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



"John Larkin" wrote in message
...
On Mon, 21 Nov 2005 23:55:26 -0800, "RST Engineering"
wrote:

Well, just for starters, what does Spice say about a 1000 pf capacitor
with
either ½" lead lengths or a total of 1" of PCB trace at 150 MHz.?

Jim



When they matter, just poke in the appropriate series L or a bit of
transmission line. I use Spice now and then to sim picosecond stuff.
Even if the simulation isn't highly accurate, it helps train your
instincts, shows you which parasitics will have which effects, so when
you build the real thing you have a jump on the complexity.

Hell, 150 MHz is slow.

John

"RST Engineering \(jw\)" writes:

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.

I am not an expert, but can you not just add a bit of series L to
approximate this? Or even transmission line for higher frequencies /
longer tracks.

I have had surprisingly good results simulating a single transistor
UHF oscillator at ~500MHz. The results agreed with reality quite well,
even *without* explicitly modelling all the parasitics. Certainly
spice was better than me at predicting what would happen (although
that is not saying much!) I guess perhaps the reason that I could get
away with this was because of the small size and high performance of
SMT parts these days.

--

John Devereux

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.

Spice, smice. You got good answers. Wake up, spice is just an
engine to run your component models in circuits of your making.
Once one gets beyond the "toy" level of using spice it becomes
necessary to vet and complete the models of all your critical
components, including parasitic circuit elements. Want a good
RF capacitor model, you do well to make it yourself, from the
manufacturer's data and info, and from your understanding of the
part, aided by theory and bench measurements. Then vet your model
with more bench measurements. Thinking about component leads and
PCB wiring? Hey, you need to explicitly add all these into your
circuit. Just don't blame any resulting shortcomings on "spice."


--
Thanks,
- Win

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.

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.

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

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

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

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

I don't know why "RST" is so aggressive but I would mention here, that the
really funny part begins if you garbage in and get meaningful output!

I like to smell the solder iron but it can be very interesting playing with
Spice and see how changing values can and CAN BE NOT have a drastic result
in the simulation. Sometimes changes are not so great in result and parts
can be simpler made. e.g. changed from 1% to 5% or more.

- Henry


schrieb im Newsbeitrag
...
On Wed, 23 Nov 2005 06:23:58 -0800, Roy Lewallen
wrote:
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

OK Jim -

I simulated your question in a first trial. Here is the result:
approx. 70MegHz resonance for 100pF, 30mOhm ESR and 5nH for the trace.
Surely I would prefer smd chips having better results.

BTW: I cannot understand why you're so aggressive and I would prefer that
the thread will go in direction of my FIRST posting. I'm doing electronics
since I was 12 years old and that is 25 years back. I have for example a
patent application made and worked in the communication industry. So you can
surely think I know what I'm doing. Thanks!

- Henry


"RST Engineering (jw)" schrieb im Newsbeitrag
...
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

"Henry Kiefer" wrote in message
...
OK Jim -

I simulated your question in a first trial. Here is the result:
approx. 70MegHz resonance for 100pF, 30mOhm ESR and 5nH for the trace.
Surely I would prefer smd chips having better results.

But that wasn't the question. THe question was to simulate a 1000 pf (1 nf)
capacitor at 150 MHz. with an inch of lead or an inch of normal pcb trace
and tell me what you get.




BTW: I cannot understand why you're so aggressive and I would prefer that
the thread will go in direction of my FIRST posting.

OK by me.



I'm doing electronics
since I was 12 years old

8 years old.


and that is 25 years back.

54 years back.


I have for example a
patent application made

I've got a few more.



and worked in the communication industry.

38 years straight now.


So you can
surely think I know what I'm doing.

Then answer the question.

Jim

jw\ wrote...

The question was to simulate a 1000 pf (1 nf) capacitor at
150 MHz. with an inch of lead or an inch of normal pcb trace
and tell me what you get.

Such a question is stupid and incomplete. First of all, in
cases where impedance matters, one wouldn't dare use an inch
of lead at 150MHz, we'd cut that short, 0.1-inch max. And
we certainly wouldn't use an inch of pcb trace unless it was
field-controlled with a ground plane. This is true whether a
1nF cap is involved or not. If you were to insist on analyzing
an inch of lead, we'd insist on knowing *all* about the ground
scene. Since you aggressively put your question without any
relevant information about what the ground is like, and where
it is, the question is intrinsically-stupid and incomplete.

Sorry, jw\, but that's the way it is.

BTW: I cannot understand why you're so aggressive ...

Indeed.

Then answer the question.

Answered repeatedly already.


--
Thanks,
- Win

"Winfield Hill" wrote in message
...

jw\ wrote...

The question was to simulate a 1000 pf (1 nf) capacitor at
150 MHz. with an inch of lead or an inch of normal pcb trace
and tell me what you get.

Such a question is stupid and incomplete.

No, Win, the question is well formed and quite complete. Sleep off the ten
shots of Old Rammycackle and let's have the discussion when you are sober.



First of all, in
cases where impedance matters, one wouldn't dare use an inch
of lead at 150MHz, we'd cut that short, 0.1-inch max.

No, Win, neither you nor I would do such a thing. But somebody who is (as
the OP posted) new to the RF world would do so without a second thought.
You and I have been playing this game all our lives and take self-resonance
into account without even thinking about it. However, a student new to the
field (as my freshman engineering students are) makes the mistake
repeatedly, even when using a decent text called ... um ... The Art Of
Something Or Other. When their RF amplifier starts squeeging or
motorboating, I tell them that the power supply isn't bypassed well enough,
and I'll be damned if the first thing they do is put a BIGGER capacitor on
the supply line.

I'll then ask them what they think the bypass impedance is and get the stock
answer "1/(2*pi*f*c)". Hm, says I, how about the three inches of wire
between the capacitor and the supply line. Oh, says them, that's a direct
short. Straight wire doesn't have a reactive component. Hm, says I, let's
see what the network analyzer says about that. Hm, says student, it says 60
nanohenries. How can that be? Mm, 20 nanohenries per inch for #20 wire
sounds about right, so what does that series circuit look like? Hm.
Inductive at the frequency of interest. Now, grasshopper, tell me about
self-resonance of capacitors with long leads.




And
we certainly wouldn't use an inch of pcb trace unless it was
field-controlled with a ground plane.

That's not always an option in commercial gear, Win.



This is true whether a
1nF cap is involved or not. If you were to insist on analyzing
an inch of lead, we'd insist on knowing *all* about the ground
scene. Since you aggressively put your question without any
relevant information about what the ground is like, and where
it is, the question is intrinsically-stupid and incomplete.


If I didn't say what the ground is, then we can assume that I formulated the
question without ground plane. 99% of the commercial products run this way.



Sorry, jw\, but that's the way it is.

Sorry, Win, that's NOT the real world.

Jim