# Thread: SOLT cal short or open fringe parameters

Replies: 73 - Pages: 5 - Apr 24, 2012 12:43 PM Last Post By: Lucux Threads: [ | Next ]
 Posts: 54 Registered: 08/21/09
Re: SOLT cal short or open fringe parameters
Posted: Jul 1, 2011 5:38 PM   in response to: Lucux
 thanks a lot Joel. this is very clear to me.yes, you are right. the impedance was not precisely 50 ohms. it is 49.987 ohms according to the approximation equation for coax transmission line with ideal perfect conductors for both outer and inner contact. i'm curious on how you know that the impedance is off by 0.01 ohms? how did you come to this conclusion from the factor of the ripple on the plot?thanks again.
 Posts: 2,793 Registered: 12/01/05
Re: SOLT cal short or open fringe parameters
Posted: Jul 2, 2011 1:46 PM   in response to: Lucux
 First, I supposed that any mechanism that made the inductance of the short non-ideal would have a monotonic behavior; that is, if some aspect of EM cause the inducance to be effectively reduced, or enhanced, as frequency increased, it would do so in a way that caused the value of the inductance to vary from ideal but always the variation would increase. For the ripple in the phase to be a correct model of the inductance, the whatever mechanism that caused the deviation would have to come and go with frequency, which would be unuasual. However, if there is an impedance mismatch, it is very common to see such phaseing in and out, as the electrical delay gets long. So I simply loaded up a into ADS your inductance model, and created the curve for phase that matched your model (which I already was suspect of, as the DC value was negative which defies logic and indicates an overall misapplication of the inductance model). Then I changed the inductance to 0 and the impedance to 50.1 ohms, and saw and inverse ripple, from which I inferred that the impedance was less than 50 ohms. I then tried 49.95 ohm, the ripple had the correct shape but was too large, and so I continued iterating until I was within 0.01 ohm and obtained a similar ripple, that being 49.99 ohms (Note, an earlier post had listed 44.99 in one spot, that I just corrected). By the way, this is almost exactly the process I went through to develop the 75 connector open model for the 8752A, about 15 or 20 years ago. It was the first application of finite element analysis to a calkit modeling process. I also modeled my expectatoin for minimum and maximum capacitance based on manufacturing tolerances: minimum is for shortest center conductor, smallest diameter center conductor, largest diameter outter conductor. Opposite for the maximum. This gives a max and min gamma phase. The model should choose a value for capacitance in the middle, even if it is not nominal, so you get the smallest worst case deviation. And the artan of the phase gives the residual sourcematch of the cal kit as well as residual phase deviatio for transmission tracking. Residual source match also needs to add in worst case load return loss, or course.
 Posts: 54 Registered: 08/21/09
Re: SOLT cal short or open fringe parameters
Posted: Jul 4, 2011 12:30 AM   in response to: Lucux
 many thanks Dr. Joelthis is very helpful. there are two places that i did not fully understand. you said, 1) ... and created the curve for phase that matched your model (which I already was suspect of, as the DC value was negative ...what value of DC is negative make you think ...an overall misapplication of the inductance model..?2) ...And the artan of the phase gives the residual sourcematch of the cal kit as well as residual phase deviatio for transmission tracking...what does this statement mean exactly? what is artan of the phase?my understanding is that the transmission line before the open or short happens has to be well optimized to have minimum reflection. otherwise the phase plot (with mark function, delay) would have high peak to peak ripples which does nothing good to either the curve fitting or residual sourcematch error. is that the right understanding? thanks again.
 Posts: 54 Registered: 08/21/09
Re: SOLT cal short or open fringe parameters
Posted: Jul 4, 2011 1:31 AM   in response to: Lucux
 2.JPG (119.0 KB)
 for plot with relatively high ripple, do you think that this is due to the reflection before the open happens? see the one attached here. in this case, does the curve fitting still work well? if not, do you think data based characterization is better to do this?in one of Agilent application note, it says,"The data-based standard model is a new feature in Agilent’s network analyzers. It allows a calibration standard to be defined by a data file that contains frequency data, S-parameter data and uncertainty data. The data file may be created using actual measured data from a reference metrology laboratory, model data from device modeling software or combinations of both. See Appendix D on page 37 for details on data file formats."do you have any comment on this?thanks again.
 Posts: 2,793 Registered: 12/01/05
Re: SOLT cal short or open fringe parameters
Posted: Jul 5, 2011 4:35 PM   in response to: Lucux
 "many thanks Dr. Joelthis is very helpful. there are two places that i did not fully understand. you said, 1) ... and created the curve for phase that matched your model (which I already was suspect of, as the DC value was negative ...what value of DC is negative make you think ...an overall misapplication of the inductance model..?2) ...And the artan of the phase gives the residual sourcematch of the cal kit as well as residual phase deviatio for transmission tracking...what does this statement mean exactly? what is artan of the phase?my understanding is that the transmission line before the open or short happens has to be well optimized to have minimum reflection. otherwise the phase plot (with mark function, delay) would have high peak to peak ripples which does nothing good to either the curve fitting or residual sourcematch error. is that the right understanding? thanks again."I don't know if I understood your extraction but you have a value for L0 of -3pH, it should never be negative if it is a realizable short. THis is a result of trying to polynomial match the ripple. You have a bad value for L0 that is over come (except at very low frequencies) to give a nearly correct value of L overall frequencies. But a strong clue that the inductance model is wrong. Mostly, the principle contribution should come from the first term.2) I should have said "arc-tan", sorry. take Arctan of the phase and you will receiver the magnitude error that caused the phase shift.
 Posts: 2,793 Registered: 12/01/05
Re: SOLT cal short or open fringe parameters
Posted: Jul 5, 2011 4:38 PM   in response to: Lucux
 "for plot with relatively high ripple, do you think that this is due to the reflection before the open happens? see the one attached here. in this case, does the curve fitting still work well? if not, do you think data based characterization is better to do this?do you have any comment on this?thanks again."Hard to say for sure, but you can do a similar measurment to find out...but to correct for this and the other error, if the impedance of the open delay offset is not 50 ohms, then change it in the model for the open (or short) to represent the true value. Then the ripple we be there as a result of the proper understanding the delay line impedance, and not some model-matching of the L or C terms to accomodate the error caused by the offset impedance. At the end of the day, it doesn't matter how the model is generated, just that the phase which it returns matches the phase response of the standard.
 Posts: 54 Registered: 08/21/09
Re: SOLT cal short or open fringe parameters
Posted: Jul 6, 2011 12:22 AM   in response to: Lucux
 many thanks Dr. Joelsorry that i might have not asked clearly, my question was,as you know there might be some mismatches before the actual open/short happens, such as unpreventable meachnical support or sometimes the interface itself, while the rest of the transmission line is still 50 ohms, in that case does the curve fitting still works well? or otherwise we can use data based standards characterization? or say is this a better approach to this problem?thanks again.
 Posts: 54 Registered: 08/21/09
Re: SOLT cal short or open fringe parameters
Posted: Jul 7, 2011 2:08 AM   in response to: Lucux
 OF.s1p (80.2 KB) 111.JPG (74.3 KB) 222.JPG (97.4 KB)
 hello Dr. Joeli tried to recall a s1p file. it is attached here. the plot frequency range is weirdly up to 28.9741GHz. the s1p file was made up to 6GHz by Ansoft HFSS. is this a bug? my PNA FW is A.09.33.09. can you help look into this? thanks in advance.
 Posts: 6 Registered: 09/08/09
Re: SOLT cal short or open fringe parameters
Posted: Jul 12, 2011 7:27 PM   in response to: Lucux
 Hello;Looking at the long line ripple after cal is only partial validation of your model, the source match. The other part you need is a way to evaluate reflection tracking. Having calibrated with offset standards, you can then attach a flush short and look at reflection tracking directly. Set up 2 traces, one in magnitude and the other in phase. Add 180 degree offset to the phase trace because a flush short should be a reflection of 1 at 180 degrees. So perfect reflection tracking would be flat lines on both traces. Match ripple approximations from peak to peak in dB: 20*Log10(ABS(1-(10^((pkpk/2)/20))))pk-pk ~source match in dB .1 44.7.2 38.7.3 35.2.4 32.6.5 30.7Drew
 Posts: 54 Registered: 08/21/09
Re: SOLT cal short or open fringe parameters
Posted: Jul 12, 2011 8:23 PM   in response to: Lucux
 thansk Drewwhen you say "attach a flush short", do you mean a short which is at the same plane as the reference plane? if yes, what about those interface where there are PTFE insulators? in that case, the flush short cannot be realized due to physical constraints. is this the right understanding?
 Posts: 6 Registered: 09/08/09
Re: SOLT cal short or open fringe parameters
Posted: Jul 13, 2011 9:29 AM   in response to: Lucux
 Yes, a flush short just terminates at the reference plane. SMA connectors with PTFE dielectric should not have any of the dielectric extending beyond the outer conductor, but SMA is quite often a sloppy connector. If the PTFE is trimmed properly, a flush short will still give you a good look at reflection tracking. If the PTFE does extend beyond the reference plane, the flush short will compact it back in and change the response of the port. But a similar thing can happen when you put 2 SMA connectors together. Drew
 Posts: 54 Registered: 08/21/09
Re: SOLT cal short or open fringe parameters
Posted: Jul 13, 2011 7:43 PM   in response to: Lucux
 thanks Drewif the PTFE does extend the reference plane, then in reality a flush short cannot be made. what do you mean by "the flush short will compact it back in"?
 Posts: 2,793 Registered: 12/01/05
Re: SOLT cal short or open fringe parameters
Posted: Jul 14, 2011 1:29 PM   in response to: Lucux
 If the PTFE extends past the reference plane, then it will interfer with the flush short, and presumably any other DUT that is connected to it, so that the connector is bad, and should be disposed of. But sometimes if the PTFE extends only slightly, the other DUT connected will push it back into the connector, and compress it back. other times it will cause the PTFE to "mushroom" over the mating surface, in which case the connector is ruined. But if the PTFE is extending past the reference plane, it is , by definition, ruined already.
 Posts: 54 Registered: 08/21/09
Re: SOLT cal short or open fringe parameters
Posted: Jul 15, 2011 1:03 AM   in response to: Lucux
 hello Dr. JoelI understand what you are saying. i meant the interface by definition has PTFE extended beyond the reference plane, such as TNC, BNC and MCX. in that case, since you cannot get a flush short how do you check reflection tracking?aslo, can you please answer my question i asked in the post of Thu Jul 07, 2011 2:08 am, where i asked why the plots look weird?thanks again.
 Posts: 2,793 Registered: 12/01/05
Re: SOLT cal short or open fringe parameters
Posted: Jul 15, 2011 8:26 AM   in response to: Lucux
 Yes, the plots look weird. I cannot reproduce the problem in latest firmware. I see from your plot that the last freq is the same as the last data. Maybe you opened the file and added as space or something? I downloaded your S1P file, recalled it on the PNA running A.09.42.01 and it recalled fine.