Anritsu 37 C Series, Operation Manual

Page: 126 / 603

Calibrating for a
Measurement
Let us assume that we want to correct for three errors in the reflection
measurement: source match, directivity, and tracking. We accomplish
this using three standards.
Shorts are the easiest to visualize. They totally reflect all of the inci-
dent RF energy output at a precise phase. The terms zero-ohms im-
pedance, voltage null, and 180° phase all define an RF Short.
Opens are similar to Shorts, but their response is more complex. The
terms voltage maximum, infinite impedance, and 0° phase all define a
perfect Open. A perfect Open, however, is only a concept. In reality
Opens always have a small fringing capacitance.
To account for the fact that the Open will not predictably reflect im-
pedance at an exact 0° phase reference, we alter its response using co-
efficients that accurately characterize the fringing capacitance. The co-
efficients are different for each coaxial line size, since each size has a
different fringing capacitance. To maximize accuracy, ensure that
these coefficients are installed prior to the calibration (Menu U3).
As Opens and Shorts provide two references for a full reflection, Z
0
terminations provide a zero-reflection reference.
Ideal Z
0
terminations must consist of two parts, a perfect connector
and an infinite-length perfect transmission line that absorbs all of the
RF energy that enters it (no reflections).
Infinite length transmission lines are unwieldy at best, so you must
use less-than-ideal terminations. For calibration purposes there are
two common types: broadband loads and sliding terminations.
37XXXC OM 7-9
MEASUREMENT CALIBRATION DISCUSSION
CALIBRATING FOR A REFLECTION
MEASUREMENT USES THREE
STANDARDS:
Short
Open
Termination
IDEAL TERMINATIONS
Reflectionless
Perfect Connector
Infinite-Length, Dimensionally Ex-
act, Reflectionless Transmission
Line