RST Engineering wrote:


"Henry Kiefer" wrote in message
...

Ok Jim. You're asked about the stray components hidden in the real
parts/circuit.

There is nothing hidden at all. Strays are a part of the real life at VHF
and above. Sometimes strays have more effect than the part itself.
You have to put the strays into the spice model if you want an accurate
result, just like you would have to if you work it out with a pencil and
paper.

If you take the values from the datasheets for die and/or
package or give it a guess if not available, and add values for the pcb
lines, you have a good simulation result.

Horsefeathers. What "values" do you add for the pcb lines?

You can work out the inductance, resistance (including skin effect) and
capacitance of arbitrary structures using the free programs FastCap and
FastHenry, though these consider inductive and capacitive effects
separately so you have to break the problem into several parts sometimes,
e.g. if the structure is a significant fraction of a wavelength in size.
Constructing the model files can be tedious, and you may end up having to
write some code to automate it. You can view the model files using a
viewer program which used to be and hopefully still is available from
www.fastfieldsolvers.com You could also buy one of the commercial
full-wave solvers if you have more spare money than time.

The pcb by itself is not of much
interest if you simulate not much over 100MegHz. Even starting a
oscillator
with thermal noise is possible!
Sure, you must know what you're doing :-)

As with anything in the world.



Spice by itself can simulate almost all if you have the right models
included - even if you wanna simulate a mechanical system. There is no
frequency limit at least to 100GHz.

The problem is getting the right models, which is an art in and of itself.
Right but you have to do that one way or another anyway even if the model is
inside your head, unless you design purely by trial and error.

Do you had bad result doing Spice?

To come back to your question: Spice will give you better results than
your
real circuit! You can connect a probe with no interaction to the circuit!
Try this with your real parts! Don't forget the component variations in
real
circuits coming from the manufacturing processes of the parts.

Again I ask the question: What does Spice say about a 1000 pf capacitor
with 1" total lead length at 150 MHz.? Don't give me the BS about Spice
being "better than a real circuit". Until you can give me a ferrite slug
on one end of a toothpick and a brass slug on the other end to increase or
decrease inductance in Spice, the physical circuit is the "real" circuit.

Jim
The real advantage of spice is in situations where trial and error is more
expensive than getting it right the first time (where you can justify
spending a long time making good models). He does also have a point about
being able to probe components inside a circuit that you could never probe
on a real one due to loading effects of the real probe. It seems to me
that your real objection is not with computer simulation of circuits, but
rather with poor models for components. Fair enough, garbage in garbage
out, but I would consider using bad models to be a form of 'user error'.

Chris