"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