[SI-LIST] Re: Diff Impedance and its noise immunity.
- From: Geoff Stokes <gstokes@xxxxxxxxx>
- To: "'si-list@xxxxxxxxxxxxx'" <si-list@xxxxxxxxxxxxx>
- Date: Fri, 17 Oct 2003 12:15:26 +0100
Hi
This is one of several questions we've had which don't seem to acknowledge
that *all* the dimensions of the physical structure affect the radiation (RE
for Radiated Emission in EMC documents, and conversely the sensitivity to
other signals coupled in, known as Radiated Susceptibility or RS). For a
linear reciprocal network (e.g. packages, PCBs cables and connectors)
signals can couple in as much as they are radiated. Whether lines are
differential, single-ended, matched or unmatched there will *always* be some
radiation and coupling. It may sometimes be too small to measure, but the
effect is there. An exception is the very rare occasion where a screening
enclosure is complete.
As an experiment, set up a microstrip line with a 50 ohms characteristic
impedance in an EM simulator (Sonnet Lite is free). Analyse the response
over say, 0.2 to 2 GHz. Vary the size of the conducting box, starting with
an inch or two. The reflection will change. The line impedance is affected
by wave reflections from various parts of the box. Equivalently we can say
the box wall is coupled to the line. Go on and set up two lines and observe
the coupling, varying heights, widths and separations. You now have a diff
pair (edge coupled). If you see, say, 40 dB coupling, it means that very
roughly, a 1 volt swing on one line will couple as 10 mV on another,
depending on the impedance at that point (which in a more complicated case
may have been transformed to an unexpected value by a transmission line
effect).
Add a third line. Vary the distance. You can now measure the coupling from
a line to diff pair. Add a fourth and you measure the coupling between two
diff pairs. I think you will find that the coupling depends on just about
everything. You have a multi-dimensional problem. This doesn't solve your
problems, but makes them appear worse I suppose. Maybe the message is that
all assumptions must be tested in detail. Some good advice has been given,
e.g. by Chris Cheng and others, showing how the whole situation is further
complicated by interconnections and vias.
Cheers
Geoff
> -----Original Message-----
> From: =AA=FC=A5=C8 [mailto:binchuan2000@xxxxxxxxxxxx]
> Sent: 16 October 2003 02:38
> To: si-list@xxxxxxxxxxxxx
> Subject: [SI-LIST] Diff Impedance and its noise immunity.
>=20
>=20
>=20
> Dear all,
>=20
> I have a question which was confused me for a long time, that is=20
> which one is important between Diff. Impedance and its trace length
> matched/spacing(Noise immunity)?
> As my viewpoint I supposed that trace length matched and spacing is=20
> more important compared to Diff. Impedance. Because sometimes we=20
> focus on impedance for its signal quality as well as EMC concern.
>=20
> But in Diff signal I am not so sure that whether the Impedance is=20
> so important as Single-Ended signal, because if the impedance=20
> doesn't mismatch so seriously then I prefer to focus on trace match=20
> and Spacing for my direct intuition.
> =20
> And I also confuse that sometimes we want to match its Impedance=20
> then sacrifice its Spacing, in this way maybe D+ and D- can't=20
> reference to each other and lose its noise immunity.
> =20
> Does it correct or not? I'll appreciate if you give me a correct=20
> conception, thanks. =20
>=20
> Regards,
>=20
> Binchuan.Wang
> HW EE
>=20
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>=20
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