HP 8753E manual Where, Selections under this menu, Or match &n&d ~~~~~~~~ .gQ or to the system

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) =

where:

fc = center frequency Thus, at 50 MHz,

7500 x VF

fc

Length (cm) =	7500	= 150 cm or 1.5 m
	(MHz)
50

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 inkitely 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 Zo 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 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 Chapter 5, “Optimizing Measurement Results.”

TRL options
	i::::i	i	.:..:.:.:.:::.:..:.. i ;_/__,.,.,.,.,.,.,.............,.,.i ,_
me ~:,;;,%:~~~~~~~~::$;E ... ... “‘.‘.” “’ “.....“‘.““’ ” softkey provides access to the TRL/LRM options menu. There are two
	..z::: ..	A.J.........._	......._	;;; +.<.;..A.;;
selections under this menu:
n ~~~~~:~ (c-&bration &,)
....:	,,	,,,. .,	..f..
.._.

The characteristic impedance used during the calibration can be referenced to either the line
::::~~::‘j::~~~~~~~::~:::::.~~~~~:.:.:.~~~~~~:.~~~~~~~~~~~~~~~-~;~		,:,..:..: :... >,. ,.,~,:,~,~,~,~,~,. B.‘:,:,.: (,.,.,Z	(~~.;:.;~~~~~~~~~~). me analyzer
(or match) &n&d (~~~~~~~~ .;gQ) or to the system
.._.......... ._	-..	._

defaults to a calibration impedance that is equal to the line (or match) standard.

~.~~.~~..~:::......:....:.:::~~::::::::~~::~,:~~::.:.:::.::.:::.:,:::::::.::::,.,,:::::.:::::::::::.:::::::::::~::~:~

men the ~~~~~~~~~~~~2~~ is &e&d, the impedance of the he (or mat&) standard is

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
.,.	.,		.........	is selected, l&e values entered for ~~~~~ (under Cm
Smith ch&. men ~~~~~~;~;,,~~~~~
?.				................ ..~~~...l.......i

menu) and ~~~~~~~~. (with the defie timdad menu) me ignored.

::...:,.,.:::::.. :::...

6-100 Applicationand Operation Concepts