/..../.;,.,.,.,.,.,.,.,.,.;..........~ ;..,.,.,.,.,..~~_........:..-

~~~,;~~~~~~~~~~~~:i;%l is selected when the desired measurement impedance differs from the

.;: i.. “../;: :. :..:: .i.,.;;;;,-d:.:

impedance of the line standard. This requires a knowledge of the exact value of the Z0 of

the line. The system reference impedance is set using B”J.: $..:G:: $fl~..:::... under the calibration menu.

Fe actual impedance of the line is set by entering the real part of the line impedance as the

,..../

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

~~~~~~:~~; within the define standard menu. For example, if the line was known to have i..:.....i.... .::::: I ........A(.~.;;..~...:~~~~~.:.:.~~:

a characteristic impedance of.,., 51 Q (~~~~~~~~~ = 51 Q), it could still be used to calibrate for a 50 Q measurement (#$$ I:.#@: = 50 a). After a calibration, all measurements would be

referenced to 50 Q, instead of 51 Q. When the line standard is remeasured, the center of the ::. ::s, .:3:; ,;..j,,:w,

Smith chart is at the current value of :$m$@’ (in this case, 50 Q). Since only one value of offset ZO can be selected for the line standard, the value of ZO should be a constant value over the frequency range of interest in order to be meaningful.

:

,:

:,.,.,.

,

_,..... _,.............:.

_..

:

,,..,.

 

 

............

/,.,,,

me location of the reference plane is d&e&& by the xl&,ion of'~~.~~~~~~ a&

:'.;'::$.:$<zc .! : c:<<< ., i.: ,,./::...................,,,i;;TT. ..A...,..:,:...,,*,,. ""'::::'::.:::...: '.,,I

.: T ~.~.~.~...~..i ii: ;.:i:::.~......~.::~..~:~~~......::.

~~~~~~. By d&a&, the reference plane is set Mththeth standard wK&

*

must have a known insertion phase or electrical length. If a non-zero length thru is specified to have zero delay, the reference plane will be established in the middle of the thru. The reflect standard may be used to set the reference plane instead of the thru provided the phase response (offset delay, reactance values and standard type) of the reflect standard is known and is specified in the calibration kit dehnition.

Note Dispersion Effects

Dispersion occurs when a transmission medium exhibits a variable propagation or phase velocity as a function of frequency. The result of dispersion is a non-linear phase shift versus frequency, which leads to a group delay which is not constant. Fortunately, the TRL calibration technique accounts for dispersive effects of the test fixture up to the calibration plane, provided that:

1. The thru (zero or non-zero length) is defined as having zero electrical length

/ : i....::...

......a .i ..L

..A...... ..a.. I...

i il .i

and is used to set the reference plane (~~~~~~~:..;~~~).

.” ..-

-.

-

_

2.The transmission lines used as calibration standards have identical dispersion characteristics (i.e., identical height, width and relative dielectric constant).

When a non-zero length thru is used to set the reference plane, it should be dellned as having zero length in the TRL standards definition, even though it has physical length. The actual electrical length of the thru standard must then be subtracted from the actual electrical length of each line standard in the TRL calibration kit dell&ion. The device must then be mounted between two short lengths of transmission line so that each length is exactly one-half of the length of the non-zero length thru standard. In this configuration, the measurement will be properly calibrated up to the point of the device.

Application and Operation Concapts 6-101

Page 382
Image 382
HP 8753E manual Is used to set the reference plane ~~~~~~~..~~~