[SI-LIST] Re: Impedance Measurements of PCB signals

> Hi All,
> 
> I would like to ask a few questions regarding Imdepence 
> measurement requirements by end users.
> 
> 1.  Are the end users of hi speed circuits looking to reduce 
> the allowed tolerance of the PCB mfg for impedance 
> measurements?  Typically what we see is a +/- 10% allowed 
> tolerance where most designs can handle +/-20%, but the end 
> user is requesting 10.  I am hearing rumors that some 
> designers are requesting +/-5%.
> 

The justification of tighter impedance tolerance is often
a trade-off between line, driver, packaging, and termination
impedance.   By tightening the tolerance on the board
impedance, you can sometimes allow a little more wiggle-room
on the other areas.

In addition, some circuits are more sensitive to impedance.
A good example are analog circuits with high-resolution
sampling, where impedance discontinuities degrade the system-level
effective resolution (ENOB).

10% is acceptable for most of our systems, but 5% would sure
make life easier....

> 2.  Would end users benefit by receiving actual on board 
> measurements, along with data pts on the measured circuitry 
> before assembly?
> 

Yes.

I've been surprised several times at the way some folks
interpret TDR measurements to extract impedance.  I've
had boards come in at 70 ohms (!!!), where the manufacturer's
data sheet stated the impedance was 50.2 ohms.  We asked
for and eventually received the TDR plot, and we quickly
realized the manufacturer was looking at the wrong part
of the curve.  

In addition, with fine-line technologies, the TDR curves
have a "slope" due to loss, and I prefer to make my own
judgement of how to interpret the slope versus letting
someone else do it for me.

> 3.  Do multiwire designs have an advantage over Cu signals 
> from an impedance stand pt, or do they just have the ability 
> to reduce high layer counts?

Personal experience: no.  That said, my experience is
limited (one stud project), so it would be worth asking
others as well.

Pat

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