The Offset menu defines a length offset and a loss for the test ports. The offset parameters complement the system error correction, compensating for the known length and loss of a (non-dispersive and perfectly matched) transmission line between the calibrated reference plane and the DUT.
Length offset parameters: Definition
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 εrand mechanical length Lmech the delay and the electrical length are calculated as follows:
Electrical Length, Mechanical Length or Delay are coupled parameters. When one of them is changed, the other two follow.
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 length 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 calculated taking dispersion effects into account.
Loss parameters: Definition
The loss L is the attenuation of a wave 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:
The DC loss LossDC, the reference frequency fref, and the loss at the reference frequency Loss(fref) are empirical parameters for the transmission lines connected to each port which can be entered into any of the dialogs in the Offset menu (see figure below). For a lossless transmission line, LossDC = Loss(fref) = 0 dB. In practice, the frequency-dependent loss often represents the dominant contribution so that LossDC can be set to zero.
The entries in the One-Way Loss section of the offset dialogs have the following meaning: DC loss LossDC(at DC), total loss at the reference frequency Loss(fref) (at Freq), reference frequency fref (Freq). Experimentally, the two loss values LossDC and Loss(fref) are determined in two separate measurements at a very low frequency (f --> 0) and at f = fref.
Offset parameters: Application and effect
Offset parameters can be particularly useful if the reference plane of the calibration cannot be placed directly at the DUT ports, e.g. because the DUT has non-coaxial ports and can only be measured in a test fixture. Offset parameters can also 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 equivalent to embedding the DUT by numerically adding a (perfectly matched) transmission line at that port.
The offset parameters are also suited for length and delay measurements; see Auto Length. In contrast to embedding/deembedding by means of the Channel – Mode – Virtual Transform functions, the offset 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 S11, S21, S12, S31... Some quantities (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 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 (λ/4), then the phase of S21 increases by 90 deg, whereas the phase of S11 increases by 180 deg. Equivalent relations hold for the loss.
If the trace is displayed in 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 group delay.
A negative offset parameter causes a negative phase shift of the measured parameter and therefore increases the calculated group delay.
Offset parameters for balanced ports
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.
The offset parameters can be defined separately for each port.
Reset Offsets restores the default values for all length offsets, i.e. it resets all values to zero.
Electrical Length defines electrical length offsets and loss parameters at all test ports.
Mechanical Length defines mechanical length offsets and loss parameters at all test ports.
Delay defines delay times and loss parameters at all test ports.
Auto Length determines a length offset for the receiving port of the active measured quantity with the condition that the residual group delay of the active trace is minimized.
Auto Length and Loss determines a length offset an loss for the receiving port of the active measured quantity with the condition that the residual group delay of the active trace is minimized and the measured loss is minimized as far as possible across the entire sweep range.
Fixture Compensation, corrects the measurement result for the effects of a text fixture.
The Zero Delay at Marker function overwrites the Offset parameters.
Remote control (for Reset Offsets):
[SENSe<Ch>:]CORRection:OFFSet<port_no>[:STATe] <numeric _value>
Opens a dialog to define the length offset parameters for the physical test ports as electrical lengths. The dialog also contains the DC loss, the loss at the reference frequency Loss(fref), and the reference frequency fref; see Loss parameters: Definition.
Compensate Meas's Transm. indicates the ports for which a direct fixture compensation has been carried out, replacing the offset parameters.
Remote control:
[SENSe<Ch>:]CORRection:EDELay<port_no>:ELENgth [SENSe<Ch>:]CORRection:LOSS<port_no> [SENSe<Ch>:]CORRection:LOSS<port_no>:FREQuency [SENSe<Ch>:]CORRection:LOSS<port_no>:OFFSet [SENSe<Ch>:]CORRection:OFFSet<port_no>:DFComp[:STATe]?
Opens a dialog to define length offset parameters for the physical test ports as mechanical lengths and permittivities. The dialog also contains the DC loss, the loss at the reference frequency Loss(fref), and the reference frequency fref; see Loss parameters: Definition.
The Mechanical Length dialog contains the three editable columns Mech. Length, Permittivity (εr) and Velocity Fact. The velocity factor is 1/sqrt(εr) and is a measure for the velocity of light in a dielectric with permittivity εr relative to the velocity of light in the vacuum (velocity factor < 1); see Offset parameters: Definition. Permittivity and velocity factor are coupled parameters.
Click Same Dielectric at Each Port to change the permittivity or velocity factor for all ports by entering a single value.
[SENSe<Ch>:]CORRection:EDELay<port_no>:DISTance [SENSe<Ch>:]CORRection:EDELay<port_no>:DIELectric [SENSe<Ch>:]CORRection:LOSS<port_no> [SENSe<Ch>:]CORRection:LOSS<port_no>:FREQuency [SENSe<Ch>:]CORRection:LOSS<port_no>:OFFSet [SENSe<Ch>:]CORRection:OFFSet<port_no>:DFComp[:STATe]?
Opens a dialog to define the length offset parameters for the physical test ports as delays. The dialog also contains the DC loss, the loss at the reference frequency Loss(fref), and the reference frequency fref; see Loss parameters: Definition
[SENSe<Ch>:]CORRection:EDELay<port_no>[:TIME] [SENSe<Ch>:]CORRection:LOSS<port_no> [SENSe<Ch>:]CORRection:LOSS<port_no>:FREQuency [SENSe<Ch>:]CORRection:LOSS<port_no>:OFFSet [SENSe<Ch>:]CORRection:OFFSet<port_no>:DFComp[:STATe]?
Adds an electrical length offset to the active test port with the condition that the residual 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 around zero. In phase format, the Auto Length corrected trace shows the deviation from linear phase. The effect of a dispersive connector type (i.e. a waveguide; see Offset Model dialog) assigned to the receiving port of the measured quantity is taken into account.
If the measured quantity is a ratio, or if it is derived from a ratio, its receiving port is given as the index of the wave quantity in the numerator. If the active trace shows an S-parameter Sij, then Auto Length adds a length offset at port i.
Length and delay measurement, related settings
Auto Length is suited for length and delay measurements on transmission lines.
Connect a (non-dispersive) cable to a single analyzer port no. n and measure the reflection factor Snn.
Select Auto Length.
The delay is displayed in the Delay dialog. The cable length, depending on the velocity factor, can be read in the Mechanical Length dialog.
It is also possible to determine cable lengths using a transmission measurement. Note that Auto Length always provides the single cable length and the delay for a propagation in one direction.
The analyzer provides alternative ways for delay measurements:
Measure the reflection factor and select Trace – Format – Delay. This yields the delay for propagation in forward and reverse direction and should be approx. twice the Auto Length result. For transmission measurements, both results should be approx. equal.
Measure the reflection factor and select Trace – Format – Phase. Place a marker to the trace and activate Trace – Trace Funct – Trace Statistics – Phase Delay / El Length. This yields the delay in one direction and should be approx. equal to the Auto Length result.
The measurement results using trace formats and trace statistics functions depend on the selected delay aperture and evaluation range. Auto Length is particularly accurate because it uses all sweep points. For non-dispersive cables, aperture and evaluation range effects are expected to vanish.
Use Zero Delay at Marker to set the delay at a special trace point to zero.
Preconditions for Auto Length, effect on measured quantities and exceptions
Auto Length is enabled if the measured quantity contains the necessary phase information as a function of 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. The magnitude of the measured quantity is not affected. 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.
Auto Length for logical ports
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.
[SENSe<Ch>:]CORRection:EDELay<port_no>:AUTO ONCE
Determines all offset parameters such that the residual group delay of the active trace (defined as the negative derivative of the phase response) is minimized and the measured loss is minimized as far as possible across the entire sweep range. Auto Length and Loss involves a two-step procedure:
An Auto Length correction modifies the phase of the measured quantity, minimizing the residual group delay. The magnitude of the measured quantity is not affected.
The Auto Loss correction modifies the magnitude of the measured quantity, leaving the (auto length-corrected) phase unchanged.
If Auto Length and Loss is used with a line connected to a test port, the end of the line should be left open.
Preconditions for Auto Length and Loss, effect on measured quantities and exceptions
Auto Length and Loss 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:
The effect of Auto Length and Loss on S-parameters, wave quantities and ratios is to eliminate a linear phase response and account for a loss as described above. Converted admittances or impedances are calculated from the corresponding Auto Length and Loss corrected S-parameters. Y-parameters, Z-parameters and stability factors are not derived from a single S-parameter, therefore Auto Length and Loss is disabled.
Calculation of loss parameters
The loss is assumed to be given in terms of the DC loss LossDC, the reference frequency fref, and the loss at the reference frequency Loss(fref). The formula used in the Auto Loss algorithm is similar to the formula for manual entry of the loss parameters (see Loss parameters: Definition). The result is calculated according to the following rules:
The reference frequency fref is kept at its previously defined value (default: 1 GHz).
The DC loss c is zero except for wave quantities and for S-parameters and ratios with maximum dB magnitude larger than –0.01 dB.
Auto Length and Loss for a wave quantity centers the corrected dB magnitude as close as possible around 0 dBm.
Auto Length and Loss for S-parameters and ratios centers the corrected dB magnitude as close as possible around 0 dB.
The resulting offset parameters are displayed in the Electrical Length, Mechanical Length, and Delay dialogs.
Auto Length and Loss for logical ports
The Auto Length and Loss function can be used for balanced port configurations as well. If the active test port is a logical port, then the same offset parameters are assigned to both physical ports that are combined to form the logical port. If different offset parameters have been assigned to the physical ports before, they are both corrected by the same amount.
[SENSe<Ch>:]CORRection:LOSS<port_no>:AUTO ONCE