Atek HP 8753D, Руководство пользователя, Страница: 352 / 677

How TRL*/LRM* Calibration W orks
The TRL*/LRM* calibration used in the HP 8753D relies on the characteristic impedance
of simple transmission lines rather than on a set of discrete impedance standards . Since
transmission lines are relatively easy to fabricate (in a microstrip , for example), the impedance
of these lines can be determined from the physical dimensions and substrate's dielectric
constant.
TRL* Error Model
Figure 6-51.
HP 8753D functional block diagram for a 2-port error-corrected
measurement system
F or an HP 8753D TRL* 2-port calibration, a total of 10 measurements are made to quantify
eight unknowns (not including the two isolation error terms). Assume the two transmission
leakage terms , E
XF
and E
XR
, are measured using the conventional technique . The eight TRL
error terms are represented by the error adapters shown in Figure 6-52. Although this error
model is slightly dierent from the traditional Full 2-port 12-term model, the conventional
error terms may be derived from it. F or example , the forward reection tracking (E
RF
) is
represented by the product of "
10
and "
01
. Also notice that the forward source match (E
SF
) and
reverse load match (E
LR
) are both represented by "
11
, while the reverse source match (E
SR
)
and forward load match (E
LF
) are both represented by "
22
. In order to solve for these eight
unknown TRL error terms , eight linearly independent equations are required.
The rst step in the TRL* 2-port calibration process is the same as the transmission step for
a Full 2-port calibration. F or the thru step , the test ports are connected together directly
(zero length thru) or with a short length of transmission line (non- zero length thru) and the
transmission frequency response and port match are measured in both directions by measuring
all four S-parameters .
F or the reect step , identical high reection coecient standards (typically open or short
circuits) are connected to each test port and measured (S
11
and S
22
).
F or the line step , a short length of transmission line (dierent in length from the thru) is
inserted between port 1 and port 2 and again the frequency response and port match are
measured in both directions by measuring all four S-parameters .
In total, ten measurements are made , resulting in ten independent equations . However , the
TRL error model has only eight error terms to solve for . The characteristic impedance of the
6-86 Application and Operation Concepts