Agilent Technologies 8753ET User Manual page 412

Operating Concepts
TRL*/LRM* Calibration (ES Models Only)
For microstrip and other fabricated standards, the velocity factor is significant. In those
cases, the phase calculation must be divided by that factor. For example, if the dielectric
constant for a substrate is 10, and the corresponding "effective" dielectric constant for
microstrip is 6.5, then the "effective" velocity factor equals 0.39 (1 ÷ square root of 6.5).
Using the first equation with a velocity factor of 0.39, the initial length to test would be
1.95 cm. This length provides an insertion phase at 1000 MHz of 60 degrees; at 2000 MHz,
120 degrees (the insertion phase should be the same as the air line because the velocity
factor was accounted for when using the first equation).
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
7500 VF
( )
= ------------------------- -
Length cm
where:
• fc = center frequency
Thus, at 50 MHz,
( )
=
Length cm
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 or very broad band measurements, fabrication of a "match" or
termination may be deemed more practical. Since a termination is, in essence, an infinitely
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 Z
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 fixtures with
contacts that are at a fixed physical distance from each other.
For information on how to modify calibration constants for TRL*/LRM*, and how to
perform a TRL or TRM calibration, refer to
Analyzers Only)" on page
7-74
×
fc
7500
---------------------- - =
( ) 150 cm or 1.5 m
50 MHz
6-51.
"Calibrating for Non-Coaxial Devices (ES
termination instead of
O