Specifications

4.7 Calibration and compensation in RF region
4.7.1 Calibration
Whether the RF I-V method or network analysis, the open, short, and load calibration minimizes
instrument inaccuracies. To perform calibration, open, short, and load reference terminations are
connected to the test port and each of the terminations is measured. This calibration data is stored
in instrument’s memory and used in the calculation to remove the instrument errors. Impedance
values of these reference terminations are indicated in both vector impedance coordinates and a
Smith chart in Figure 4-14.
Note: A 7-mm coaxial connector has a fringe capacitance of typically 0.082 pF when terminated
with Open. This fringe capacitance value has been memorized in the instrument and is used to
calculate accurate open calibration data.
Figure 4-14. Calibration standard values
Though all three terminations are indispensable for calibration, the load termination impedance
(50 Ω) is particularly important for precise calibration and has a large influence on resultant mea-
surement accuracy. The uncertainty of the load termination impedance is represented by a circle
that encloses the error vector (see Figure 4-14 (a).) The uncertainty of its phase angle increases with
frequency and becomes a considerable error factor, especially in measurements of high Q (low ESR
or low D) devices at high frequencies.
To improve accuracy for the high Q (low loss) measurement, the RF I-V measurement instrument
can be calibrated using a low loss capacitor (LLC) termination in addition to the open/short/load
terminations. The LLC provides a reference for calibration with respect to the 90°-phase component
of impedance. As a result, the instrument can measure high Q (low dissipation factor) devices more
accurately than when basic open/short/load calibration is performed. The LLC calibration takes
place only in the high frequency range (typically above 300 MHz) because the phase angle of the load
impedance is accurate at relatively low frequencies.
4-16
Note: Open impedance is infinite,
so it is not shown in the graph.
(a) Vector impedance plane (b) Smith chart