Reference Plane

In a VNA measurement, the reference plane is known as the location within the system where the user calibration is performed and is therefore the plane where the measurement is typically made.

VNA measurements are commonly made using cables connected from the device front panel to the device under test (DUT). When calibrating the VNA, you must account for the length and characteristics of those cables to remove any effects the cables could have on your measurement, such as time delay, phase shift, or attenuation. Establishing a reference plane is most commonly done by attaching calibration standards to the ends of the cables during calibration, which establishes the end of those cables as your calibration reference plane.

Reference Plane Extensions

The reference plane exists at the point where calibration was performed, but if needed, it is possible to extend the reference plane. A reference plane extension, shown in the following figure, is one technique you can use when it may not be practical to use custom calibration standards that match the test setup.

In the preceding figure, distance (d) refers to the measured distance between the calibrated reference plane and the DUT input. This distance could represent an additional cable or test fixture that was not included in the calibration. If you are using a reference plane extension, d refers to the extended distance of the reference plane as you move it toward the DUT. The wavelength (λ) is inversely proportional to the frequency of the measurement.

To manually move or extend the reference plane after calibration, you must specify the properties of the cable or test fixture you want to account for, such as distance, time, phase offset, and loss. To specify these properties automatically, you can insert an open or short calibration standard at the location in the test setup where you want to relocate the reference plane. The figure above shows a full reflection condition when the DUT is replaced by an open or short, and the incident voltage is reflected back to the VNA. The VNA can then perform the calculations necessary to move the reference plane and adjust subsequent measurements to match the desired measurement setup.

You can apply reference plane extension to your system by navigating to the NI-VNA Soft Front Panel (SFP) and selecting Measurement»Reference Plane from the NI-VNA SFP Menu.