# differential impedance

F.Hart ([email protected])
Wed, 9 Apr 1997 12:46:32 -0500

We've seen specifications on differential impedance, but no one has
been able to define what that number represents. We use the rule of
thumb that the differential impedance is 2 times the odd mode
impedance. Does that make sense? Can someone give me a real
definition of differential impedance and it's relation to odd mode
impedance?

Thanks,

```--
Weston Beal                Signal Integrity Engineer
[email protected]     [email protected]
"A little bit of knowledge is a dangerous thing"
------------- End Forwarded Message -------------

Weston,

I'll be interested in all responses to this question because I'd like
to know if my thinking on this is correct.

Here's my two cents:

In a coupled microstrip construction, odd mode TEM propagation is the
transmission of equal magnitude but opposite polarity signals.  The
calculations are fairly straightforward and equivalent to "differential"
transmission.  Typically, the impedance will be approximately twice
that of even mode transmission (equal magnitude/same polarity). This is
because of the impact of coupling between the conductors dominates
the coupling to ground (my thoughts).

_____       Zo      _____
|_____| ---/\/\/--- |_____|     Conductors
|                   |
<                   <
< Ze                < Ze
|                   |
______________________________   Ground

However, when you add conductors, there are multiple modes of propagation
and the rule of thumb is less reliable for predicting impedance.

There are several resources which do a better job of explaining this, but
this is the short answer as it relates to the mathematics.  As I
understand it, the testing of even (single-ended) and odd (differential)
impedance relies on how the far end of the transmission is terminated, and
I'll have to leave that explanation to someone else.  (To add to the confusion -
these may also be refered to as balanced and unbalanced transmission).

I'd appreciate any feedback.
Thanks,
Fran

____________________________________________________________________
|__________________________________________________________________|
||                              ||                                ||
||  Frances G. Hart             ||  e-mail:  [email protected]       ||
||  3M                          ||                                ||
||  Austin, Texas               ||  Phone:   (512)984-6869        ||
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```