[SI-LIST] Re: TDR equipments -- comments or reviews

  • From: "Loyer, Jeff" <jeff.loyer@xxxxxxxxx>
  • To: Dan <signal.integrity2@xxxxxxxxx>, "si-list@xxxxxxxxxxxxx" <si-list@xxxxxxxxxxxxx>
  • Date: Thu, 4 Nov 2010 07:06:13 -0700

I find the Tek equipment much more intuitive than the Agilent.  I usually can 
get what I need out of the Agilent, but it takes much longer.  And, sometimes I 
haven't been able to get what I want,  but I'll leave open the possibility I'm 
missing something that a more experienced user can find and/or my equipment is 
out of date.  I know others have echoed my feelings; I look forward to seeing 
comments from folks who have had positive experiences w/ the Agilent and/or 
Sparq and have reasons for preferring them.

I've witnessed it proven that the Tek can provide very accurate 2-port 
S-parameters using IConnect, see
http://www.tek.com/products/oscilloscopes/sampling/interconnect_analysis/customer_papers/vna_tdr_correlation.pdf
I expect these results could also be replicated on the Agilent and/or Sparq.  
Of course, there's going to be a difference in the dynamic range between the 
TDR and VNA, so don't expect to measure massively lossy systems with a TDR.  

The Tek provides differential excitation; the Agilent performs single-ended 
measurements and derives the differential response from those (Sparq too?).  
Theoretically nothing wrong with that - that's what VNA's do also.  But, on the 
practical side, I've been able to use Tek's differential probes (P80318) to 
measure SDD21 of actual traces on product boards.  This wouldn't be practical 
if I couldn't take advantage of the purely differential signaling to preclude 
the need for ground connections.  
Some caveats: 
 * I measured extremely long traces to minimize the effect of launch 
discontinuities on the results
 * I used traces close to the top and backdrilled vias so their effects were 
minimal
 * I wouldn't want to quote my results very precisely, especially after about 
10GHz
 * Don't try this with cruddy cables - the skew has to be in the sub picosecond 
range
But, the measurements were enough to clearly discern the SDD21 difference 
between "bad" and "good" boards.

The P80318 probes are also nice for quick differential impedance measurements 
since you don't need a ground connection; it would only work with the Tek.

Also, you didn't mention how many ports you're hoping to measure, though you 
imply >2 since you're getting a separate sampler.  As Cherry's paper (link 
above) demonstrates, TDR/TDT works just fine for 2-port measurements.  If 
you're going above 2 ports, you have a completely different beast to wrestle: 
femtoseconds of skew will influence your measurements.  For some limited cases 
or coarse measurements, I've been able to make the Tek work for >2 ports, but 
wouldn't want to push it too far.  I haven't heard of a calibration scheme from 
Tek to accommodate this; I've crafted my own but it takes a lot of effort.  
Perhaps Agilent or Sparq do better here.  4-port VNAs have calibration 
algorithms to solve the problem.

I think folks would find risetimes of about 15 ps out of both the Agilent and 
Tek; PicoSecond Pulse labs has modules if you need faster, and I think the 
Sparq is faster.

Of course, this is purely opinion, and I'd probably be loyal to the Agilent or 
Sparq if I'd used them more.

My $0.02...

Jeff Loyer


-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On 
Behalf Of Dan
Sent: Tuesday, November 02, 2010 3:15 PM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] TDR equipments -- comments or reviews

Hi all,
I am planning to buy TDR equipments. I have shortlisted two equipments, one
from Agilent and anther Tektronix, along with the mainframe. Can anybody let
me know your comments or reviews about these equipment and your advice or
idea to buy this?

I am also planning to do s-parameter extraction from the TDR measurement. So
i need to TDT also. So i am adding a samping module.

Agilent:
mainframe: 86100C (option202 for extarcting s-parameter touchstone)
TDR: 54754A (can anyone tell me the achievable rise time or frequency in
this module)
Sampling module: 86112A

Tektronix:
mainframe: DSA8200 (80SSPAR Iconnect for s-parameter extraction)
TDR: 80E04 (can anyone tell me the achievable rise time or frequency in this
module)
Sampling module for TDT: 80E03

Is TEK better than Agilent. Is TEK's Iconnect outperforms Agilent's option
202 in the mainframe to extract s-parameter touchstone?

Thank You for your time.

DAN


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