In theory, all hams are slim, handsome, great conversationalists at
any cocktail party, and have over-achieving kids. In practice, ....

In theory, all capacitors are pure and stable, strictly obeying the
impedance formula, x = 1/2piFC. In practice, they change value with
temperature, they have lossy dielectrics, they have resistive
contacts, and they also have a bit of inductance in the leads and
plates.

If you remember the impedance formulas from your ham exam, you will
recall that the impedance of a fixed value of capacitance decreases as
you increase the frequency of the signal you are passing through it.
The impedance of a fixed value of inductance increases with frequency.
What happens, as you increase the frequency of the signal, to the
impedance of a typical ceramic capacitor which has a little bit of
stray inductance? At low frequencies, the inductive part contributes
very little impedance, so the capacitive impedance predominates --
that is, it behaves like a capacitor! As you increase the frequency,
the capacitive impedance decreases and the inductive impedance
increases. Would there be a particular frequency at which the
capacitive impedance equals the inductive impedance? Absolutely! And
if you increase the frequency still further, the inductive impedance
becomes higher than the capacitive impedance, and your poor capacitor
behaves just like an inductor! The frequency at which both impedances
are equal is known as the self-resonant frequency. This frequency is
set by the materials used and the construction of the capacitor. It
can also be affected by how the capacitor is installed, which is why
all the kit instructions tell you to mount the cap very close to the
board, with the minimum lead length necessary. In general, for
capacitors of the same type and general construction, the larger value
cap will have a lower self-resonant frequency.

The ideal bypass component would have infinite resistance at DC, and
zero impedance everywhere else. Unfortunately, this component is not
yet available, but a capacitor comes close. It has very high
impedance at DC and decreasing impedance at higher and higher AC
frequencies -- this is, until you hit its self-resonant frequency. At
that point and beyond, it becomes less and less effective as a bypass
component because its impedance is becoming larger. The capacitor is
a very effective bypass at some frequencies but useless at frequencies
somewhat above its self-resonant frequency.

How do we handle this problem? If we use a large value cap which is a
good bypass at low frequencies but useless at high frequencies, and
put a small value cap in parallel with it, the small cap will not be
very effective at the low frequencies, but since its self-resonant
frequency is much higher, it will be effective at the high frequencies
where the big cap is useless. Simple!

If you pull out some data sheets for capacitors, you will often find
graphs of this and other characteristics of the component (impedance
vs. frequency showing the self-resonant point, equivalent series
resistance, etc). Keeping all these practical limitations of real
components in mind and adapting the design to accommodate them is part
of the engineering process.

Incidentally, this is also the reason why you won't find a cell phone
built with anything other than surface mount components. The
self-resonant frequency of chip caps is much higher than leaded caps.

BTW - inductors have similar flaws. The latest QRPp has an
interesting article about the RF choke used in the Back-to-the-Future
project. It's an interesting education in the real impedance of an
inductor at HF frequencies.

Mike K1MG

Another reason I did not mention in the prior post.

Smaller caps drift in proportionately smaller steps and are somewhat self
compensating. I.e.., A 10% 10pf cap will drift up/down by 1 pf. A 10% 100 pf
cap will drift up/down by 10 pf. The ultimate result of 10x10 pf caps to
equal 100 pf may seem the same however, 5 of the 10 pf caps may drift up and
5 of the 10 pf caps may drift down resulting in a net change of zero whereas
a single 100 pf cap will always drift in one direction by the tolerance
spec. Of course, it is improbable that you will get symmetrical results like
5 up and 5 down however, the odds are that you will NOT get 10, 10pf caps
that all drift in the same direction.
( You thought this was a science? :-) )

I did not mention Silver Micas that are commonly used in VFO's. The Silver
Mica film tends to be more rigid than ceramic film and are less likely to
flex with the same intensity under the same proportionate degree of heat
however, when they do flex, they flex at a higher ratio than ceramics. Plus,
the drift direction is unpredictable. In tightly controlled temperature
environments, they *can be* more stable than an NPO ceramic. In the real
word this generally does not hold true.

Roger, hope this is somewhat an answer to your question, and decided to add
some Elmer info too, hope I don't offend you in the scope of the answer....

On the topic of Varactors, they are virtually all 'good' BUT they are not
the same as a variable capacitor.

You cannot substitute them directly and achieve equivalent performance, but
you can nearly always incorporate a varactor into a circuit and achieve
good performance if you take all the effects into account.

These different effects include:
- temp coefficient
- junction conductance
- non-linearities with respect to instantaneous voltage, that is the sum of
both the tuning voltage and the RF voltage at any time but especially the peak
- V.tune impeadance

Others have already addressed the different temperature coefficient, so
I'll leave well enough alone.

All solid state juntions suffer from one significant difference from a
'real capacitor'. IF the RF voltage is large enough the diode junction
will conduct during that portion of the cycle that exceeds the diode 'on'
voltage. This introduction of non-linear effects includes changing the RF
impeadance of the circuit, changing bias if the circuit is designed wrong,
and changing the instantaneous capacitance and hence the tuning. This is
to be avoided at all costs. sometimes you will see two varactor diodes in
series with opposite polarity. the tune voltage will be feed to the
junction of the cathodes. this is to eliminate conduction in the circuit.
(one will always be non-conducting..) Preferably, the designer accounts
for the peak voltages and ensures that the diode is not going to go into
conduction.

In the same vein. A varactor diode (ALL!!!) exhibit non-linear
relationships between V and C. This is still true with respect to the
instantaneous voltage we talked about above. The C you really get is based
on the sum of both the tuning voltage and the RF voltage at any instant of
time. This results in spurious modulation, oscillators which will start
and run for a few cycles, stop for some small portion of the RF cycle, and
then restart (this is the voice of experience, and a REAL pain to find at
Ghz freq's!!), and many other nasty behavior. Good designs will ensure
that the instanteous variation does not substantatially deviate from the
tuning point.

Although not a function of the Varactor, another potential problem
introduced by the use of a varactor is the impeadance of the tuning voltage
connection. this should be very high to ensure that no extraneous effects
(circuit loading, stray rf/audio coupling, etc) will be introduced to the
VFO. VFO's can make great mixers, product detectors, etc under the right
conditions conditions. This line (V.tune) is generally a 100k resistor to
each tuned point for this reason.