7- 67
Operating Concepts
TRL*/LRM* Calibration (ES Models Only)
TRL* calibration is a modified form of TRL calibration. It is adapted for a receiver with three samplers instead
of four samplers. The TRL* calibration is not as accurate as the TRL calibration because it cannot isolate the
source match from the load match, so it assumes load match and source match are equal.
How TRL*/LRM* Calibration Works
The TRL*/LRM* calibration used in the analyzer 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 7-43 Functional Block Diagram for a 2-Port Error-Corrected Measurement System
For the analyzer 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 7-43. Although this error model is slightly different from the traditional Full 2-port 12-term
model, the conventional error terms may be derived from it. For example, the forward reflection 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 first step in the TRL* 2-port calibration process is the same as the transmission step for a Full 2-port
calibration. For 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.
For the reflect step, identical high reflection coefficient standards (typically open or short circuits) are
connected to each test port and measured (S
11
and S
22
).
For the line step, a short length of transmission line (different 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.
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