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Sliding Load Performance Verification

Question asked by AVCM on May 22, 2012
Latest reply on Jul 25, 2012 by AVCM
I’m new to the forum, lots of good info here.

I work in a metrology lab and I am having the same issues regarding verification/calibration of sliding loads. I’m relatively new to metrology and currently I am trying to master the art of calibrating VNA mechanical calibration kits. I’d like to come up with a method of establishing some Pass/Fail criteria that would be somewhat simpler than the process suggested in the 85052B O&M manual requiring calibrating a system, performing a system verification, and then getting a printout of the residual errors after a calibration has been performed; or, using an airline, precision termination, and time domain gating to measure directivity directly.

I encounter a variety of sliding loads, predominantly HP 911C/D/E and Maury 8035’s in addition to sliding loads from the same manufacturers for N-Type, APC-7, and 2.4mm. I’ll focus on 3.5mm sliding loads as I suspect that whatever solution reveals itself (assuming there is a solution) will be applicable to the other connector style sliding loads. I’ve also noticed that Agilent (HP) used Maury 8035 series sliding loads in the early 85052A calibration kits. I’ve seen one 85052A kit with 911C loads. Most 85052B kits appear to use 911D/E sliding loads, although I have seen a few with Maury 8035 series sliding loads as well. Maury uses 8037 series sliding loads in their kits.

I’ve read Agilent AN 117-1 (great primer for folks like me who are just getting into this) as well as the _Sliding Load_, _Sliding load check_, _Sliding load calibration_, and _Sliding Load Verification_ threads here. The threads point out that the specs for sliding loads are different for each manufacturer, in particular Maury.

911C     (Reflection Coefficient)
Moveable Load Element: < 0.1, 2GHz to 10GHz
                                         < 0.035, 10GHz to 26.5GHz
Connector and Transmission Line: < 0.02, 2GHz to 26.5GHz (Male)
                                                        < 0.02 + 0.001*F(GHz), 2GHz to 26.5 GHz (Female)
911D/E (Parameter not stated)
Load Stability:     <0.004, 3GHz to 26.5GHz (Not sure what this means. If this is reflection coefficient, then return loss would be >48dB, which conflicts with the 85052B kit spec below)
85052A Kit (1250-1891, Return Loss)
Sliding Load: >42dB (Airline portion only) (Similar to Maury specification format)
85052B Kit (911D/E, Return Loss)
Sliding Load: >44dB (residual return loss after calibration)

Maury 8035 series:
Terminating Element (VSWR): 1.090 maximum, 2 — 4 GHz (<1.06 typical)
                                                 1.050 maximum, 4 — 34 GHz (<1.03 typical)
Connector (VSWR): <1.02 + 0.002ƒ GHz female
                                   1.01 + 0.001ƒ GHz female (I suspect a typo in here somewhere…)
Airline Accuracy (Return Loss): >44dB

Maury 8037 series:
Terminating Element (VSWR): 1.090, 2 to 4 GHz
                                                  1.05, 4 to 34 GHz
Airline Accuracy (Return Loss): 50 dB min return loss (Equivalent return loss of air line impedance)

The formats for the sliding load specifications are different not only between manufacturers, but also within manufacturer models.

Three responses from ‘daveb’ and ‘Dr_joel’ appear to offer a solution:

daveb     Re: Sliding Load Posted: Aug 29, 2007 9:36 AM
“…After calibration when measuring the sliding load, you will see the reflection coefficient of the sliding load element. As described previously, the variation in the magnitude of the sliding load element provides an indication of the quality of the calibration.”

Dr_joel     Sliding Load Posted: Aug 30, 2007 1:22 PM
Take the difference in linear magnitude (worst case is 0.002), and then take 20*log10(.002)=-54 dB. So, this says that a combination of your sliding load error and your directivity is worst case -54 dB (we can't separate the two). Good cal.

Dr_joel     Sliding load calibration Posted: Jan 23, 2009 9:22 AM
“To really measure a sliding load, you would need to meaure the relfection at each slide point, then compute the differences (worst case) to come up with the min-max vswr of the load. The difference is the quality of the load, since it should provide exactly the same gamma at each slide point. The only reason it won't is if the transmission line (airline) is not 50 ohms. The actual value of the return loss does not substantially contribute to the error of a sliding load.”

I calibrated our 8510C using a TRL kit (85052C) and took data for a 911C set of sliding loads, two sets of 911D/E sliding loads, and four sets of 8035 series sliding loads. If I apply ‘Dr_joel’s expression: 20*log10(reflection coefficient) to the reflection coefficient max-min difference across the six slide positions for each measurement frequency, to what do I compare the result to establish proper performance? It appears that I can use the 85052B kit 44dB spec as the minimum performance for 911D/E sliding loads (Only one set of 911D/E’s exceed 44dB across 3GHz to 26.5GHz), but I’m not sure what to use for the other devices. Am I on the right track? 
If using the 44dB specification from the 85052B kit is not the correct quantity for evaluating the performance of the 911D/E sliding loads, what should I use? More generally, what would constitute a ‘good cal’ threshold when evaluating any of the sliding loads I listed?