The Measurement Manifesto – Becoming a VNA Master

About 22 years ago I decided to get my fly fishing mojo together.    Understanding that casting is fundamental to my goal I took a class with a guy donned the “Casting Nazi” by the local Fly fishing club.   I got good, practiced casting, dealt with the instructors salty personality, fished, for a couple of years I was a bit unbalanced.   Two years later I went on a salt water fly fishing trip for redfish in a guided flats boat in Florida.    The wind was howling +20mph, the boat was moving, the fish were moving, the guide was shouting over the wind in frustration trying to help me place the fly with about dinner plate accuracy typically 20-50 feet from my position to traveling visible fish.   I felt so defeated.    I had a long journey to be a fly fishing “Master”, I wasn’t even close.

Assuming your past the basic issues with making a good measurement, what does it take to be a Master at making VNA measurements from a Signal Integrity Practitioners perspective?

A good fraction of the S-parameters I encounter have serious issues – they are plagued with passivity violations, the VNA wasn’t set up properly, fixturing issues, the calibration was not validated or even  checked, loose cables… the list goes on.    Around 2002 I proposed a Test/Measurement track in DesignCon, and after a bit of fanfare it was adopted and has turned out to be a valuable track.   I co-chair the track with a few other folks, so I see a lot of energy focused on getting a good S-parameter models.

My essential background includes making a lot of measurements to 50GHz, including fast TDR, designing platforms to validate S-parameters, and corresponding them to 3D-EM analysis.   My Company has conducted quite a bit of VNA training and we market products for the SI type engineer.   I want to share my observations in this blog.   Although I consider myself good, I am still no Master, buy I keep trying.

The Beatty Standard

Beatty_Standard

Beatty_Standard_Graph

 

 

 

 

The Beatty Standard is an invaluable tool for calibration validation, material extraction, 3D EM analysis.   Ill be discussing this in upcoming blog.

The first consideration is to discuss all the relevant elements of what is required to be the VNA Master, and my list (your list will be different of course) :

  • Complete understanding of types of calibrations and terminology, for example insertable versus non-insertable calibration, mixed-calibrations, etc., also versed in de-embedding methods in addition to classic
  • Extrapolating to DC without destroying the S-parameter causality and distorting time-domain convolutional-based simulation
  • Understand the underpinnings of SOLT, TRL, LRM, SOLR, and perform those calibrations
  • Understand limitations of VNA including IF bandwidth, number of points, averaging impact on S/N, and associated Source/Load matching, Directivity, and general error models
  • Design on-board TRL calibration that works to 50GHz
  • Ability to perform consistent calibrations ( even hybrid and multi-tier), resolve issues with calibration problems, and verify the calibration with external standards
  • Understand the measurement requirements based on the DUT and reasonable requirements
  • Ability to interpret single-ended, differential, and modal S-parameters based on a structures topology and layout
  • Technically not part of the VNA, but correspond S-parameters with TDR and VNA time domain methods. I would add ability to establish time-domain measure-based models as well
  • Interpret frequency domain of classic structures like stubs, resonators, Beatty Standards, etc.,
  • Ability to prevent passivity issues and minimize causality problems, correcting minor passivity issues
  • Able to generate Rational Compact models of S-parameter data, and understand why this is a useful tool
  • Ability to create multi-port S-parameter matrix from 2 and 4-port analyzer (for 12-port crosstalk aggressed systems for example)
  • Ability to gauge connectors/adapters and identify bad ones, ability to buy the right VNA cables and create a complete adapter kit

Let’s digress and discuss a more global concept:  How long does it take to become an expert in any given area of human activity?   Well, a popular book by Malcom Gladwell called “Outliers” he put forth some a basic maxim of mastery; it takes 10,000 hours to become an expert in anything. If this is true then that means you can become good at anything given you put in some serious time.   Maybe the time issue is really a reflection of the level of commitment of the individual.

 

 

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