The Application of Time Domain Measurements

Several years ago, we decided to purchase a 20 GHz Vector Network Analyzer (VNA) to improve the quality of our calibrations and to perform site validation measurements. Previously, we had been using a 500 MHz VNA to calibrate LISNs, CDNs, and current probes; but we only had a basic understanding of network analysis. We didn’t really grasp the benefits of time-domain reflectometry (TDR) that are available in modern VNAs.

It wasn’t until we witnessed a TDR demonstration by Mike Windler of UL, that we began to understand the possibilities. Mike performed site validation above 1 GHz using a proposed revision of ANSI C63.4 (circa April 2005) in one of our 10m chambers. The benefits were immediately obvious: the TDR method was much faster than the Site Voltage Standing Wave Ratio (SVSWR) method, it was much less labor intensive, and troubleshooting with the TDR method was far superior. In fact, it appeared to be the only way to identify the source of non-compliance.

Despite the many benefits of using the TDR method for site validation, as an accredited test laboratory we are compelled to also perform site validation measurements using the CISPR 16-1-4 SVSWR method. The SVSWR method is also cited in the latest edition of CISPR 22 and the EuroNorm version (EN55022) will be a requirement in Europe and Japan next year (note at the time of this writing there may be some delay). We decided to conduct a study where both methods were used on a variety of test sites (10m, 5m, and 3m chambers, and a 10m OATS). Our hope was to correlate the results, so we would use the TDR method to not only troubleshoot problems with our test sites, but also confirm their continued compliance.

The study was conclusive in many aspects:

  • Both TDR and SVSWR methods correlate extremely well in determining compliance
  • Our test facilities which were measured with the SVSWR and the TDR techniques passed the respective site validation requirements. This was considered very useful in accepting the TDR technique as well as the SVSWR technique.
  • RF absorber type, the coverage area of the absorber, and the chamber volume are all factors in meeting site validation requirements
  • The SVSWR method is more labor intensive and utilizes more of the existing lab equipment
  • The TDR method is an excellent tool in identifying the source of non-compliance and is much faster

So in addition to using our VNA to perform equipment calibrations, it is used on an on-going basis to improve the performance of our test sites. The TDR function is terrific in identifying the exact fault location in cables and fixtures. It is also the best tool to measure absorber performance.

We also make the VNA available to our clients as a product development tool. It is extremely well suited to evaluate antenna matching networks as well as antenna performance. As a result, we’ve been able to grow our business while lowering our cost for calibrations and site verifications.

Greg Kiemel is the Director of Engineering at Northwest EMC, Inc. He has 23 years experience in the EMC field. Mr. Kiemel is a NARTE-certified EMC and ESD engineer, as well as a Senior Member of the IEEE. He is active in the ANSI ASC C63® and TCBC committees. Mr. Kiemel recently completed his tenure as a Distinguished Lecturer for the IEEE EMC Society. Prior to his fifteen years with Northwest EMC, he worked as the lead regulatory engineer in the personal computer division at Epson Portland, Inc. and as an EMC engineer at Tektronix, Inc. He earned his BS in Engineering from Weber State University. Mr. Kiemel may be reached via email at gkiemel @mwemc.com or phone at 888-364-2378.

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