VNA0440
–
VNA0440e
VNA0460
–
VNA0460e
user manual V3.1
- 20 -
Reference plane
It is important for a Calkit that all
impedances are defined at exactly the same
point in or after the connector. This point is
called the Reference Plane or Calibration
Plane. The calibrated measurements will
be defined from that point on to the DUT.
For connector Calkits the reference plane is
inside the connector, which makes them
useful mostly for cable measurements and
system measurements.
For PCB based Calkits the reference plane
is at the point where the termination is
located. These are indicated here in the
UFL picture. The Through calibration has
some length between the ports. The Through measurements will be compared to this
length. If the length in the measurement is shorter than the result can have a negative
phase.
It is important that the reference plane is as close as possible to the measuring point.
Calibration Kits
For the Open, Short and Load calibration you need calibration elements that define these
impedances. Since the whole measurement depends on the quality of these elements they
need to be as precise as possible. In addition they need to calibrate at the same reference
plane. Also the through measurement needs to be calibrated with a well defined through
element.
The combination of calibration elements is usually sold together and is called a Calibration
Kit, or Cal Kit. However, these elements are never ideal and have a deviation from the
ideal values and sometime contain a significant delay because the reference point is
located behind some transmission line.
To get the calibration more precise the Cal Kit usually comes with a set of coefficients or
parameters that can be used by the VNA software to compensate with the calibration
element’s deviation.
For test fixtures and PCB based measurements there are usually no coefficients because
the measurement gets calibrated through the structure, which is hard to model or
characterize.
