Hi Allen,
I suppose I can take a stab at this one.
(1) How much, if any, of the energy will be radiated into free space when it
gets to the end of the open transmission line? To me, this looks like a
monopole antenna. I don't have a very good understanding of antenna
theory, so this could very well be an invalid assumption.
>>I am not really sure, maybe someone else can shed some light on this.
(2) If radiation does take place as stated above in question (1), which is
better for reducing the radiation, termination at the source or termination
at the load of the transmission line, or does it matter?
>> If you do get a lot of emission form this it will probably be due to
standing waves on the
line and I would say that a termination at the end of the line would be
most useful to stop
the occurrence of reflections (which will cause your standing wave).
Ideally what you want
is to have your source and load matched to the characteristic impedance
of your line.
(3) If the characteristic impedance of the trace on the printed circuit card
differs from the characteristic impedance of the trace on the backplane, how
is this handled? Is a termination needed at each end in this case?
>> In this case I would not worry too much about the length of the trace
from the driver to the
edge connector (unless you are working with a several GHz signal). At
this point if you are
going to match something I think you would have to consider matching the
PCB to the
characteristic impedance of the backplane.
Good luck!
Jeff Bailey
EMC Technologist
SST Division of Woodhead Canada
Phone: (519) 725 5136 ext. 363
Fax: (519) 725 1515
Email: [email protected]
Web: www.sstech.on.ca
All comments here are my own and do not necessarily express
the views of SST.
-----Original Message-----
From: Allen Tudor [mailto:[email protected]]
Sent: Wednesday, May 12, 1999 5:31 PM
To: [email protected]
Subject: Transmission Line Theory
Greetings group,
I am trying to draw a parallel between transmission line theory and radiated
emissions.
>From what I understand, a transmission line can be terminated at the source
or the load with an impedance that equals the characteristic impedance of
the transmission line itself.
With this in mind, consider this scenario. A printed circuit card drives a
clock signal down a trace on a backplane. The length of the backplane trace
is long enough to be considered a transmission line. The driver on the
printed circuit card is located within ½ inch of the edge connector (mating
with the backplane) and is terminated with an impedance equal to the
characteristic impedance of the backplane trace. However, the backplane
trace is open ended (there is nothing connected to the end of the trace).
Transmission line theory says the signal integrity will be maintained in
this case.
Now for the questions:
(1) How much, if any, of the energy will be radiated into free space when it
gets to the end of the open transmission line? To me, this looks like a
monopole antenna. I don't have a very good understanding of antenna
theory, so this could very well be an invalid assumption.
(2) If radiation does take place as stated above in question (1), which is
better for reducing the radiation, termination at the source or termination
at the load of the transmission line, or does it matter?
(3) If the characteristic impedance of the trace on the printed circuit card
differs from the characteristic impedance of the trace on the backplane, how
is this handled? Is a termination needed at each end in this case?
I look forward to your responses.
Thanks.
Allen Tudor, Compliance Engineer
PairGain Technologies tel: (919)875-3382
2431-153 Spring Forest Rd. fax: (919)876-1817
Raleigh, NC 27615 email:
[email protected]
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