On Wed, 23 Nov 2005 06:23:58 -0800, Roy Lewallen
wrote:

RST Engineering (jw) wrote:
Agreed, John, 150 MHz is damned near DC for a lot of us, but as yet I have
no answer from the "spice" folks for the 1 nf capacitor question. A lot of
fancydancing but no answers.

Jim

I didn't realize that this question was addressed to all "'spice'
folks", but was directed only to the OP. That's probably why the
"'spice' folks" haven't leapt to respond. Anyone who uses SPICE
professionally, and indeed a great number of amateur users should be
able to answer the question easily.

At 150 MHz, a capacitor lead can generally be handled as a single lumped
inductance. The amount depends on the wire's diameter and environment,
but around 7 nH is a good working number for a half inch lead. The
inductance of the capacitor body itself might have to be included in the
model if high accuracy is important. Capacitance to ground, from both
the leads and the capacitor body, might also be important if the
impedance of the circuit to ground is high. If so, it can be included.
The way I'd approach inclusion of the C would be to calculate the
impedance and length of the transmission line comprised of the lead and
ground plane or capacitor body and ground plane, then convert those
values to a single series L and shunt C rather than just directly using
a transmission line model -- very short transmission lines in a model
can greatly slow SPICE calculations unless there are also other very
short time constants involved.

The problem is the higher the frequency the more second and third
order effects are noted. A 1000pF cap at 1mhz is a cap for all
intents. At 150mhz it's now a complex RLC.

On the other hand, if the "leads" are PC traces over a ground plane,
shunt capacitance will be higher, and the approach I mentioned with the
transmission line is the way I'd always do it. The model for the leads
would include both L and C. As an example, if the "lead" is a .010"
trace on .032" FR4 material (er ~ 5) over a ground plane, it makes a
transmission line of Z0 = 105 ohms, velocity factor 0.55. The equivalent
L and C of a half inch of this line are 8 nH and 0.73 pF respectively.

There is a point here where you have to ask is a PCB trace a cap,
inductor or more realisticly a transmission line. Each has it's model
and they are different. With two sided (or more layers) board the
transmission line model is likely the one to use. Especially if your
working with VHF transistors at any power level as impedences
are in the range of a few hundred ohms on the high side and fractions
of an ohm at the low side. Typical 30-100W devices have an imput
imedence under 2 ohms ( and Xc) and output in the sub 4 ohms range
depending on power. In those cases everything counts and getting
hard numbers requires some work.

It hits a hard nail on what model to use and when because spice will
only give the results you asked for be they real or imagined.

Other characteristics of the capacitor such as ESR might have to be
included in the model depending on the application.

In most I'd naturally assume though at low VHF it's less signigicant
than at 2.4ghz.

You gave a capacitor value in your question -- an ideal capacitor of
that value would of course be the other part of the model.

And depending on frequency it may be a open ended transmission line
on the etch.

SPICE is used daily, as it has been for decades, by professionals and
produces strikingly good results in the hands of someone who is skilled
at modeling and has a good understanding of the circuitry being modeled.
I've personally used SPICE for modeling linear to highly nonlinear
circuits up to 50 GHz, where even tiny SMT components were often modeled
as transmission lines and every pad and solder blob is significant and
included. The results were used in the design of products which have
been successfully produced by the thousands and sold for years.

Roy Lewallen, W7EL

Spice is a great tool. Like every tool one must know how it works,
how to use it to its fullest and knowing the tool can hurt you if
abused. When introduced to spice for the first time I was told
garbage in, garbage out and never assume that computer
crunched garbage is anything other than composted garbage.
What I've found is that Spice does allow you the luxury of saying
"what if" or "how does that affect xxx" even if you are not sure it's
real.

Allison
KB1GMX