[SI-LIST] Re: Diff. Bal. to Unbal. Transition
- From: "Loyer, Jeff" <jeff.loyer@xxxxxxxxx>
- To: <si-list@xxxxxxxxxxxxx>
- Date: Wed, 10 Sep 2003 09:03:21 -0700
Hi Scott,
While I strongly agree with your first point, I'm not clear on your =
second point. Some clarification, please...
If I understand you correctly, you're saying that a "long" transition =
(relative to the signal risetime) represents a problem? I.E., when =
going from differential to single-ended, if I slooooowly transition the =
traces, that will induce an impedance discontinuity due to the modal =
conversion effects. So, if you take your 2 traces that are very close =
(say 5mils) together, and then decouple them (say 20mils), if you make =
that transition between tightly coupled to loosely coupled (so they're =
each effectively single-ended) very gradual, that can be a problem.
The alternative is to use a short transition section - separate them =
very quickly. This avoids the issue.
Is that what you're saying, or am I misunderstanding it?
_____________________________
What I would have said is that, when you transition a tightly-coupled =
differential pair to 2 single-ended traces, you:
1) May have to compensate for any impedance change you induce. If a =
tightly coupled diffential pair has a differential impedance of 85ohms, =
each single-ended trace should have an impedance of 85/2, or 42.5ohms. =
That probably means your traces will be a different width when they're =
single-ended (a bit wider) than when they were differential. This only =
applies if the single-ended length is significant, relative to the =
rise-time (for instance, if you're separating your traces to go around a =
via, the discontinuity may be short enough to be ignored).
2) Lose the advantages of routing a differential pair tightly-coupled =
for the length they are routed single-ended. The advantages I know of =
(in no particular order) are :
a) less susceptibility to noise - noise that's injected on 1/2 of the =
pair gets injected on the other half (hopefully, equally), and that =
"common mode" noise has less effect on the receiver than on a =
single-ended signal.
b) less EMI - radiated waves of the 2 halves cancel each other.
c) less noise induced on other traces - radiated waves of the 2 =
halves cancel each other
d) differential signals are much less susceptible to problems caused =
by reference plane changes
e) differential pairs can be AC-coupled - allowing different DC =
levels for the transmitter and receiver.
3) May induce more subtle problems. If 2 halves of a differential pair =
are routed over dielectric with significant differences in their =
respective properties, you might inadvertently induce =
differential-to-common mode conversion. For instance, the 2 halves =
might be perfectly differential when they transition to 2 single-ended =
signals. Then, one trace might be routed over a portion of the board =
with a different effective dielectric constant than the other (due to =
processing variation). (Or, you might do something very silly and route =
them on different layers). When the 2 halves came back together, they =
might now be out of alignment - the purely differential signal now has =
some common-mode component. That would be a bad thing for the =
differential receiver. But, I haven't heard of this being a significant =
problem. Anyone who has information showing otherwise, please correct =
me. In the case of routing the two halves on different layers, that =
would be asking for trouble.
So, I would have said that, theoretically, it is acceptable for a =
differential pair to be routed both tightly-coupled and loosely-coupled, =
as long as no significant impedance discontinuity is introduced. The =
caveat is that the advantages of routing tightly-coupled are lost for =
the length of the single-ended portion, and you must be careful to not =
route the two halves in significantly different media.
And finally, I'm not sure how a balun is applied in this situation. A =
single balun would transition the differential pair to a single =
single-ended signal (say that 3 times quickly!), and then another one of =
the far end would convert that back to a differential pair? That may be =
done, but I haven't heard of it in PCB applications. But, I wouldn't be =
surprised to find out it's done for applications I'm not familiar with.
Jeff Loyer
-----Original Message-----
From: Scott McMorrow [mailto:scott@xxxxxxxxxxxxx]
Sent: Wednesday, September 10, 2003 7:27 AM
Cc: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Diff. Bal. to Unbal. Transition
Brad,
There are two issues that need to be considered in a differntial to=20
single ended transition.
1) Differential impedance must be matched on both sides of the=20
transition. For differential wire to single ended trace transitions,=20
this can be accomplished easily with loosely coupled 50 ohm traces,=20
where the differential impedance is approximately 100 ohms.
2) Modal converstion control. (Yes, you did use the correct term.) In a=20
balanced pure differential wire line the field distribution is=20
significantly different than that in PCB traces. Maximum field strength =
is in the area between the lines. For single ended traces, the maximum=20
field strength is contained between the trace and the plane. So, a=20
differential to single ended transition will facilitate the transfer of=20
energy from one field pattern to the other. At most edge rates, this=20
happens naturally as the fields find a plane in proximity to the=20
traces. The fields transfer first from the wire line to the traces,=20
and then they are redistributed between the traces and the plane. At=20
high edge rates, this constitutes a discontinuity. (Actually, it is=20
always a discontinuity. At low edge rates the discontinuity is small=20
with respect to the edge rate, and so is generally invisible.) To=20
smooth the transition, the traces can be designed to be highly=20
differential coupled in the region near the two-wire line, along with=20
some ground removal underneath the traces, and then smoothly vary until=20
they become decoupled 50 ohm single ended traces. I'd recommend this=20
for signals switching in the sub 100 ps region.
In general, if the modal conversion region is << risetime, then little=20
has to be done. But when the region approaches a reasonable fraction of =
the risetime, adjustments should be made. In reality, differential=20
transitions are a bit easier to accomplish than single ended=20
transitions, such as with coax cable to board transitions. In the=20
single-ended coax case, the field patterns are significantly different. =
Here the transition must be accomplished in 3 dimensions.
These transition optimizations are not easily done by hand, although it=20
is possible to get quite close by a Zen-like "being the fields" and=20
imagining where they might want to go. Unfortunately, we humans tend to =
miss some of the places where electromagnetic fields like to travel. In =
that case, I'd recommend a full wave 3-dimensional field solver like CST =
Microwave Studio, Ansoft HFSS, or many of the others, that will=20
correctly solve (and allow us to visualize) these sorts of problems.
best regards and happy transitioning,
scott
--=20
Scott McMorrow
Electromagnetic Field Wrangler
Teraspeed Consulting Group LLC
2926 SE Yamhill St.
Portland, OR 97214
(503) 239-5536
http://www.teraspeed.com
Bradley S Henson wrote:
>
>
>For years I've encountered systems with balanced differential signals =
that
>transition to 2 single ended lines in a PWB. For example, an RS422 =
signal
>on a 100 ohm diff. pair wire that connects to a PWB that then routes it =
as
>a loosely-coupled pair of 50 ohm striplines. This seems to work fine =
for
>these slower edge rate systems.
>
>I'm starting to see attempts to do the same thing with sub ns edge rate
>signals in the 100s of MHz and GHz range. This bothers me seeing a
>transition between the 2 modes (probably poor term to select) without
>seeing a transformer or other balun-like device. Should I be concerned? =
If
>this is a problem, how do you simulate it (tools)?
>
>Thanks,
>Brad Henson,Raytheon
>
>
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--=20
Scott McMorrow
Teraspeed Consulting Group LLC
2926 SE Yamhill St.
Portland, OR 97214
(503) 239-5536
http://www.teraspeed.com
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