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August 1997

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Subject:
From:
Doug McKean <[log in to unmask]>
Reply To:
TechNet Mail Forum.
Date:
Tue, 5 Aug 1997 09:05:21 -0400
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> From: Jack Olson <[log in to unmask]>
> To: [log in to unmask]
> Subject: Re: Via Inductance
> Date: Friday, August 01, 1997 1:25 AM
>
> From: [log in to unmask] (Doug McKean)
> >
> > One question:  Analog or digital signals at 1.9 GHz?
> >
> > Regards,  Doug
>
> Well, I WARNED you I was the new guy, I'm not sure and
> kind of embarrassed to ask (but I'll ask tomorrow).
>
> We make amps for cellular communications and now this
> is a PCS project. Does that help? sorry...

Ok.  I hope to keep this simple but not so much it insults people.

It's digital.  That means the chips are all switching ON/OFF
producing alot of switching noise.  Noise that is represented
by very quick rising and falling spikes of current and voltage.
This noise can be felt all over the place to include the power and
ground planes.  Inductance opposes changes in current flow.
The change in current gets magnified by the inductance to
produce a voltage.  If you want to know how to run the actual
numbers, here goes -

Voltage(from a spike) = L * (change in Current) / (change in Time)

L (inductance) is in Henries,  Current is in Amps,  Time is in Seconds.

So, you have an inductance of  1 micro-henry (10^-6),
a current spike of 10 amps,
and the current changes from zero to 10 amps in 1 micro-second (10^-6).
The voltage you will see is

V = (10^-6 Henries) * (10 Amps) / (10^-6 Seconds) = 10 volts.

Let's say the rise time for your spike is now in the nano-second range.

V = (10^-6 Henries) * (10 Amps) / (10^-9 Seconds) = 10,000 volts.

Anyway,  now that I've totally bored you,
on to your question about reducing inductance by widening the hole.
I don't believe it really does reduce inductance.  The inductance per
length of regular wire remains *fairly* constant over a wide range of
gauges.  Andy said it best with increasing hole to increase
capacitance.  Capacitance is the electrical flip side (so to speak)
of inductance.  Increasing capacitance will help counteract
the previous inductive effects.

Regards,  Doug ...


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