[SI-LIST] Re: Quarter wavelength transmission lines
- From: Rob Hinz <rob@xxxxxxxxxx>
- To: richard2636@xxxxxxxxxx
- Date: Wed, 14 Nov 2001 12:02:44 -0800
Hi Rich,
If I understand your question correctly, you do not need to be concerned if
the line is properly terminated, that is, terminated in an impedance
matching the characteristic impedance of the line. The "resonance" issue
you are hearing about has to do with the physics of transmission lines. In
the case of a quarter wave section connecting a driver to a load (or a stub
off a bus), the driving point impedance will be the inverse of the
normalized load impedance times the characteristic impedance of the line.
If your load impedance is almost an open circuit, your driving point
impedance will be almost zero. The impedance as a function of frequency for
this system, operating near the quarter wave frequency of the section, will
behave much like series RLC for an open circuit load and a parallel RLC for
a short circuit at the load. Thus the often cited resonant circuit concept.
While it is similar to a classical RLC tank circuit it is not exactly the
same. The impedance function of a line section is periodic in frequency.
The impedance function of a RLC tank circuit is not.
This transmission line effect is brought about by wave reflections on the
line. For a quarter wavelength line terminated in an open circuit, the
reflected voltage wave arrives back at the driving point 180 degrees out of
phase because it has traversed the quarter wave section twice. This will be
true for any odd multiple of a quarter wavelength terminated in an open
circuit. Of course if you match the termination to the line, there is no
reflection and no issue.
For signal integrity, you must carefully manage reflected waves. It is not
just quarter wave sections (and their multiples) that are problematic,
although they can be particularly bad. Any large, long delay reflection has
the potential to create co-symbol or inter-symbol interference directly or
indirectly through cross talk.
I would highly recommend looking at the transmission line section in any
one of a number of basic texts on signal integrity, EM fields, or RF and
microwave engineering. The fundamentals usually require a chapter or less
and are not too difficult.
I hope I answered your question. Good luck!
Rob Hinz
Senior Electromagnetics Specialist
SiQual Corporation
rob@xxxxxxxxxx
phone (503)885-1231 x30
fax (503)885-0550
http://www.siqual.com
At 10:38 AM 11/14/2001 -0800, richard hill wrote:
>Hi all,
>
>When laying out a high speed trace which is properly terminated
>is it necessary to keep in mind the quarter wavelength scenario.
>What exactly is the significance of the quarter wavelength transmission
>line. I know it has something to do with a resonant
>circuit but do not understand it fully. Any input will be very
>educational.
>
>Thanks,
>Rich
>
>
>
>
>
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Rob Hinz
Senior Electromagnetics Specialist
SiQual Corporation
rob@xxxxxxxxxx
phone (503)885-1231 x30
fax (503)885-0550
http://www.siqual.com
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