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.