Concepts and Features
R&S
®
ZNB/ZNBT
184
User Manual 1173.9163.02 ─ 53
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
;
In the CHANNEL – [OFFSET EMBED] > "Offset" softtool tab, "Delay","Electrical
Length" and "Mech. Length" are coupled parameters. When one of them is changed,
the other two are adjusted accordingly.
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
+
-
=
)
(
)
(
The "Loss at DC" Loss
DC
, the reference "Freq for Loss" f
ref
, and the "Loss at Freq"
Loss(f
ref
) are empirical parameters for the transmission lines connected to each port
which can be entered in the CHANNEL – [OFFSET EMBED] > "One Way Loss" soft-
tool tab.
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
.
5.6.1.3
Auto Length
The "Auto Length" function (CHANNEL – [OFFSET
EMBED] > "Offset" > "Auto
Length") adds an electrical length offset to a test port with the condition that the resid-
ual delay of the active trace (defined as the negative derivative of the phase response)
is minimized across the entire sweep range. If "Delay" is the selected trace format, the
entire trace is shifted in vertical direction and centered on zero. In phase format, the
"Auto Length" corrected trace shows the deviation from linear phase.
Offset Parameters and De-/Embedding
