Agilent Technologies 8753ET User Manual page 441

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
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 "Calibrating for Non-Coaxial Devices (ES Analyzers Only)" on page

fc
7500
--------------------- - =
150 cm or 1.5 m
 
50 MHz
termination instead of a transmission line for the third
O
Operating Concepts
TRL*/LRM* Calibration (ES Models Only)

6-50.
7- 73