VNA concepts and features
R&S
®
ZNL/ZNLE
283
User Manual 1178.5966.02 ─ 19
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,
which is equivalent to deembedding the DUT by numerically removing a (perfectly
matched) transmission line at that port.
●
A negative offset moves the reference plane away from the DUT, which is equiva-
lent to embedding the DUT by numerically adding a (perfectly matched) transmis-
sion line at that port.
The offset parameters are also suited for length and delay measurements; see
on page 279. In contrast to the embedding/deembedding
functions (see
Chapter 9.6.2, "Embedding and deembedding"
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
, S
31
... Some quan-
tities (like the Z-parameters) 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.
To account for the propagation in both directions, the phase shift of a reflection param-
eter due to a given length offset is twice the phase shift of a transmission parameter. If,
at a frequency of 300 MHz, the electrical length is increased by 250 mm (λ/4), then the
phase of S
21
increases by 90 deg, whereas the phase of S
11
increases by 180 deg.
Equivalent relations hold for the loss.
If the trace is displayed in "Delay" format, changing the offset parameters simply shifts
the whole trace in vertical direction.
The sign of the phase shift is determined as follows:
●
A positive offset parameter causes a positive phase shift of the measured parame-
ter and therefore reduces the calculated group delay.
●
A negative offset parameter causes a negative phase shift of the measured param-
eter and therefore increases the calculated group delay.
9.6.1.7
Offset parameters for balanced ports
The offset parameters can be used for balanced port configurations:
●
Offset parameters must be assigned to both physical ports of a logical port.
●
"Auto Length" corrects the length offset of both physical ports of a logical port by
the same amount.
Offset parameters and de-/embedding
