PCB trace with controlled impedance

[Zhiheng Cao]

Hi,

I found references on how to calculate characteristic
impedance for "microstrip" line (with ground plane
underneath) but in that case the trace is not
surrounded by ground plate on the upper layer.
Is there any reference on how to calculate Zo
when the top plate is surrounded (horizontally)
by ground plate.

Thanks,
Cao

 

[PeteS]

Hi,

I found references on how to calculate characteristic
impedance for "microstrip" line (with ground plane
underneath) but in that case the trace is not
surrounded by ground plate on the upper layer.
Is there any reference on how to calculate Zo
when the top plate is surrounded (horizontally)
by ground plate.

Thanks,
Cao

BaSed on my empirical experience, provided the surrounding plane is
more than 3 track widths clear, it makes a negligible difference to Zo.

Are you trying to make this yourself, or are you having the board made
by a faBricator? if it is being made at a fab, tell them the impedance
requirements and they will tell you the track widths, spacing and the
stackup.

Cheers

PeteS

 

[Noway2]

The buried signal line would be the "stripline" configuration and has a
formula similar to the microstrip one. You may want to consider
looking at www.sigcon.com and search the archives by topic. There are
plenty of articles on both microstrip and stripline configurations
there.

 

[Mikko Kiviranta]

Is there any reference on how to calculate Zo
when the top plate is surrounded (horizontally)
by ground plate.

This structure is sometimes called a "Coplanar
Waveguide with Ground". Eq 3.4.3.1 from Waddell
says

Zo = 188.5ohm / [ sqrt(er,eff) ( K(k)/K(k') + K(k1)/K(k1') ) ]

where

1 + er K(k') K(k1) / (K(k) K(k1'))
er,eff = ------------------------------------
1 + K(k') K(k1) / (K(k) K(k1'))

k = a/b, where a is the trace width and b is the width
of the groove (trace plus the two insulating gaps)

k1 = tanh( Pi a /(4 h)) / tanh( Pi b /(4 h))
where h is the substrate thickness

k' = sqrt(1 - k^2)
k1' = sqrt(1 - k1^2)

The function K(x) is the elliptic integral.

Regards,
Mikko

 

[Rene Tschaggelar]

Hi,

I found references on how to calculate characteristic
impedance for "microstrip" line (with ground plane
underneath) but in that case the trace is not
surrounded by ground plate on the upper layer.
Is there any reference on how to calculate Zo
when the top plate is surrounded (horizontally)
by ground plate.

Search the Agilent website for AppCAD, this free
tool does it for you.

Rene

 

[bg]

Go to http://www.taconic-add.com/en--downloads.php
Download TXline


Zhiheng Cao wrote in message ...

 

[Zhiheng Cao]

Thanks for the link, but this tool can only
calculate single ended transmission line, how
could I use it to calculate differential traces?

 

[Joel Kolstad]

Thanks for the link, but this tool can only
calculate single ended transmission line, how
could I use it to calculate differential traces?

Hmm... does the link get you TxLine 2003? It knows about coupled
(differentla) microstrips and striplines.

For many applications the separation between your differential traces ends up
being large enough such that the coupling coefficient between them ends up
being pretty small (e.g., less than 5%), in which case you can treat each
trace as a single-ended line and not worry too much about the true
differential impedances (even and odd modes).

---Joel Kolstad

 

[James T. White]

Hmm... does the link get you TxLine 2003? It knows about coupled
(differentla) microstrips and striplines.

Joel,

The TxLine off the Taconic web site is Version 1.1 and has a 1996
copyright. Where can you download TxLine 2003?

Thanks.

 

[John Larkin]

Joel,

The TxLine off the Taconic web site is Version 1.1 and has a 1996
copyright. Where can you download TxLine 2003?

Thanks.

Tx2003 seems gone from the AWR website. I could email it to you...
it's a 500K zipfile. It's a nice complement to Appcad.

mail me:

jjlarkin/at/highlandtechnology/dot/com/

John