R&S ZVB - Page 73

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ZVT System Overview
Calibration Overview
Operating Manual 1145.1084.12 – 30 58
TRL calibration is especially useful for DUTs in planar line technology (e.g. test fixtures, on-wafer
measurements) where it is difficult to design and connect accurately modeled open, short or match
standards. If TRL is not practicable, TNA may be an alternative.
The system of equations solved to derive the error terms is such that singularities occur whenever the
length difference ΔL between the through and the line is an integer multiple of half of the wave length:
Refer to TRL Extensions to learn how to avoid singularities (extension with two lines, combination with
TRM).
NIST Multiline TRL Calibration
A NIST (National Institute of Standards and Technology) Multiline TRL (Through – Reflect – Line)
calibration requires one or more two-port line standards in addition to a through standard. The lengths of
line and through standards must be known exactly and should be different from each other. Lossy line and
through standards are permitted and can increase the overall accuracy. The propagation constants need
not be known, however, they must be identical across all transmission (Through and Line) standards. A
(preferably accurate) estimate for the effective relative permittivity of the transmission standards must be
provided.
Furthermore, a reflecting one-port standard (reflect) is needed. The magnitude and phase of the reflection
coefficient is required only to be approximately known, however it is assumed to be identical for both
ports.
The calculation of the error correction terms is based on a publication by Roger B. Marks (R. B. Marks, "A
Multiline Method of Network Analyzer Calibration", IEEE Transactions on Microwave Theory and
Techniques, Vol. 39, No. 7, July 1991). The conventional TRL calibration is typically limited to a bandwidth
no larger than 8:1. To accommodate for a wider bandwidth, a second line is normally employed and the
band is split, where only one line measurement is evaluated for each portion of the band. In contrast, the
NIST Multiline TRL method uses all line measurements simultaneously at each frequency point and
appropriately weighs their contribution based on the expected error. In doing so this method uses a large
amount of data that the conventional split-band TRL neglects. In addition it also eliminates calibration
discontinuities at the frequency break points where in conventional TRL a transition from one line standard
to another is required.
TNA Calibration
A TNA (Through – Network – Attenuation) calibration requires two-port standards only. Again, the through
standard must be ideally matched and lossless. The symmetric network must have the same properties as
the reflect standard used for a TRL calibration, i.e. the magnitude of its reflection coefficient can be
unknown but must be nonzero; its phase must be roughly known ( 90 deg). The magnitude and phase of
the reflection coefficient must be the same at both test ports. The attenuation standard must be well
matched on both sides and cause an attenuation different from 0 dB; the exact value of the transmission
coefficient is not important.
As with TRL, TNA calibration is especially useful for planar DUTs. If TNA is not practicable, TRL may be
an alternative.
Full n-Port Calibration with Reduced Number of Through Connections
The analyzer can calculate the error terms for an full n-port calibration (TOSM, UOSM, TOM, TRM, TRL,