Concepts and Features
R&S
®
ZNA
168
User Manual 1178.6462.02 ─ 12
5.6.1
Offset Parameters
Offset parameters compensate for the known length and loss of a (non-dispersive and
perfectly matched) transmission line between the calibrated reference plane and the
DUT.
The analyzer can also auto-determine length and loss parameters, assuming that the
actual values should minimize the group delay and loss across the sweep range.
5.6.1.1
Definition of Offset Parameters
The
delay
is the propagation time of a wave traveling through the transmission line.
The
electrical length
is equal to the delay times the speed of light in the vacuum. It is a
measure for the length of the transmission line between the standard and the actual
calibration plane. For a line with permittivity ε
r
and
mechanical length
L
mech
the delay
and the electrical length are calculated as follows:
r
mech
r
mech
L
Length
Electrical
c
L
Delay
;
For a non-dispersive DUT, the delay defined above is constant over the considered fre-
quency range and equal to the negative derivative of the phase response for the fre-
quency (see mathematical relations). The length offset parameters compensate for a
constant delay, which is equivalent to a linear phase response.
5.6.1.2
Definition of Loss Parameters
The
loss
L is the attenuation of a wave when 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 due to the skin effect;
the total loss can be approximated by an expression of the following form:
[
]
DC
ref
DC
ref
Loss
f
f
Loss
f
Loss
f
Loss
+
-
=
)
(
)
(
For a lossless transmission line, Loss
DC
= Loss(f
ref
) = 0 dB. In practice, the frequency-
dependent loss often represents the dominant contribution so that Loss
DC
can be set to
zero.
Experimentally, the two loss values Loss
DC
and Loss(f
ref
) are determined in two sepa-
rate measurements at a very low frequency (f --> 0) and at f = f
ref
.
Offset Parameters and De-/Embedding
