[SI-LIST] Re: Q on TDR coupon measurements

Folks:

The TDR visual trace you see one the scope face (assuming you are using a=20
TEK, LeCroy or Agilent TDR) has only a mild dependence on risetime of the=20
excitation pulse. As the pulse travels down the interconnect, it loses=20
risetime and tends to smear (average) any effects which are proportional to=
=20
a time derivative (parasitic or real capacitors and/or inductors for=20
instance).  If you look at a coupon with the usual messy launch on each=20
end, the nearest the launch point will show a lot of discontinuity while=20
the far one will show much less. This is because the risetime is slowed by=
=20
the losses.  The lower the loss the interconnect, the larger the far end=20
discontinuity will affect the results.

In the frequency domain, the presence to launch discontinuities causes=20
standing waves at specific frequencies corresponding to half and full=20
wavelength (in the dielectric) distances along the interconnect.  For low=20
loss materials, the resonance effects are more pronounced.

So, if you are using a surface pad to launch onto a PCB, the effect of=20
parasitics must be taken into account.  To a first approximation, the=20
risetime loss is approximately equal to the time constant of the=20
parasitic.  In the case of a shunt Capacitance, the transmission line=20
impedance is divided by two and for a series inductance, the transmission=20
line impedance is multiplied by 2. Thus for a capacitor, time constant =3D=
=20
0.5*Zo*C and for an inductor, time constant =3D L/(2*Zo).  As typical=20
numbers, C =3D (2-3) pF and Zo =3D 50 ohms, time constant ~ (50-75)ps.  This=
 is=20
more than enough to corrupt the eye on a good 5 Gbps SERDES device.

TDR traces are wonderful for seeing discontinuities for diagnostic=20
purposes, far better than the Return Loss.  A TDR is not suited for=20
measuring propagation delay to any accuracy since the loss of risetime or=20
equivalently, the additional delay.  These introduce a 2-way error into the=
=20
measurement. For that measurement either a TDT through measurement is=
 required.

Thanks for letting me share these ideas with you.

ed



At 12:27 PM 3/24/2006 -0500, Scott McMorrow wrote:
>ah ... now we know where the term "averaging mean" method comes from.
>Averaging refers to placing the TDR instrument into averaging mode.
>Mean refers to the segment of the TDR waveform over which the mean is
>measured.
>
>This method is reasonable, considering that most TDR coupon launches are
>terrible, and attempts to remove the launch artifacts from the
>measurement result.  But, it is incorrect, and does not provide a
>measure of the actual impedance of the PCB trace.
>
>
>Scott McMorrow
>Teraspeed Consulting Group LLC
>121 North River Drive
>Narragansett, RI 02882
>(401) 284-1827 Business
>(401) 284-1840 Fax
>
>http://www.teraspeed.com
>
>Teraspeed=AE is the registered service mark of
>Teraspeed Consulting Group LLC
>
>
>
>Loyer, Jeff wrote:
> > One source for this kind of information:
> >
> > http://www.intel.com/design/chipsets/applnots/29817901.pdf=3D20
> >
> > It's pretty old, though - much has been learned since 2000...
> >
> > Jeff Loyer
> >
> >
> > -----Original Message-----
> > From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
> > On Behalf Of Ivor Bowden
> > Sent: Thursday, March 23, 2006 5:07 PM
> > To: si-list@xxxxxxxxxxxxx
> > Subject: [SI-LIST] Re: Q on TDR coupon measurements
> >
> > I've been enjoying the discussion on TDR measurements. While I
> > understand the principle, I don't have experience with them beside
> > evaluating TDR printouts.
> >
> > I was wondering if anyone knows a web site or other resource that has a
> > variety of example waveforms and their proper interpretations? Including
> > touching on some of the details Scott and Eric mention.
> >
> > tia,
> >
> > Ivor Bowden
> > Curtiss-Wright Controls Embedded Computing
> >
> >
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