FreeLists / si-list / [SI-LIST] Re: Diff. Bal. to Unbal. Transition

[Scott McMorrow <scott@xxxxxxxxxxxxx>]

Brad,

There are two issues that need to be considered in a differntial to 
single ended transition.

1) Differential impedance must be matched on both sides of the 
transition.  For differential wire to single ended trace transitions, 
this can be accomplished easily with loosely coupled 50 ohm traces, 
where the differential impedance is approximately 100 ohms.

2) Modal converstion control. (Yes, you did use the correct term.) In a 
balanced pure differential wire line the field distribution is 
significantly different than that in PCB traces.  Maximum field strength 
is in the area between the lines.  For single ended traces, the maximum 
field strength is contained between the trace and the plane.  So, a 
differential to single ended transition will facilitate the transfer of 
energy from one field pattern to the other.  At most edge rates, this 
happens naturally as the fields find a plane in proximity to the 
traces.   The fields transfer first from the wire line to the traces, 
and then they are redistributed between the traces and the plane.  At 
high edge rates, this constitutes a discontinuity.  (Actually, it is 
always a discontinuity.  At low edge rates the discontinuity is small 
with respect to the edge rate, and so is generally invisible.)  To 
smooth the transition, the traces can be designed to be highly 
differential coupled in the region near the two-wire line, along with 
some ground removal underneath the traces, and then smoothly vary until 
they become decoupled 50 ohm single ended traces.  I'd recommend this 
for signals switching in the sub 100 ps region.

In general, if the modal conversion region is << risetime, then little 
has to be done.  But when the region approaches a reasonable fraction of 
the risetime, adjustments should be made. In reality, differential 
transitions are a bit easier to accomplish than single ended 
transitions, such as with coax cable to board transitions.  In the 
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 
is possible to get quite close by a Zen-like "being the fields" and 
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 
correctly solve (and allow us to visualize) these sorts of problems.


best regards and happy transitioning,

scott

-- 
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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-- 
Scott McMorrow
Teraspeed Consulting Group LLC
2926 SE Yamhill St.
Portland, OR 97214
(503) 239-5536
http://www.teraspeed.com




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