R&S ZVL
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Operating Manual 1303.6580.32-06
227
Electrical Length, Mechanical Length
or
Delay
are coupled parameters. When one of them is
changed, the other two follow.
The velocity factor is 1/sqrt(
]
r
) and is a measure for the velocity of light in a dielectric with
permittivity
]
r
relative to the velocity of light in the vacuum (velocity factor < 1). Permittivity and
velocity factor are coupled parameters.
For a non-dispersive DUT, the delay defined above is constant over the considered frequency
range and equal to the negative derivative of the phase response with respect to the frequency
(see mathematical relations). The length offset parameters compensate for a constant delay, which
is equivalent to a linear phase response.
If a dispersive connector type (i.e. a waveguide; see
Offset Model
dialog) is assigned to a test port
that is related to a particular quantity, then the phase of the quantity is calculated taking dispersion
effects into account.
Loss parameters: Definition
The loss
L
is the attenuation of a wave traveling through the offset transmission line. In logarithmic
representation, the loss can be modeled as the sum of a constant and a frequency-dependent part.
The frequency dependence is essentially due to the skin effect; the total loss can be approximated
by an expression of the following form:
The DC loss Loss
DC
, the reference frequency f
ref
, and the loss at the reference frequency Loss(f
ref
)
are empirical parameters for the transmission lines connected to each port which can be entered
into any of the dialogs in the
Offset
menu. For a lossless transmission line, both Loss
DC
and
Loss(f
ref
) are zero. In practice, Loss(f
ref
) often represents the dominant contribution so that Loss
DC
can be set to zero.
Offset parameters: Application and effect
Offset parameters can be particularly useful if the reference plane of the calibration cannot be
placed directly at the DUT ports, e.g. because the DUT has non-coaxial ports and can only be
measured in a test fixture. Offset parameters can also help to avoid a new complete system error
correction if a cable with known properties has to be included in the test setup.
A positive length offset moves the reference plane of the port towards the DUT, a negative offset
moves the reference plane away from the DUT. The offset parameters cannot compensate for a
possible mismatch in the test setup.
•
A positive length offset moves the reference plane of the port towards the DUT, which is
equivalent to deembedding the DUT from a (perfectly matched) transmission line at that
port.
•
A negative offset moves the reference plane away from the DUT, which is equivalent to
embedding the DUT into a (perfectly matched) transmission line at that port.
In contrast to the embedding/deembedding procedure, the offset parameters cannot compensate
for a possible mismatch in the test setup.
Each offset parameter is assigned to a particular port. The delay parameters affect the phase of all
measured quantities related to this port; the loss parameters affect their magnitude. An offset at
port 1 affects the S-parameters S
11
, S
21
, S
12
. Some quantities depend on the whole of all S-
parameters, so they are all more or less affected when one S-parameter changes due to the
addition of an offset length.
