Most Common Measurement Errors – Probing, VNA

As the consultants in my company and I go about helping clients with making more accurate and efficient measurements, we see a number of common mistakes over and over across all applications (Data I/O, RF/Wireless, etc.). Very often the errors are controllable and require only some simple solutions but yet have cost the operators hours of frustration and the company thousands in wasted development or test time. Here are the top two we’ve seen and how to prevent them…
1. Problem: Inaccurate jitter results due to improper probing
Proper de-skewing is key! Be sure to use the correct signal for your reference; also, any wires used to facilitate probing need to be short and equal in length. Be sure to use the grounding pin or lead which is nearest the signals being probed.

For devices with have differential inputs/outputs (i.e. DDR) then it is critical to measure and take into account the static phase offset which will be present as shown below.

Calculation of Static Phase Offset in a Differential DDR Circuit

Calculation of Static Phase Offset in a Differential DDR Circuit

Each instrument will have a different approach to implementing this so be sure to understand it if you want accurate measurements. One last word on jitter, realize that you really need to utilize a variety of tools if understanding jitter in your device is critical. Spectrum Analyzers, Oscilloscopes, BERTs and Time Interval Analyzers all give you a different view and insight to the various jitter parameters; don’t buy into the “one tool does it all” approach.

2. Problem: Inaccurate or non-repeatable Vector Network Analyzer phase measurements

Vector Network Analyzers (VNAs) are the workhorse of many test labs yet I repeatably see incorrect techniques being used especially in regards to phase measurements which include Group delay and AM/PM Distortion. I had a client spending many hours and thousands of dollars on sophisticated algorithms to reduce Spectral Regrowth which were primarily a result of the additive nature of the signal phases. These were not being correctly measured due to the incorrect calibration and measurement techniques used on the VNA, specifically, the lack of, or improper use of, Phase Equal Insertable (PEI) adapters. Anritsu has a great application note on this and other techniques (de-embedding and port extension). Once this issue was corrected, the spectral regrowth issue was quickly addressed using a fairly straight-forward waveform clipping and FIR filtering approach.

Also highly misunderstood are group delay measurements on VNA, specifically, proper Video Bandwidth (VBW), Aperture, Averaging and Smoothing settings. A proper phase reference calibration is critical for Group Delay measurements as is using the proper settings for the actual measurement. Have your local Field Application Engineer (FAE) from the VNA manufacturer spend some time going through these fundamentals, even if you consider yourself a seasoned RF professional, instrument advances are such that you may learn a trick or two. If getting an FAE onsite is not feasible, contact me to see about some some of our remote training or consulting solutions which over 90% of our clients said immediately saved them time and frustration immediately on their very next measurement.

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