[SI-LIST] Using standard scope and single ended pulse to estimate differential impedance

Dear List members,

I have done some experiments and have gotten what seems to 
be reasonable results.  I seek the list's wisdom to comment on 
whether the technique is valid or it was just a coincident that 
the results looked right but can't be trusted.

My application is very limited.  We are making board assemblies 
with HDMI receivers.  One of the HDMI compliance requirements 
is that the differential impedance must be between 85 and 115 ohm.  
We do not own a differential TDR scope, but we do have a high 
performance DSA70804 with a 300 psec risetime FAST EDGE 
output from the scope and ~100 psec risetime probe+front end.
I thought I could press it into giving a reasonable estimate if 
the line length is more than a couple of inches.

Starting from the 2x2 impedance matrix and the relationship 
between the two voltages and the two currents as shown on 
page 14 of this very nice article by Daniel Wu:

http://www.ansoft.com/hfworkshop02/Danile_Wu.pdf

It seems that even if the -phase is not driven, we should be able 
to observe the induced voltage and thus infer the differential 
impedance.

Assuming symmetry and rewriting the equations:

V1 = i1 * Z11 + i2 * Z12
V2 = i2 * Z11 + i1 * Z12

Rearranging and combining:

Z11 = (V1 - i2/i1 * V2) * (i1) / (i1^2 - i2^2)
Z12 = (V1 - i1/i2 * V2) * (i2) / (i2^2 - i1^2)

V1 and V2 are the instantaneous voltages and 

i1 = (V1open - V1) / 50ohm
i2 = (V2open - V2) / 50ohm

Since all unknown variables can be measured, we 
can compute Z11 and Z12.  Then:

Zdiff = 2 * (Z11 - Z12);
Zcm = (Z11 + Z12) / 2;

I have made some measurements and plotted some results here:

http://alfred1520.wiki.zoho.com/HomebrewTdrMeasurements.html

The first graph is the V1, V2, and the computed Zdiff and Zcm 
of stuffed board.

The second graph is the Zdiff of 12 measurements.  They all seem 
to give believable results.

I'll be more than happy to share more details about the procedure 
here or else where if this is really a valid technique.


Best Regards,
Alfred Lee

------------------------------------------------------------------
To unsubscribe from si-list:
si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field

or to administer your membership from a web page, go to:
http://www.freelists.org/webpage/si-list

For help:
si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field


List technical documents are available at:
                http://www.si-list.net

List archives are viewable at:     
                http://www.freelists.org/archives/si-list
 
Old (prior to June 6, 2001) list archives are viewable at:
                http://www.qsl.net/wb6tpu

Other related posts: