[SI-LIST] Re: Diff Pairs

  • From: "Scott McMorrow" <scott@xxxxxxxxxxxxx>
  • To: cgrassosprint1@xxxxxxxxxxxxx
  • Date: Sun, 05 Oct 2003 09:53:58 -0700

Charles,
Actually, these were edge coupled differential stripline pair that I 
presented in my table.

To better read the columns, they are as follows in order from left to right:

trace width
trace edge-to-edge separation
dielectric height (always a constant 14 mils)
routed width (= 2*trace width + separation)
conductor loss (dB/in)
dielectric loss (dB/in)
total loss (dB/in)
loss for 20" trace
loss for 30" trace

frequency = 1.5625 GHz
Impedance = 100 ohms
Er = 4.2
tanDelta = 0.02

bottom line, the difference in loss for 30" of trace that is routed 
differentially with 7.5 mil lines and 2.5 mil lines, with impedance held 
at a constant 100 ohms, is -7.317 dB vs. -8.658 dB.  This is a 
difference of -1.34 dB, which may or may not be significant for a given 
application.  However, the difference in linear space required by the 
two are  45 mils vs. 8 mils for an  improvement of 560%.

So, to say that closely spaced differential pairs are always a "bad" 
design practice is just not true.  If you can tolerate some slight 
increase in loss, they may be quite good.


regards,

scott


Charles Grasso wrote:

>Scott,
>thanks for soing this. I have a question though. I infer
>that this a stripine broadside coupled pair?
>
>How about a similar study for an edge coupled pair
>ustrip configuration?
>
>
>-----Original Message-----
>From: si-list-bounce@xxxxxxxxxxxxx
>[mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of Scott McMorrow
>Sent: Friday, October 03, 2003 3:53 PM
>To: silist
>Subject: [SI-LIST] Re: Diff Pairs
>
>
>All,
>So, "just for fun"  I decided to run a differential pair trace through a
>field solver and put some numbers to our musings on differential pair
>loss vs. width and spacing.  As Lee Richey likes to point out, it is
>always good to add a bit of actual science to our conjectures about
>these things.
>
>For this particular experiment I assumed FR-4 with an Er = 4.2 and a
>loss tangent of .02, which is representative of some of the resent
>designs we have built and measured here at Teraspeed.  The dielectric
>width from plane to plane was kept at a constant 14 mils, and the
>differential pair was constructed as stripline in the center of the
>layer.  Finally, the pair width and spacing was adjusted from weak
>coupling to tight coupling, always keeping differential impedance
>constant at 100 ohms.  Here are the results for a frequency of 1.5625,
>which is the Nyquist frequency for 3.125 Gbps binary data streams::
>
>width   separation      height          routed   width          conductor loss 
>dB/in
> dielectric loss dB/in          total loss dB/in        20"     30"
>
>
>
>
>
>
>
>
>
>7.5     30      14      45      0.0958          0.1481          0.2439         
> 4.878   7.317
>6.79    13.89   14      27.47   0.0973          0.1481          0.2454         
> 4.908   7.362
>6.05    9.62    14      21.72   0.101   0.148   0.249   4.98    7.47
>5.4     7.55    14      18.35   0.1052          0.1481          0.2533         
> 5.066   7.599
>4.82    6.25    14      15.89   0.1098          0.1481          0.2579         
> 5.158   7.737
>4.3     5.35    14      13.95   0.1147          0.148   0.2627          5.254  
> 7.881
>3.84    4.67    14      12.35   0.1197          0.148   0.2677          5.354  
> 8.031
>2.5     3.07    14      8.07    0.1406          0.148   0.2886          5.772  
> 8.658
>
>There are several things to note here:
>
>1) Since impedance is kept constant, dielectric loss is constant.
>2) As pointed out by others, as trace spacing becomes smaller, and trace
>width becomes narrower, and conductor losses increase.
>3) Loss is dominated by dielectric loss.
>4) For long traces there is just a titch less than 20% improvement in loss
>by using a 7.5 mil conductor vs. a 2.5 mil conductor, with constant
>impedance.
>5) Trace linear density is improved by greater than 550% by using a narrower
>conductor.
>6) A wide trace is easier to manufacture and has better impedance control
>than a narrow trace.
>
>
>So, as with all things in engineering, there are tradeoffs.  We can trade
>off loss for density by changing the conductor width.  We can also trade off
>manufacturability, since a narrow conductor with small space is harder to
>produce and control.  We can trade off poor vs. good impedance control.  All
>of these must be evaluated for any design.
>
>One assumption that everyone makes is that loss is bad.  This is not always
>the case.  Given a choice, I generally appreciate having a bit of conductor
>loss in a design, as it tends to de-Q resonant circuits quite nicely.
>Little things like package, connector, blocking capacitor and via
>discontinuites can benefit from a bit of increased resistive loss.
>
>If you require the density, close spaced, narrow width differential pairs
>win hands down, at a slight sacrifice in loss performance.  The interesting
>thing to consider will be the overall system performance.  Due to other
>discontinuities, it may be that overall performance is actually improved by
>a little additional loss here and there.
>
>
>regards,
>
>scott
>(Who doesn't have a book.)
>
>--
>Scott McMorrow
>Teraspeed Consulting Group LLC
>2926 SE Yamhill St.
>Portland, OR 97214
>(503) 239-5536
>http://www.teraspeed.com
>
>More than that, it does not have any benefit.  Tight coupling of
>differential pairs forces the traces to be narrower increasing the skin
>effect losses.  Also, this tight coupling is going to result in good old
>cross talk that actually degrades the edges.
>
>How the notion of tight coupling of differential pairs as beneficial got
>started is a mystery to me.  There are several references that show that
>tight coupling is not beneficial, one of them is Howard Johnson's latest
>book, at least one column he has written and my recently released book.
>
>Lee Ritchey
>
>
>
>  
>
>>>[Original Message]
>>>From: Duane Takahashi <duanet@xxxxxxxxxxxxxxxxxxxxxx>
>>>To: <si-list@xxxxxxxxxxxxx>
>>>Date: 10/2/2003 3:58:59 PM
>>>Subject: [SI-LIST] Re: Diff.Pairs
>>>
>>>Hi Juergen:
>>>
>>>Aligning the stack up for the broadside coupled diff lines is expensive.
>>>   You can do this, but it drives up the cost of the board.
>>>
>>>Duane
>>>
>>>      
>>>
>>    
>>
>>>>>Hi Juergen,
>>>>>You can find lots of  application notes
>>>>>especially with respect to process variation
>>>>>on differential pairs here:
>>>>>
>>>>>
>>>>>www.polarinstruments.com/support/cits/cits_index.html
>>>>>
>>>>>In particular this one may be of interest:
>>>>>
>>>>>
>>>>>How measured impedance may vary from field solver calculations when
>>>>>using woven glass reinforced
>>>>><http://www.polarinstruments.com/support/cits/AP139.html>laminates
>>>>>
>>>>>www.polarinstruments.com/support/cits/AP139.html
>>>>>
>>>>>
>>>>>And this note:
>>>>>
>>>>>Copper thickness, edge coupled lines and
>>>>>characteristic
>>>>><http://www.polarinstruments.com/support/cits/AP151.html>impedance
>>>>>
>>>>>
>>>>>www.polarinstruments.com/support/cits/AP151.html
>>>>>
>>>>>
>>>>>
>>>>>Hope this helps....
>>>>>
>>>>>
>>>>>Kind regards
>>>>>Martyn Gaudion
>>>>>www.polarinstruments.com
>>>>>T: +44 1481 253081
>>>>>F: +44 1481 252476
>>>>>M: +44 7710 522748
>>>>>E: martyn@xxxxxxxxxxxxxxxxxxxx
>>>>>
>>>>>============================================
>>>>>  Controlled Impedance & Signal integrity tools
>>>>>  for the Printed circuit fabrication industry
>>>>>============================================
>>>>>          
>>>>>
>
>
>
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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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