[SI-LIST] Re: Stripline more lossy than Microstrip?

  • From: olaney@xxxxxxxx
  • To: jeff.loyer@xxxxxxxxx
  • Date: Tue, 28 Aug 2007 09:41:42 -0700

The starting premise for many designs (concious or otherwise) is that you
can get away with a lot if connections are short and at least nominal
attention is paid to proper layout.   What I'm hearing in all this is
that if the difference between microstrip and stripline becomes a real
issue, it's time to question the starting assumptions, e.g. floor
planning, # of layers, board dimensions, to see if trace lengths can be
moderated or other improvements made.

Thanks for a lively discussion.

Orin

On Tue, 28 Aug 2007 08:42:57 -0700 "Loyer, Jeff" <jeff.loyer@xxxxxxxxx>
writes:
> It sounds like we're in violent agreement - thanks.  And an 
> excellent
> article, too.  As always, great to hear from you...
> 
> Jeff Loyer
> 
> -----Original Message-----
> From: Eric Bogatin [mailto:eric@xxxxxxxxxxxxxxx] 
> Sent: Tuesday, August 28, 2007 8:35 AM
> To: Loyer, Jeff; olaney@xxxxxxxx
> Cc: si-list@xxxxxxxxxxxxx
> Subject: RE: [SI-LIST] Stripline more lossy than Microstrip?
> 
> Jeff-
> 
> The article you mention is my Feb 2006 PCD&M column, "Microstrip or
> Stripline". This is posted for free download on my web site,
> www.BeTheSignal.com, under the Feature Articles and columns tab. 
> Look on
> the
> right side of the page for the columns.
> 
> Bottom line is microstrip, due to some of the field lines in air, 
> does
> have
> about 30% lower dielectric loss than stripline. However, if the 
> lines
> are 5
> mil wide, this 30% lower dielectric loss may contribute less than 
> 10%
> difference in total loss below 3 GHz. Add a little solder mask which 
> can
> have dissipation factors of 0.04 and the difference in attenuation 
> is
> even
> less.
> 
> This analysis was done with ADS, and is a very straightforward
> simulation. I
> also have a section on this effect in my book in the chapter on 
> lossy
> lines.
> 
> --eric
> 
> **************************************
> Dr. Eric Bogatin, President
> Bogatin Enterprises, LLC
> Setting the Standard for Signal Integrity Training
> 26235 w 110th terr
> Olathe, KS 66061
> v: 913-393-1305
> f: 913-393-0929
> c:913-424-4333
> e:eric@xxxxxxxxxxxxxxx
> www.BeTheSignal.com 
> **************************************** 
> 
> -----Original Message-----
> From: si-list-bounce@xxxxxxxxxxxxx 
> [mailto:si-list-bounce@xxxxxxxxxxxxx]
> On
> Behalf Of Loyer, Jeff
> Sent: Tuesday, August 28, 2007 10:10 AM
> To: olaney@xxxxxxxx
> Cc: si-list@xxxxxxxxxxxxx
> Subject: [SI-LIST] Stripline more lossy than Microstrip?
> 
> Title changed and old thread deleted.
>  
> 
> OOPS - you're right.  I had "microstrip" and "stripline" backwards 
> in my
> last e-mail.  I meant to question your assertion that stripline is 
> more
> lossy than microstrip.
> 
>  
> 
> Thanks for the links, Orin.  Looking these over and my old data
> (including postings to this forum), here's what I think:
> 
> 1)       The IEEE paper is for microstrip w/o soldermask.  My own 
> data
> agrees that microstrip w/o soldermask is slightly less lossy than
> stripline.
> 
> 2)       Regarding the 2nd link (Eric B.'s posting), I'll ask Eric 
> to
> share his source data for the assertion that microstrip provides 
> less
> attenuation than stripline (perhaps he's referring to the same 
> paper).
> 
> 3)       I'm including some verbiage from a previous posting of 
> mine
> below, including empirical data demonstrating that the difference 
> in
> loss was negligible.  In looking at the data again, there does seem 
> to
> be more loss on the stripline at 20 GHz, but not until then (the
> difference between microstrip and stripline at frequencies less 
> than
> 20GHz are in the noise range).   
> 
>  
> 
> Overall, I think the difference in loss between the 2 is not 
> significant
> enough to use as a criteria in layer selection.  Though I would 
> stress
> again that, in the practical real-world case, you might well be able 
> to
> use wider traces on microstrip and thus reduce your loss 
> significantly.
> That has to be balanced against more crosstalk and worse impedance
> control, etc.
> 
>  
> 
> Enjoyed the chat - cheers!
> 
>  
> 
> NOTE: I apologize to those besides Orin, since I know this data 
> below is
> going to be converted into "plain text" and the table will look 
> like
> garbage.  With a little work, you can turn it back into reasonable 
> data.
> 
> ______________ Previous posting verbiage ___________________
> 
>  
> 
> Eric's comments got me looking at my data again (that lead me to 
> believe
> the losses of microstrip and stripline are equivalent).  That data 
> was
> based on simulations (XFX -> Hspice), which my past experience has 
> shown
> 
> to correlate well to measurements.  Assuming the trace width and
> thickness is kept constant (adjusting the dielectric thickness to
> achieve the proper impedanct), it showed little difference between
> microstrip and stripline for our current frequencies of interest 
> (say,
> up to ~5GHz).  I also validated this using ADS, but would love to 
> see
> anyone's conflicting data.  I should point out that this is with
> soldermask, which I'm assuming most designs will be forced to use.
> Similar experiments with pure microstrip (no soldermask) showed
> microstrip to be slightly less lossy than stripline.
> 
>  
> 
> I also went searching for empirical data comparing microstrip vs.
> stripline (with trace width/thickness kept constant), and could 
> only
> come up with differential insertion loss data (which my experience 
> shows
> to be about the same as single-ended) from some previous test 
> boards.
> Again, this data suggests there isn't a significant difference in
> insertion loss between microstrip (ms) and stripline (sl).  
> 
>  
> 
> designator  ms/sl   length          SDD21 (dB/inch)     SDD21 
> (dB/inch)
> SDD21 (dB/inch)    SDD21 (dB/inch)     SDD21 (dB/inch)
> 
>                                    (inches)        @ 2GHz
> @ 5GHz                   @ 10GHz                     @ 15GHz
> @ 20GHz
> 
> tx                     ms       3.70                -0.31
> -0.64                          -1.09         
> 
> ty                     ms       6.72                -0.33
> -0.68                          -0.93                           
> -1.41
> -1.95
> 
> tz                     ms    14.73                -0.29
> -0.65                          -1.24                          -1.78
> -1.94
> 
> sx                    sl        4.70                 -0.32
> -0.84                          -1.39                         -2.82
> -2.89
> 
> sy                    sl      10.70                 -0.31
> -0.70                         -1.39                          -2.40
> -2.48
> 
> sz                    sl      20.00                 -0.32
> -0.69                         -1.10                          -1.72
> -2.18
> 
>  
> 
> When I look at this data, here's what I see:
> 
> (1) at 2GHz, there's no difference (as expected - conductor loss
> dominates)
> 
> (2) at 5GHz, "sx" has significantly more loss than the rest - but 
> hard
> to believe it's solely because it's stripline.
> 
> (3) at 10GHz, 2 of the striplines exhibit more loss, but the 
> longest
> stripline ("sz", which is probably the most reliable, due to its 
> long
> length), shows the same as the microstrips (actually less than the
> longest microstrip, "tz").
> 
> (4) roughly the same trend as (3) continues up to 20GHz (the data 
> from
> the shortest microstrip, "tx", was too noisy to make sense of).
> 
>  
> 
> The dimensions (in mils) were:
> 
> ms: h=3.5, w=5, s=5, t=1.4; microstrip had soldermask on it.
> 
> sl: h1=h2=6, w=5, s=5, t=1.4
> 
> Standard FR4 was used.
> 
> ______________ End Previous posting verbiage ___________________
> 
>  
> 
> Disclaimer:
> 
> The content of this message is my personal opinion only and although 
> I
> am an employee of Intel, the statements I make here in no way 
> represent
> Intel's position on the issue, nor am I authorized to speak on 
> behalf of
> Intel on this matter.
> 
> Jeff Loyer 
> 
> 
> 
> 
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