Rohde & Schwarz ZNB20, Руководство пользователя

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Concepts and Features
R&S
®
ZNB/ZNBT
176 User Manual 1173.9163.02 ─ 38
4.6.1.1 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 and 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


 


;
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 with respect
to the frequency (see mathematical relations). The length offset parameters compen-
sate for a constant delay, which is equivalent to a linear phase response.
4.6.1.2 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 essentially 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" softtool
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
.
4.6.1.3 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 residual delay of
the active trace (defined as the negative derivative of the phase response) is mini-
mized across the entire sweep range. If "Delay" is the selected trace format, the entire
Offset Parameters and Embedding