Rohde & Schwarz 1145.1010.04/05/06, Operating Manual

Page: 150 / 813

Calibration Overview R&S
®
ZVA/ZVB/ZVT
1145.1084.12 3.40 E-1
Implicit verification
The two one-port standards of the TOM calibration are connected to both ports. Together with the four
S-parameters of the through standard, this results in 8 S-parameter measurements.
Despite the fact that TOM uses fewer standards than TOSM – the short is missing –, the 3 fully known
standards provide more information than necessary. This is due to the system error modeI that TOM
and the other TXX calibration procedures (see below) are based on. This model has only 7 error terms
in contrast to the TOSM model, which contains 10 terms.
The redundancy of TOM is used for an implicit verification of the solution. Implicit verification helps to
detect and avoid calibration errors, e.g. errors due to defective cables or loose connections. In the case
of failure of that implicit verification a warning message appears. The user may then continue or cancel
the calibration.
TRM Calibration
A TRM (Through – Reflect – Match) calibration requires a low-reflection, low-loss through standard with
an electrical length that may be different from zero, a reflect and a match. The magnitude of the
reflection coefficient of the reflect standard 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.
TRM calibration is especially useful for DUTs in test fixtures.
TRL Calibration
A TRL (Through – Reflect – Line) calibration requires the two-port standards through and line, which
are both assumed to be ideally matched. Beyond that, the through must be lossless. Furthermore, a
reflecting one-port standard (reflect) is needed. The magnitude of the reflection coefficient of the reflect
standard 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.
Frequency restrictions
The system of equations solved to derive the error terms is such that singularities occur whenever the
length difference
∆
L between the Through and the Line is equal to an integer multiple of half of the
wave length:
2
λ
n L
≠ ∆
As a rule, singularities are avoided with sufficient accuracy if the phase shift resulting from the length
difference between through and line standard is between 20° and 160°. This corresponds to a ratio of
1:8 for the start and stop frequency of the calibrated sweep range.
To shift the calibrated sweep range to smaller or larger frequencies, you can use a longer or
shorter line.
Example: Suppose you want to perform a TRL calibration using coaxial air lines for the sweep range
between 100 MHz and 4 GHz. The through standard has zero length. The ratio of the stop frequency to
the start frequency is 40, therefore an accurate calibration with a single line standard is not possible.
Proceed as follows:
1. Divide the sweep range into two subranges that meet the TRL calibration criterion, e.g. [100
MHz, 800 MHz] and [800 MHz, 4 GHz].