[SI-LIST] Re: Single-ended S-para plot of 2 microstrip traces
- From: "Loyer, Jeff" <jeff.loyer@xxxxxxxxx>
- To: <sam.sim@xxxxxxx>
- Date: Tue, 11 Mar 2003 15:01:50 -0800
Interesting that you'd come across this. I was baffled by the same =
thing, except in my case it originally came from measurements in the =
lab, which I then reproduced in simulation. By putting 2 traces close =
enough together that they couple to each other, we create "Coupled Line =
Couplers". The single-ended S-parameters start doing strange things. =
You have to look at the differential S-parameters (as you observed). =20
The stripline version (much simpler math than microstrip) of these are =
covered in Pozar's book (Microwave Engineering, page 389); I haven't =
found the formulas for the microstrip cases, but I've heard that =
microwave folks have been using them for directional couplers.
Pozar's book only covers the low frequency effects (half-wavelength of =
the entire trace length) on stripline, while the phenomena I saw was for =
much higher frequencies on microstrip (and could be eliminated by simply =
putting Kapton tape over the traces to make them stripline).
Bottom line, it's real.
Below is some of the text of my original posting. You can search the =
archives on the responses I got.
_______________________________________
While measuring and simulating long coupled T-lines, I came across a =
"resonance" (sudden increase in S21 magnitude loss) at a much higher =
frequency than I expected. In the simulation below, a 13.655" coupled =
T-line with a Tp of ~160pS/in has a resonance for S21 at 3.54GHz.
By my calculations, 13.655" at 160pS/in is ~2.2nS, or 457MHz. Does =
anybody know where a resonance at 3.54GHz would come from?
Jeff Loyer
-----Original Message-----
From: sam.sim@xxxxxxx [mailto:sam.sim@xxxxxxx]
Sent: Tuesday, March 11, 2003 2:41 PM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Single-ended S-para plot of 2 microstrip traces
Hello, Experts
When I plot a S31 for 2 microstrip traces, say insertion loss, I'm =
seeing
suspicious notchs as the trace have a stub.
The microstrip geometry was 5 mil width, 5 mil space and 6 mil FR4
dielectric thickness, and I got RLGC model from Ansoft 2D Solver. The =
S31
graph was plotted by Hspice using w-element model.
Those notchs did not be seen at the case of one trace with same =
microstrip
geometry. When I calculated differential S-parameter, SDD21, those =
notchs
were disappeared. I could not find such notchs at stripline cases =
either.
=20
I'm very curious why the unpredictable notchs at graph of the =
single-ended
s-parameter happen at microstrip case.
Is it the limit of RLGC transmission line model for microstrip modeling =
?
Sam=20
AMD Boston
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