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In some cases—depending on the fixture or the device being meas
-
ured—this is satisfactory. But when it is not, we need to employ other
techniques.
One of the reasons that moving the reference plane out to the device
does not always work, is that the test fixture includes a transition
from coax to a structure such as microstrip, coplanar waveguide, or
stripline (Figure 8-24).
Engineers have come to grips with the general problem. However,
there is no established standard approach. Two of the more common
approaches are to calibrate the fixture as a part of the analyzer, and to
characterize the fixture and compute the desired result.
ACTIVE DEVICE MEASUREMENTS
8-26 37xxxE OM
G P C - 7 D U T
5 0 O H M L I N E
N O L O S S , R E F L E C T I O N L E S S
Figure 8-23. Simple Example of De-Embedding
G R O U N D
I N P U T
F E T
C O P L A N A R
L A U N C H E R
I N P U T
G R O U N D
G R O U N D G R O U N D
C O P L A N A R
L A U N C H E R
A L U M I N A
C A R R I E R
Figure 8-24. Coax-to-Substrate Transition