[SI-LIST] Re: Measuring common mode return loss
- From: Peter.Pupalaikis@xxxxxxxxxx
- To: "Juan G Fuentes" <jgfnolasco@xxxxxxxxx>, "si-list" <si-list@xxxxxxxxxxxxx>
- Date: Fri, 18 Mar 2011 21:11:01 -0500
Hi Juan:
Your problem is that your probe probably does not have a ground connection
(I.e. Two tips that touch down only). It works for differential because
theoretically no current flows in the ground if you can drive it differentially
and it comes out right even when driven single-ended and the conversions are
done. For common-mode signals, current wants to flow in the ground and the
probes look like an abrupt open circuit for common-mode.
In summary, to perform common mode measurements, you need a different probing
arrangement.
Pete
From Pete Pupalaikis's Blackberry
----- Original Message -----
From: Juan G Fuentes [jgfnolasco@xxxxxxxxx]
Sent: 03/18/2011 07:54 PM CST
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Measuring common mode return loss
Hi All
I am trying to measure the differential (DRL) and common return loss
(CMRL) of a differential driver an receiver in my SOC using a TDR but
I don't think I am following the right procedure, specially for the
CMRL
I am using a Tek sampling scope with the TDR module and Iconnect to
postprocess the TDR waveforms. I went through the process outlined in
some white papers from Gigaprobes on how to extract the differential
S-parameters (SDD11) and I think the data I got is good. However for
the CMRL (SCC11), I just cannot find any reference documentation on
how to do it. The web is full of theoretical docs about differential
s-parameters but focusing on VNA-based approaches.
My main issue here is that Iconnect requires a reference signal (to an
open or to a short) to do the S-parameters calculation; this works
fine when ODD signals are sent through the tighly coupled
differential calibration traces of my board where the DUT resides,
but when EVEN mode signals are launched, these get fully destroyed
(cancelled out, see attached files) by the time it reaches the TDR
sampler thus I cannot get reference step to use it in Iconnect.
Does anybody knows how to workaround this? VNA is not an option
Thanks
Gabriel
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