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.

You might have come across some bad parts. Or perhaps you don't realize
that many different types of ceramic are used for making capacitors.
Three general classes are most common. The "general purpose" class
(something of a misnomer) is a very high-k ceramic used for capacitors
of Z5U, Y5V and similar types. Those are very good for bypassing because
of their small physical size, but terrible for nearly anything else.
They're microphonic, hygroscopic, piezoelectric, and highly temperature
and voltage dependent.

Another class is used for X7R and related types, sometimes called
"stable". These are much more stable in all respects, but are physically
larger due to the lower dielectric constant (k) of the ceramic. They're
suitable for a wider variety of uses, but still not for high Q tuned
circuits. They should be used in low Q circuits only after evaluating
the potential effects of temperature and voltage dependence, at the least.

The third common class is used for making capacitors with near-zero
temperature coefficients, such as C0G (formerly and still often called
NPO). These *are* an excellent choice for tuned circuit applications,
and are often better than silver mica (whose temperature coefficient and
Q are variable and unpredictable). They're sometimes identified with a
black dot -- on "dog bone" capacitors, it's on one end.

Many years ago I published an article describing a stable, VFO
controlled QRP transceiver which used NPO ceramic capacitors for the
oscillator tank. Scattered feedback indicated that some people had
gotten "NPO" capacitors from less-than-reliable vendors and were
experiencing an objectionable amount of frequency drift. That didn't
happen with name-brand parts. So there are some flakey capacitors out
there. If you need to depend on the quality, get name brand parts from a
reputable vendor. And you'll find that NPO ceramics are a very good
choice for tuned circuits.

Roy Lewallen, W7EL