HP 8712C User Manual page 263

Calibrating for increased Measurement
Accuracy
Choose an Appropriate Calibration Method
About Calibration Standards.
A calibration standard is a specific, well-deGned, physical device used
to determine systematic errors. Each standard has a precisely known or
predictable magnitude and phase response as a function of frequency. The
response of each standard is mathematically defined in the error models used
by the network analyzer.
A standard type is one of four basic types that define the form or structure of
the model to be used with that standard (e.g. short or load).
Standard characteristics are the numerical, physical characteristics of the
standards used in the model selected.
Typical calibration standards are cables (or throughs), opens, shorts, and
loads. They are used singly or in combination, depending on the type of Cal.
In essence, for each type of measurement error that is to be corrected, one
standard is measured.
Open defines the standard type of an open circuit used
for calibrating reflection measurements. As a reflection
standard, an open circuit offers the advantage of broadband
frequency coverage. At high frequencies, however, an open
rarely has perfect reflection characteristics because the
fringing capacitance effects cause phase shift that varies
with frequency.
These effects are impossible to eliminate, but the calculation
built into the analyzer includes an open circuit capacitance
model. This capacitance model is a cubic polynomial as a
function of frequency, where the polynomial coefficients are
user-definable.
The capacitance model equation is:
c = (CO) + (Cl"f) + (c2*f2) + (c3*P)
where f is the measurement frequency.
The terms in the equation are defined when specifying the
open as follows:
CO is used to enter the CO term, which is the constant
term of the cubic polynomial and is expressed in Farads.
Cl is used to enter the Cl term, expressed in F/Hz
(Farads/Hz).
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