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Controlled impedance (stripline etc.) boards

S

Spehro Pefhany

Jan 1, 1970
0
Hi,

Does anyone have references for crosstalk and impedance of stripline
and microstrip designs that have analog signal traces relatively close
to ground strips (on the same layer as the signal traces)?

The spacing I get for the desired max level of crosstalk without this
is uncomfortably large.

Thanks in advance.


Best regards,
Spehro Pefhany
 
Look for an article by Dr. Eric Bogatin in latest Printed Circuit
Design. Don't know if it's on the web yet or not.
Paul Mathews

Spehro said:
Hi,

Does anyone have references for crosstalk and impedance of stripline
and microstrip designs that have analog signal traces relatively close
to ground strips (on the same layer as the signal traces)?

The spacing I get for the desired max level of crosstalk without this
is uncomfortably large.

Thanks in advance.


Best regards,
Spehro Pefhany
http://www.speff.com
 
Johnson and Graham's "High Speed Digital Design" (ISBN-0-13-395724-1)
talks about this in section 5.6 - Guard Traces - on pages 201-204 in my
copy.

The message is that running your traces over a solid ground plane is
the way to substantially reduces cross-talk. Adding guard traces
between acitve tracks already running over a ground plane roughly
halves the residual cross-talk, and you can halve this again by putting
lots of vias along the guard trace to tightly couple it to the ground
plane.

"Lots of vias" (actually "vias at frequent intervals") is referred back
to J.A.Coekin "High Speed Pulses Techniques" Pergamon Press, Oxford,
1975 pages 203-205 - a book that I've never seen nor heard of.

Hope this helps. I've got a pair of text books specifically covering
microstrip, but they weren't a good investment - I could poke around in
them if you are desperate, but I don't like my chances of coming up
with anything useful.
 
S

Spehro Pefhany

Jan 1, 1970
0
Johnson and Graham's "High Speed Digital Design" (ISBN-0-13-395724-1)
talks about this in section 5.6 - Guard Traces - on pages 201-204 in my
copy.

The message is that running your traces over a solid ground plane is
the way to substantially reduces cross-talk. Adding guard traces
between acitve tracks already running over a ground plane roughly
halves the residual cross-talk, and you can halve this again by putting
lots of vias along the guard trace to tightly couple it to the ground
plane.

"Lots of vias" (actually "vias at frequent intervals") is referred back
to J.A.Coekin "High Speed Pulses Techniques" Pergamon Press, Oxford,
1975 pages 203-205 - a book that I've never seen nor heard of.

Hi, Bill:-

Okay, thanks, I've found some info in "High-Speed Circuit Board Signal
Integrity" (Thierauf) which suggests vias every 1/10 of a rise time
in the guard trace(s). An example shows crosstalk reduced by 4:1 for
the same *signal* trace spacing. References for double guard traces
("coplanar transmission line") are to (neither of which I have easy
access to at the moment):

Gopinath "Losses in Coplanar Waveguides" IEEE Trans. Microwave Theory
and Techniques, Vol. MTT-30, No. 7, July 1982, pp. 1101–1104.

and

Knorr "Analysis of Coupled Slots and Coplanar Strips on Dielectric
Substrate" IEEE Trans. Microwave Theory and Techniques, Vol. MTT-23,
No. 7, July 1975, p. 541.
Hope this helps. I've got a pair of text books specifically covering
microstrip, but they weren't a good investment - I could poke around in
them if you are desperate, but I don't like my chances of coming up
with anything useful.

If there's anything (simple) on the effect of the guard traces on the
characteristic impedance, I'd really appreciate having it. This isn't
picosecond stuff I'm working with.

Thanks,


Best regards,
Spehro Pefhany
 
J

John Larkin

Jan 1, 1970
0
Hi, Bill:-

Okay, thanks, I've found some info in "High-Speed Circuit Board Signal
Integrity" (Thierauf) which suggests vias every 1/10 of a rise time
in the guard trace(s). An example shows crosstalk reduced by 4:1 for
the same *signal* trace spacing. References for double guard traces
("coplanar transmission line") are to (neither of which I have easy
access to at the moment):

Gopinath "Losses in Coplanar Waveguides" IEEE Trans. Microwave Theory
and Techniques, Vol. MTT-30, No. 7, July 1982, pp. 1101–1104.

and

Knorr "Analysis of Coupled Slots and Coplanar Strips on Dielectric
Substrate" IEEE Trans. Microwave Theory and Techniques, Vol. MTT-23,
No. 7, July 1975, p. 541.


If there's anything (simple) on the effect of the guard traces on the
characteristic impedance, I'd really appreciate having it. This isn't
picosecond stuff I'm working with.

You can probably treat the guards as being very wide, since there
won't be much field on the far side of a guard trace. So just use the
signal trace dimensions and the gap distance to the guard traces and
pretend it's coplanar waveguide. Appcad or Txline or one of those
progs will then calculate the trace impedance.

We had a crosstalk problem a while back and did a bunch of
measurements to determine trace-trace capacitances among rows of
parallel runs and a couple of cases with guard traces. This was slow
stuff, so we were only concerned about capacitance, not full e-m
coupling. On the surface of a typical multilayer with three
closely-spaced traces, the effective c between the two outer traces
dropped about 4:1 if the middle one was grounded, as opposed to
floating or just not there.

John
 
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