 1% C0G caps.

o A C0G is probably about the cheapest electronic component you'll
find with a maximum 30ppm/C temperature coefficient (though beware,
some C0Gs I've seen are rated up to 60ppm/C).

o There are special high-Q ceramics that are better than standard
C0Gs for use at microwave frequencies...generally above 1GHz.

Cheers,
Tom