Because test cables experience loss and delay, vector network analyzers (VNAs) must be calibrated to make precise measurements. Those cable losses mean that phase measurements made at the VNA ports will not be the same as those made at the device under test (DUT).
If the characteristic impedances of the test cables are not precisely 50 Ω, the effective source and load match of the VNA will be slightly off, resulting in measurement ripple. In addition, the directivity error in the VNA measurement bridges must be corrected to ensure accurate reflection measurements. This short primer will quantify the various systematic errors and introduce the 12-term error-correction model, which serves as a basis for VNA calibration.