Re: SPICEing The Inductance of a Trace Over a Ground Plane?

"D from BC" <myrealaddress@xxxxxxxxx> wrote in message
news:0daue3l50e11g711v8qpaui0qag3u1a637@xxxxxxxxxx
then using this strip model (transmission line model)
with a pulse gen and load...

pulse > ---- L ------>Hi Z load (CMOS input)
| |
C/2 C/2
| |
Gnd Gnd

..It looks like this can have severe ringing...

Yes, but this is due to your pulse having a very fast rise time. If you
want to use pulses, for a given rise time make the delay through each L-C
section around, say, 1/10th of the rise time and then you'll give a
reasonable facsimile of the pulse at the far end.

Isn't this also called a pie filter..?

Yes it is (albeit spelling "Pi" as in 3.14..., not "pie" as in "tasty treat
that's calorically dense"). This is telling you that if you take a
transmission line (like a length of coax cable), at low frequencies it will
just behave like a low pass filter.

So to stop the ringing.. Rsource needs to be 47 ohms and then the CMOS
input needs to be loaded by 47 ohms.

....and the rise time of your pulses need to be slow enough that the number
of L-C sections you use is a fair approximation of a real (pure delay)
transmission line.

Aside from the 50% signal loss, invalid CMOS levels and a hot pulse
generator...the great is the 10cm strip acts like it's not even
there....if I got that right..

In the ideal case, yes -- the 10cm strip acts just like a time delay. In
real circuits there's always those little parasitic bits that get in the way
(and there's loss that's a function frequency and eventually becomes
significant), but they can sometimes either be neglected or "fixed" in a
brute-force manner (e.g., by paralleling a termination resistor with a diode
to either rail, thereby preventing overshoot and undershot... often people
get this feature "for free" due to the ESD diodes attached to most IC pins
these days).

---Joel


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