Be confident in VNA measurements

-September 26, 2013

Vector Network Analyzers (VNAs) are the preferred workhorse for many laboratories when it comes to calibrating and characterizing passive microwave devices like attenuators, power splitters, directional couplers, band-pass filters, terminations, and other devices. Additionally, VNAs are frequently used for measuring the Standing Wave Ratio (SWR) of signal sources and amplifiers (RF power output switched off), as well as measurement devices like power sensors and spectrum analyzers.

VNAs are designed with two, four, or more measurement ports and have the ability to make independent measurements on each measurement port (reflection measurements), as well as ratio measurements between ports (transmission measurements).

VNAs provide a wealth of information from their fundamental ability to measure incident RF power, reflected RF power, and phase. Using these basic measurement functions, and employing mathematical formulas and manipulation, VNAs derive other measurement functions such as group delay, propagation delay, and complex impedance. In addition, many VNAs have the ability to transform frequency domain measurements into time domain measurements in order to create eye diagrams and derive Time Domain Reflection (TDR) responses.

VNA architectures incorporate directional coupling devices that let VNAs discern reflected RF power from incident RF power. RF power sensors measure RF power amplitudes and phase-detection circuitry measure phase values.

VNAs aren't, however, perfect in regards to measuring RF power and phase. Test equipment manufacturers recommend VNAs be calibrated prior to making a measurement using an appropriate VNA calibration kit. For a two-port VNA, a cal-kit calibration can compensate for twelve VNA error terms, six each in forward and reverse measurement paths:

  • Directivity
  • Source Match
  • Reflection Tracking
  • Load Match
  • Transmission Tracking
  • Isolation.

Following a VNA cal kit calibration, you can then proceed to make a measurement. But, what if there was a "hiccup" during the calibration, i.e. improper torque of a connector, a connector becomes loose during calibration, corruption of one or more calibration coefficients, operator error, etc?

An easy way to establish confidence after a VNA cal kit calibration is to connect a 50Ω-termination to a VNA measurement port, switch the VNA display to Smith Chart, and observe the VNA reflection response. Without going into the insightful genius of a Smith Chart (Fig. 1), for a quality 50-Ω termination (load), the VNA response should be a tightly coupled circle in the center of the Smith Chart.

Given the same scenario, but with a "Short" connected, the VNA response should be located on the left edge of the Smith Chart (Figure 1). Again with the same scenario, but with an "Open" connected, the VNA response should be located on the right edge of the Smith Chart.

Figure 1. A Smith Chart provides a polar view of an impedance.

An easy way to remember these Smith Chart responses is S-L-O (from left to right on the Smith Chart; Short, Load, Open). Any departure from the aforementioned responses is indicative of a less than optimum VNA cal kit calibration or some other problem.

Also see:
Slideshow: Vector network analyzers
How does a Smith chart work?
Smith chart and how to use it
The Smith chart: more vital after all these years

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