[SI-LIST] Re: Question about diff-probes
- From: Don DeGroot <don.degroot@xxxxxxxxxxxxxxxxxxx>
- To: Zaiyi Liao <zaiyi.liao@xxxxxxxxxxxx>
- Date: Tue, 5 Jan 2010 10:28:24 -0700
Hello Zaiyi--
The only hand-held probes we've used are those we've made ourselves.
I've not tried the Gigaprobes as a hand held solution, but want to at
some point, To be honest, our hand-held differential probing was more
than frustrating...
- We had difficulty making consistent connections to four contact pads
(we were using GSSG), and all S-parameter correction requires
identical electrical contacts for all devices and all cal standards.
The fewer the contacts the easier it is for hand-held probing.
- We also encountered in the literature a fundamental difference in
what is meant by a two-port differential circuit (like a truly
balanced system) and a four-port transmission line over which we send
differential signals. It's really hard a keep a differential pair on
PCBs to be fully balanced, so most of the time we're really working
with four-port networks.
So, we had much better luck (as others have already stated), making
four port measurements and computing the mixed-mode S. And we had much
better luck with a probe station or when using single port GS probes.
We do this with either VNAs or TDR/Ts. The more instrument samplers,
the faster it goes.
With a probe station and a TDR scope with four samplers (we were using
a LeCroy WaveExpert), we get good differential S-parameters up to 15
GHz or so, and can push to 20 GHz when we were very careful.
See if any of the papers or software listed at
http://www.pcbmeasurements.com/Resources.html
might be of some help.
We can also characterize custom cal standards if you ever get to that
point.
Cheers!
--Don DeGroot
www.PCBmeasurements.com
On Jan 4, 2010, at 8:46 PM, Zaiyi Liao wrote:
> Hello experts:
> Now I want to test S-parameter of diff-channel on PCB.
>
> I find it's hard to find portable low-impedance(50ohm) diff-
> probe for VNA. As I know,there are some diff-probes up to 15Ghz
> (such as N1020A TDR probe) for TDR test. And there are lot of high-
> impedance diff-probes for eyepattern test up to 10Ghz. In chip test,
> there are many low-impedance probes(GSG,GSGSG,GSSG,etc)
>
> I want to know whether there is any portable(no need probe
> station) diff-probe for VNA?? Why it's hard to find low-impedance
> probe for PCB test? Another question is that what is the differences
> of S-parameter test by VNA and TDR.
>
> Thank you very much!
>
> BR//Zaiyi Liao
>
>
>
>
>
>
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