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July 1998

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From:
"Rhoads, Randall E" <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Fri, 10 Jul 1998 08:22:19 -0400
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Scott and forum:

I'd like to add to the good info that Paul Gould has already given.  (This
is probably overkill, but you can stop reading anytime you want.)  :-)

"Characteristic impedance" deals with signals propagating along some uniform
path.  "Impedance" is a general term defined as opposition to the flow of
current and is mathematically the voltage divided by the current at some
point in a circuit.  It usually applies to components where signal
wavelengths are long compared to the dimensions of the circuit, like at
lower frequencies.  In contrast, "characteristic impedance" is a
high-frequency or transient thing and applies to a wavefront as it
propagates, like a high-speed (quickly changing)signal along a (relatively
long) circuit board trace.  It is the ratio of instantaneous voltage to
current at the wavefront.  It is usually measured with a single-ended (one
conductor is ground) TDR (time domain reflectometer).

"Differential" has to do with where signal currents are returned through a
structure.  In striplines and similar geometries, the signal current returns
in the ground plane conductor.  A signal is usually a voltage referenced to
ground.  In "differential" situations, the current is returning on another
conductor, but not ground.  "Differential" refers more to how signals in a
multiconductor structure are measured - that is from one conductor to
another, where there is a (different but related) signal on each.  Usually
physical symmetry is involved which allows for "balanced" signals -- equal
but opposite with respect to ground (electrical symmetry).  Differential
TDR's are a convenient way to measure differential impedance.  There is
essentially one single-ended channel for each of the two conductors being
measured.  The net signal is the difference between the two, usually done
mathematically within the instrument.  Two isolated individual 50-ohm
single-ended traces when measured differentially would be 100 ohms.

I've probably mentioned more than I should have. So if I confused you (or
anyone else for that matter), contact me off-line and I'll be happy to try
to unconfuse you.  All my explanations come with a money-back guarantee.
:-)

Randall E Rhoads
Development Engineer
Electromagnetic Technology
AMP Incorporated
Harrisburg PA USA

717-986-5201 / Telephone & Voice Mail
717-986-5643 / Facsimile
[log in to unmask]

> -----Original Message-----
> From: [log in to unmask] [SMTP:[log in to unmask]]
> Sent: Thursday, July 09, 1998 5:52 PM
> To:   [log in to unmask]
> Subject:      [TN] Impedance
>
> Could I please get some feedback as to the definitions and difference
> between
> characteristic versus differential impedance?  I have customers that ask
> for
> both but can't explain what they are actually looking for.  Any help would
> be
> appreciated.
> Thanks,
> Scott
>
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