RST Engineering (jw) wrote:


"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.

It is not a complete question. You could get all sorts of different
parasitic inductance values by putting the component near or far from other
metallic objects, which usually occurs to some extent in any practical
situation. Many other things will affect the answer.

You have not yet supplied anything other than a single dimension measurement
for the component. If you want the inductance, skin effect, etc. then you
would have to give me a dimensioned drawing showing the placement of the
wires, the plating material, plating thickness and the internal
construction of the capacitor. (You would also have to pay me enough to
make it worth me bothering to simulate it.) Your question is 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.

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.
Well as you have not specified whether this is "commercial gear" and what
type of PCB material, dielectric thickness, trace width etc. of course we
can't tell you the answer. Neither could a guy who was going to answer
your question by building one and measuring it. He could find one possible
answer but there are lots of possible answers which differ because you have
not given us a complete problem to solve.


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.
You still gave insufficient information on the wire geometry. I would
refute your claim that 99% of commercial products don't use a ground plane.
The cell-phone market is in the high hundreds of millions of units this
year, and is likely to reach 1 billion units per year next year, and I
guarantee you that every one will contain a multi-layer PCB with ground
planes and microstrip traces etc. every one of them designed using field
simulators and some version of SPICE to model the integrated circuit
packages and bondwires, as well as the antenna. I don't believe that this
one billion units would fit into the 1% minority of products that you think
have ground planes!

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

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

Jim

Anyhow, until you tell me how long a piece of string is, (to the nearest
micron or micro-inch whichever you prefer), I have had enough of this
thread.

Chris