FreeLists / si-list / [SI-LIST] Re: Differential microstrip with coplanar ground traces... unexpected results

["Scott McMorrow" <scott@xxxxxxxxxxxxx>]

Michael,

Resonance conditions are formed between sections of transmission lines 
that have discontinuities, due to reflections at the boundary between 
two impedances.  These are generally quarter-wave and half-wave 
resonances and multiples thereof, and are described quite well in books 
such as Pozar's Microwave Engineering.

As for ground shields, or ground traces, the trace portion is "just a 
transmission line" that is periodically terminated to ground through a 
via.  At the via, the impedance is 0, and causes a discontinuity at the 
point where the ground traces connects with the ground via. This 
discontinuity will set up a reflection and a corresponding resonance. If 
the other end of the ground trace is left open, then a quarter-wave 
resonator is formed.  If the other end of the ground trace is also tied 
to ground through a via, then a half-wave resonator if formed.  At the 
resonant frequency and integer multiples the trace will no longer 
operate as an effective ground shield, and instead will cause a peaking 
of crosstalk.

Much more on this phenemenona can be found by searcing the SI archives. 
Several of us, including Mike Conns and myself have written about this 
in the past.

best regards,

scott

-- 
Scott McMorrow
Teraspeed Consulting Group LLC
2926 SE Yamhill St.
Portland, OR 97214
(503) 239-5536
http://www.teraspeed.com



Michael Smith wrote:

Hi guys,

I found this thread very interesting and informative.  I was hoping
someone could explain what is meant by "exciting half-wave resonance"
and how via spacing would affect this.

Thanks for your help,

Michael Smith
Hardware Engineer
iZ Technology Corp.

-- 
Scott McMorrow
Teraspeed Consulting Group LLC
2926 SE Yamhill St.
Portland, OR 97214
(503) 239-5536
http://www.teraspeed.com



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