Issue link: https://iconnect007.uberflip.com/i/1464168
58 DESIGN007 MAGAZINE I APRIL 2022 I oen get asked why a measurement isn't as expected; without being physically pres- ent, even via a weblink, that's a tough call. What can be done to rule out as many things as possible and build confidence in the complete system? Interestingly, as soon as the user is confident that the system is okay then good measurements become simpler to achieve. is is where the Touchstone files come in handy. Simply by exporting a pair of 4 port .s4p Touchstone files for the coupon under test from the modeling soware, these two files can be fed to the long line/short line post-processing soware. With Polar Atlas you can set the input to the Delta-L to be Touchstone rather than the VNA. is sim- ple step allows you to send idealised results for the two coupons direct into the test so- ware. Post-process the results and verify that with "ideal" data the long line/short line post processing works as expected. Once that is proven, the operator can go back, recalibrate the VNA cable setup, and make the measure- ment again with increased confidence of a quality result. What kind of situations can this help with? A first time or inexperienced VNA user can be faced with a variety of unfamiliar situa- tions during insertion loss measurement: e VNA needs to be correctly warmed up and stabilised, a calibration procedure needs to be run once the cables are connected with the appropriate artefacts, and care must be taken for all the ports to be connected in the correct order to the correct artefact. With practice, "muscle memory" makes this pro- cess easy but first-time users need to take care, and it's best to do this in a quiet situa- tion without interruption. Connectors all need to be torqued to the specified level, and whilst this is recommended on TDR measurements, in practice you can get away with manual connections on a TDR. With a VNA, the need to torque connectors is far more critical. Simple errors, like connect- ing the VNA probes to the short line when you should be probing the long line, are easy to fall into. e ability to dry run the measurement with accurate simulated s-parameters removes a level of doubt and raises an operator's con- fidence that good measurements are possible and, indeed, with practice are repeatable and normal. In conclusion, the ability to insert trusted s-parameter transmission line data directly into the insertion loss processing soware in place of raw measurement data raises operator trust in the system when using with "real" data from a VNA. DESIGN007 Additional related content: • The Printed Circuit Designer's Guide to… Secrets of High-Speed PCBs, Part 1 by Martyn Gaudion • The Printed Circuit Designer's Guide to… Secrets of High-Speed PCBs, Part 2 by Martyn Gaudion Martyn Gaudion is managing director of Polar Instruments Ltd. To read past columns or contact Gaudion, click here. I often get asked why a measurement isn't as expected; without being physically present, even via a weblink, that's a tough call.