[SI-LIST] Re: SMA cable for TDR
- From: "Zabinski, Patrick" <zabinski.patrick@xxxxxxxx>
- To: "QU Perry" <Perry.Qu@xxxxxxxxxxxxxxxxxx>, <si-list@xxxxxxxxxxxxx>
- Date: Tue, 28 Oct 2008 15:01:22 -0500
Perry,
The answer depends on your application - here's a brief explanation of
"why."
At 3 feet (1 meter) in length, most common laboratory-grade SMA cables
have noticeable attenuation and dispersion. The attenuation will reduce
the vertical swing of the TDR pulse, but you can readily compensate for
it through calibration and/or post process. From that respect, we
generally do not care too much about simple/flat attenuation.
Dispersion, on the other hand, is not so easy to compensate for.
If you disconnect the cable from your TDR head, you should see a
fairly-well shaped step response, where the signal starts low,
transitions high, and goes high. Ignoring any ringing around the
transition, the signal should transition to its high state and remain at
the same high voltage level. The "high" should be fairly constant in
voltage.
If you then attach a long cable to the TDR head, the dispersion changes
the shape of the waveform. Instead of being flat, the "high" voltage
will have an asymptotic shape that quickly comes to about 80% to 90% of
full swing then ever-so-gradually increases to full 100% swing. A good
way to see this is to place a cursor near the transition (i.e., just
after the rising edge). Then, move the cursor to the right, and you'll
see the voltage (i.e., apparent reflection) increase as you move out.
You generally need to zoom way, way out before you can read 100% swing.
When you calibrate the TDR for vertical height, you generally do so at
one point in time. In some TDRs, you get to choose that point in time
using cursors. In other TDRs, you do not have control, and it chooses
where to calibrate the vertical scale. Either way, the chances of the
calibration point being aligned with your device under test (DUT) are
low, such that the vertical scale will be inaccurate.
In our lab, we have seen typical cables (on the range of 3 feet in
length) skew our measurements by 10% to 15%. When we are using the TDR
to find discontinuities (e.g., when troubleshooting), we don't care
about such inaccuracies, so we use whatever cable is convenient.
However, when we are using the TDR to validate or create models, then
such inaccuracies in apparent reflectively (when inversely scaled to
impedance) can be quite significant, and we then pull out our
more-expensive low-dispersion cables.
It comes down to how persnickety you are about accuracy and what you're
going to do with the results. If you can live with inaccuracies, then
surf the web and buy the cheapest cable you can find. If, on the other
hand, you are trying to develop/verify models, then you should budget
for more expensive cables - paying most attention to dispersion and not
so much on attenuation.
Pat
> Hi,
>
> I plan to buy some dedicated SMA cables for my TDR
> measurement that was
> used to extract S parameters for serdes channels. Rise time about 40ps
> and 3 feet flexible cable will be ideal. Is there any specific vendor
> that you recommend ? I tried W L Gore but they are expensive.
>
> Please email me if you have any suggestions.
> Thanks
>
> Perry
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