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 transmission line between the standard and the actual calibration plane. For a line with permittivity erand mechanical length Lmech the delay and the electrical length are calculated as follows:

The use of an Electrical Length, Mechanical Length or Delay is therefore equivalent; the analyzer automatically calculates all three parameters when a single value is entered.

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 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 Modeldialog) is assigned to a test port that is related to a particular quantity, then the phase of the quantity is corrected using both the (constant) specified length offset parameter and the dispersion effects due to the connector.

Offset parameters can be particularly useful if the DUT is connected to a transmission line that cannot be taken into account in a calibration, e.g. because it is fixed to the DUT and cannot be removed.

An offset is assigned to a particular port and affects the phase of all measurement parameters related to this port. An offset for port 1 affects the S-parameters S11, S21, S12, S31..., the Z-parameters Z11, Z21,Z12, Z31 etc.

To account for the propagation in both directions, the phase shift of a reflection parameter 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 (l/4), then the phase of S21 increases by 90 deg, whereas the phase of S11 increases by 180 deg.  

If the trace is converted to the Delayformat, 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 parameter and therefore reduces the calculated (residual) delay.  

• A negative offset parameter causes a negative phase shift of the measured parameter and therefore increases the calculated (residual) delay.  

The functions in the Offset menu can be used for balanced port configurations:

• If a balanced port configuration is active the logical and physical ports are shown in the Electrical Length, Mechanical Length and Delay dialogs.

• 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.  

Auto Length is enabled if the measured quantity contains the necessary phase information as a function of the frequency, and if the interpretation of the results is unambiguous:

• A frequency sweep must be active.

• The measured quantity must be an S-parameter, ratio, wave quantity, a converted impedance or a converted admittance.

The effect of Auto Length on S-parameters, wave quantities and ratios is to eliminate a linear phase response as described above. Converted admittances or impedances are calculated from the corresponding Auto Length corrected S-parameters. Y-parameters, Z-parameters and stability factors are not derived from a single S-parameter, therefore Auto Length is disabled.  

The Auto Length function can be used for balanced port configurations as well. If the active test port is a logical port, then the same length offset is assigned to both physical ports that are combined to form the logical port. If different length offsets have been assigned to the physical ports before, they are both corrected by the same amount.