HP HP 8753E, Руководство пользователя, Страница: 381 / 699

Another reason for showing this example is to point out the potential problem in calibrating at
low frequencies using TRL. For example, one-quarter wavelength is
Length (cm) =
7500 x VF
where:
fc = center frequency
Thus, at 50 MHz,
Length (cm)
7500
50 (MHz)
= 150
cm
1.5
m
Such a line standard would not only be difficult to fabricate, but its long term stability and
usability would be questionable as well.
Thus at lower frequencies and/or very broad band measurements, fabrication of a “match” or
termination may be deemed more practical. Since a termination is, in essence, an
long transmission line, it fits the TRL model mathematically, and is sometimes referred to as a
“TRM” calibration.
The TRM calibration technique is related to TRL with the difference being that it bases the
characteristic impedance of the measurement on a matched termination instead of a
transmission line for the third measurement standard. Like the TRL thru standard, the TRM
THRU standard can either be of zero length or non-zero length. The same rules for thru and
reflect standards used for TRL apply for TRM.
TRM has no inherent frequency coverage limitations which makes it more convenient in some
measurement situations. Additionally, because TRL requires a different physical length for the
thru and the line standards, its use becomes impractical for with contacts that are at a
physical distance from each other.
For information on how to modify calibration constants for and how to perform a
TRL or TRM calibration, refer to Chapter 5, “Optimizing Measurement Results.”
options
. . ........ .............
selections under this menu:
n
: . . . .
The characteristic impedance used during the calibration can be referenced to either the line
defaults to a calibration impedance that is equal to the line (or match) standard.
assumed to match the system impedance exactly (the line standard is reflectionless). After
a calibration, all measurements are referenced to the impedance of the line standard. For
example, when the line standard is remeasured, the response will appear at the center of the
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Applicationand Operation Concepts