[SI-LIST] Re: 40 GHz probe calibration
- From: "Loyer, Jeff" <jeff.loyer@xxxxxxxxx>
- To: "si-list@xxxxxxxxxxxxx" <si-list@xxxxxxxxxxxxx>
- Date: Mon, 27 Oct 2014 15:29:47 +0000
Hello Arnav,
I had an opportunity to study a similar issue for SET2DIL, which uses 450um
GSSG probes. My experience coincides with that of Gert and Alex. I believe
the fundamental issue is that there is a difference between the propagation of
energy when measuring the GSSG "thru" and that of an actual DUT.
The calibration "thru" is a bar directly between the probe tips, and allows
significant energy to be coupled directly between the two signal probes. Note
that the connection between the probes is orthogonal to the probe tips.
The DUT typically begins with a microstrip trace out of the end of the probe
pad, and forces the energy to flow parallel to the probe tip.
I believe there is a fundamental difference in the modes of propagation between
these two, causing an inherent flaw in any calculation assuming equivalence. I
think the difference is negligible up to ~15GHz (for 450um GSSG probes), but
significant after that.
If a calibration thru was designed to exit the probe pad parallel to the probe
tip and then loop back to the other probe pad, that might better mimic the DUT,
but those don't exist, as far as I know.
I agree with Alex's recommendation if you need to probe at those frequencies -
you'll want smaller probes with GSGSG configuration. At those frequencies,
you'll also need to be more careful of your calibration method, connector
types, and thoroughly test your cables for phase stability; don't be surprised
if your current 20GHz connectors and cables aren't adequate.
I would revisit the question of how high a frequency you really need. 20GHz is
easy; 40GHz is HARD!!! (to steal a quote from an expert on the subject) You
may spend 10x the time and money making 40GHz measurements as you did making
20GHz measurements. And, there's probably no point in acquiring data beyond
where return loss is greater than insertion loss.
If you decide you need 40GHz data, there are measurement experts who routinely
perform these; it might be more efficient (both in cost and time) to contract
the work out if you only need a few. Their prices may seem high until you
consider the cost of obtaining appropriate equipment and experience.
One fast/easy method to check calibration is to connect a "thru" between every
available port immediately after calibration (1->2, 1->3, 1->4, 2->3, etc.) and
ensure insertion and return loss for every similar combination are the same.
This will quickly highlight the issue you originally referred to, as well as
many others (whether the "thru" should be classified as "defined" or "unknown",
for instance). This is only a first-order check, but a flawed calibration will
usually fail this.
Jeff Loyer
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
Behalf Of Havermann, Gert
Sent: Monday, October 27, 2014 1:35 AM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] AW: 40 GHz probe calibration
Take a look at this Oracle Design Con13 paper:
13-WP5 Impact of Probe Coupling on the Accuracy of Differential VNA
Measurements It tries to quantify the amount of crosstalk caused by the PCB
probes when making probe measurements. These effects can be calibrated when the
isolation step of the calibration is performed, but this brings up even more
problems.
BR
Gert
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-----Ursprüngliche Nachricht-----
Von: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] Im
Auftrag von Ippich, Alexander
Gesendet: Montag, 27. Oktober 2014 07:39
An: arnshah@xxxxxxxxx
Cc: si-list@xxxxxxxxxxxxx; Ippich, Alexander
Betreff: [SI-LIST] AW: 40 GHz probe calibration
Arnav,
I can confirm, that when attempting to calibrate GSSG style microprobes with
1000um S/G pitch, I also do see return loss much worse than 20dB at 40GHz. The
probes were calibrated with the recommended calibration substrate from the same
supplier.
The measurement protocol that was shipped with the probes states a S11 of
around -5dB at 40dB into a 50ohm load.
I would recommend using GSG style probes and smaller pitches to alleviate the
issue.
Best regards,
alex
-----Ursprüngliche Nachricht-----
Von: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] Im
Auftrag von Arnav Shah -X (arnshah - BBI TECHNOLOGIES INC at Cisco)
Gesendet: Sunday, October 26, 2014 8:16 PM
An: si-list@xxxxxxxxxxxxx
Betreff: [SI-LIST] 40 GHz probe calibration
Hello all,
I am attempting a 40GHz calibration. I previously tried calibrating with 1000um
pitch G S S G differential probes without success. I think at 40GHz there is no
resonance, or planar waves on the cal substrate. Crosstalk is the main problem;
I will try calibrating with <500um probes. Is there a way to measure/calculate
this crosstalk? How can I verify this hypothesis? Right now, I define a good
cal as return loss below 20dB, is there any other information I can use in
determining a good cal?
Thank you!
-Arnav
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