Vector network analyzer basics
14
© 2017 Pico Technology
pv106ug r1
4.5
Displaying measurements
Input and output parameters, S
11
and S
22
, are often displayed on a polar plot or a Smith chart. The polar plot
shows the result in terms of the complex reflection coefficient, but impedance cannot be directly read off
the chart. The Smith chart maps the complex impedance plane onto a polar plot. All values of reactance and
all positive values of resistance, from 0 to ∞, fall within the outer circle. This has the advantage that
impedance values can be read directly from the chart.
The Smith chart
4.6
Calibration and error correction
Before accurate measurements can be made, the network analyzer needs to be calibrated. In the calibration
process, well-defined standards are measured and the results of these measurements are used to correct
for imperfections in the hardware. The most common calibration method, SOLT (short, open, load, through),
uses four known standards: a short circuit, an open circuit, a load matched to the system impedance, and a
through line. These standards are usually contained in a calibration kit and their characteristics are stored
on the controlling PC in a Cal Kit definition file. Analyzers such as the PicoVNA 106 that have a full S-
parameter test set can measure and correct all 12 systematic error terms.
