Atek HP 8753D, Руководство пользователя, Страница: 359 / 677

Another reason for showing this example is to point out the potential problem in calibrating at
low frequencies using TRL. F or example , one-quarter wavelength is
Leng th (cm) =
7500 2 V F
f c
where:
fc = center frequency
Thus , at 50 MHz,
Leng th (cm) =
7500
50 (M H z )
= 150 cm or 1: 5 m
Such a line standard would not only be dicult 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 innitely
long transmission line , it ts the TRL model mathematically , and is sometimes referred to as a
\TRM" calibration.
The TRM calibration technique is related to TRL with the dierence being that it bases the
characteristic impedance of the measurement on a matched Z
O
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
reect standards used for TRL apply for TRM.
TRM has no inherent frequency coverage limitations which makes it more convenient in some
measurement situations . A dditionally , because TRL requires a dierent physical length for the
thru and the line standards , its use becomes impractical for xtures with contacts that are at a
xed physical distance from each other .
F or information on how to modify calibration constants for TRL*/LRM*, and how to perform a
TRL or TRM calibration, refer to Chapter 5, \Optimizing Measurement Results ."
TRL Options
The
NNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNN
TRL/LRM OPTION softkey provides access to the TRL/LRM options menu. There are two
selections under this menu:
NNNNNNNNNNNNNNNNNNNNNNN
CAL ZO: (calibration Z
0
)
N
NNNNNNNNNNNNNNNNNNNNNNNNN
SET REF: (set reference)
The characteristic impedance used during the calibration can be referenced to either the line
(or match) standard (
NNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNN
CAL ZO: LINE ZO ) or to the system (
NNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNN
CAL ZO: SYSTEM ZO ). The analyzer
defaults to a calibration impedance that is equal to the line (or match) standard.
When the
NNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNN
CAL ZO: LINE ZO is selected, the impedance of the line (or match) standard is
assumed to match the system impedance exactly (the line standard is reectionless). After
a calibration, all measurements are referenced to the impedance of the line standard. F or
example , when the line standard is remeasured, the response will appear at the center of the
Smith chart. When
NNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNN
CAL ZO: LINE ZO is selected, the values entered for
NNNNNNNNNNNNNNNNNNNN
SET ZO (under CAL
menu) and
NNNNNNNNNNNNNNNNNNNNNNNNNNNNN
OFFSET ZO (within the dene standard menu) are ignored.
Application and Operation Concepts 6-93