Differential transmission lines and circuits are widely used, because their characteristics give them a lower susceptibility to electromagnetic interference. Linear balanced devices can be tested with sufficient accuracy using the virtual differential mode, where the vector network analyzer generates unbalanced stimulus signals and uses a mathematical transformation to convert unbalanced wave quantities into balanced S-parameters. A different behavior is expected for nonlinear balanced devices, where the transmission characteristics of the DUT may depend on how closely the stimulus signal matches real operating conditions.
With option R&S ZVA-K6, True Differential Mode, the vector network analyzer can generate true differential and common mode stimuli at arbitrary reference planes in the test setup and determine mixed-mode S-parameters, wave quantities and ratios. It is also possible to perform true differential measurements on frequency-converting DUTs or to use true differential mode in combination with external frequency converters (see background information in section Balanced Ports and Port Groups – True Diff Mode). Moreover the true differential mode provides two additional sweep types, the amplitude imbalance and phase imbalance sweeps. Like the virtual differential mode, the true differential mode requires a port configuration with at least one balanced port. The differential mode signal that the analyzer generates in true differential mode is shown below (the plot shows the waveforms of the two single-ended sources measured vs. ground).
True differential mode requires an analyzer with at least two independent internal sources, i.e. a minimum of 3 ports. To activate true differential mode, open the Balanced Ports and Port Groups dialog to define a balanced port configuration, then select the True Diff Mode tab (Channel – Mode – Port Configuration... – Balanced Ports and Port Groups... – True Diff Mode – True Differential Mode: On). Refer to the measurement examples for details.
On R&S ZVA-Z67 network analyzers, all ports have independent internal sources. You can use true differential mode with an arbitrary combination of two source ports.
Virtual and true differential mode
The analyzer uses different stimulus signals and different mathematical methods to obtain results in normal, virtual differential, or true differential mode. The following table gives an overview.
Normal (unbalanced) mode
Measurement of wave quantities and S-parameters for unbalanced ports. Unbalanced stimulus signals.
Virtual differential mode
Precondition: At least one balanced port is defined.
Measurement of unbalanced wave quantities using unbalanced stimulus signals. Unbalanced system error correction. Conversion of unbalanced wave quantities into balanced and mixed-mode S-parameters with possible renormalization of port impedances.
True differential mode
Measurement of unbalanced wave quantities using balanced stimulus signals (differential and common mode). System error correction and conversion of unbalanced into balanced wave quantities. Calculation of balanced and mixed-mode S-parameters with possible renormalization of port impedances.
True Differential mode relies on Enhanced Wave Correction. A consistent system error correction is essential for accurate balanced waves at the reference planes and accurate measurement results. You must perform a full n-port calibration (TOSM, UOSM or one of the Txx calibration types) of all physical ports involved in the true differential measurement and change the differential and common mode reference impedances, if they differ from the default settings. A subsequent source power calibration is recommended. You can also use offset parameters or additional two-port transmission factors in order to move the reference plane where the analyzer provides an accurate differential or common mode signal. See also True Differential Mode on Frequency-Converting DUTs.
Option R&S ZVA-K6 provides an alternative Defined Coherence Mode where you can generate several continuous signals with specific relative amplitude and phase. True differential mode and defined coherence mode cannot be active at the same time because they require a different source port configuration.
Measuring balanced wave quantities and ratios
Measuring mixed mode S-parameters
Performing an amplitude or phase imbalance sweep
True differential measurement on frequency-converting DUTs
A single-ended stimulus can be thought of as resulting from the superposition of a differential and a common mode stimulus, where the signals add at one single-ended port and cancel at the other (see equations in section Wave Quantities and Ratios in True Differential Mode). Therefore the differential content of a single-ended signal is 3 dB lower than the total signal power.
The stimulus power in true differential mode can be set in two different ways:
If Same Differential and Common Mode Voltages as in Single-Ended Mode is selected, the analyzer ensures that the stimulus voltages in true differential or common mode are the same as the respective stimulus components in virtual differential mode. The single ended voltages at the involved physical test ports are divided by 2 and the output powers with reference Z0 by 4 (see table below).
If Apply to Differential (Zd = 2Z0) and Common Mode (Zc = Z0/2) Waves is selected, the channel power Pch is interpreted as the power of the true differential or common mode stimulus signal, referenced to the corresponding mode impedance Zd = 2Z0 or Zc = Z0/2. The single ended voltages at the involved physical test ports are divided by sqrt(2) and the output powers with reference Z0 are divided by two (see table below).
Switching between the two stimulus power modes changes the single ended powers by 3 dB. The channel power setting Pch (Channel – Stimulus – Power) remains unchanged.
Mode
Stimulus signal
Nodal generator powers and voltages at physical ports
Port i
Port j
Unbalanced
Pch( = Ud2 / Z0) Ud /2 + Uc (= Ud)*)
0 W Ud /2 – Uc (= 0)*)
True differential mode, "Same Differential and Common Mode Voltages as in Single-Ended Mode"
Differential mode
Pch / 4 Ud / 2
Pch / 4 -Ud / 2
Common mode
Pch / 4 Uc
True differential mode, "Apply to Differential (Zd = 2Z0) and Common Mode (Zc = Z0/2) Waves"
Pch / 2 Ud / sqrt(2)
Pch / 2 -Ud / sqrt(2)
Pch / 2 Uc * sqrt(2)
*) This entails the condition Ud = 2 * Uc
Modal generator powers and voltages at balanced port
Pch / 2 Ud
Pch / 2 Uc
0 W 0 V
Pch Ud *sqrt(2)
Pch Uc *sqrt(2)
Avoiding possible problems at very large or very low source powers
1st scenario: Large source powers
The maximum output power is not always exactly the same for different physical test ports. The deviations are generally small (< 1 dB).
If the network analyzer in true differential mode is operated in the vicinity of the maximum channel power (> +5 dBm), a message "Port<n> power unleveled" may indicate that one of two combined physical ports cannot provide the required source power. The measurement is not aborted, however, the analyzer is no longer capable of providing accurate balanced waves.
If you operate the analyzer close to its maximum power, first check the reference channels (a-waves) and ensure a 1 dB reserve over the entire sweep range.
2nd scenario: Small source powers
To generate equal source powers, the signal-to-noise ratio at each port must be sufficiently high. At very low levels (<–30 dBm) and large IF bandwidths, a message "Port<n> power unleveled" may indicate that the signal-to-noise ratio at one of the ports is too low. Again, the measurement is not aborted, however, the analyzer is no longer capable of providing accurate balanced waves.
If you operate the analyzer at very low output power settings, reduce the IF bandwidth to 1 kHz or below.
The source power mode is defined in the General tab of the System Configuration dialog. It is a global setting which is not affected by a reset of the instrument.
Remote control:
SOURce<Ch>:TDIF:WAVes SENDed | DCMode
In true differential mode, the analyzer can determine the balanced wave quantities and ratios for all balanced ports. The balanced quantities appear in the More Wave Quantities or More Ratios dialog as soon as the true differential mode is active.
Assume that a balanced port numbered i comprises the two physical ports k and l. To obtain the balanced wave quantities for port i, the analyzer measures the unbalanced wave quantities at ports k and l. The differential mode waves adi0 and bdi0 and the common mode waves aci0 and bci0 can be calculated from the unbalanced waves using the following equations:
Possible modification for amplitude and phase imbalance sweep
In an amplitude imbalance or phase imbalance sweep, it is possible to compensate the a waves for the known phase imbalance φ and the amplitude imbalance r = | ak / al |. This modifies the formulas for the balanced wave quantities as shown below.
For an amplitude imbalance sweep with known imbalance r:
For a phase imbalance sweep with known imbalance φ:
The amplitude and phase imbalances are known quantities (sweep parameters). The unbalanced waves ak and al are measured in the reference channels. With ideal unbalanced waves and no additional disturbing effects, the compensated balanced stimulating a-waves remain constant over the entire sweep range.
Renormalization of port impedances
In the default scenario where the reference impedances for the differential and common mode are equal to Zd = 2 Z0 and Zc = 1/2 Z0, the waves adi0, bdi0, aci0 and bci0 correspond to the true balanced waves at port i. With arbitrary reference impedances for the balanced waves, an additional renormalization step is necessary:
The reference impedances Zc and Zd can be entered in the Balanced Ports and Port Groupsdialog.
CALCulate<Ch>:PARameter:SDEFine 'AS1D2S' |
The mixed-mode S-matrix elements that the analyzer acquires in true differential mode correspond to the mixed-mode matrix elements obtained in virtual differential mode, however, the analyzer uses true differential and true common mode stimuli at each balanced port of the DUT.
Example: 3x3 mixed-mode S-matrix
Suppose that a three-port analyzer is configured for one balanced and one single-ended port as shown below:
To obtain the complete mixed-mode S-matrix, the analyzer generates the following stimulus signals:
Differential mode signal fed to balanced port no. 1 of the DUT
Common mode signal fed to balanced port no. 1 of the DUT
Unbalanced signal fed to single-ended port no. 2 of the DUT
The DUT is fully characterized by the following mixed mode matrix:
For linear DUTs, the S-matrices acquired in virtual and in true differential mode are expected to be equal. The following figure shows a comparison for the transmission coefficient Ssd12. The red trace was measured in true differential mode. The blue trace (below, measured in virtual differential mode) is almost identical over the entire sweep range.
Differences may appear for nonlinear devices at high stimulus power levels. For example, the bias of semiconductor devices like transistors may depend on the kind of stimulus signal. The following figure shows a power sweep measured in virtual differential mode (green) and in true differential mode (blue). In true differential mode, gain peaking is smaller and compression starts at much lower stimulus power.
CALCulate<Ch>:PARameter:SDEFine 'SCD11' |
True differential mode can be combined with frequency-converting measurement modes as outlined in the measurement example. For this type of measurements, a consistent calibration is particularly important. E.g. scalar mixer measurements are typically performed at different RF and IF frequency ranges.
Calibration kits do not contain frequency-converting two-port standards so that any system error correction is performed at equal source and receiver frequencies. However, if a scalar mixer measurement is active, the analyzer automatically performs the system error correction in both the RF and the IF frequency range. The number of points for the calibration sweep is doubled. In the Calibration Manager dialog, the calibrated frequency range is referred to as a "Segmented Grid".
A subsequent source or receiver power calibration not only calibrates the source power and the receiver but is also used to adjust the normalization of the system error corrections in both power ranges. For this reason, the measurement steps must be performed in the following order:
1. Activate the frequency-converting (e.g. scalar mixer) mode.
2. Perform a full n-port calibration for all ports involved.
3. Perform a power calibration (for true differential ports, once per balanced port).
4. Activate true differential mode.
4. Connect the DUT in order to perform measurements.
To ensure proper operation, differential devices need a pure differential stimulus signal. Unequal attenuation or loss in the conductors of the balanced input or output line leads to a phase or amplitude imbalance of the stimulus signal. If the device under test is linear, the effects of this imbalance can be calculated from the mixed-mode S-parameters without performing an additional measurement. For nonlinear devices, however, the effects may be unpredictable. Therefore, the option True Differential Mode incorporates the ability to generate differential and common mode signals with a physical amplitude or phase imbalance. Moreover, the imbalance can be swept over a user-defined range.
Characteristics of the sweep
The amplitude imbalance sweep is performed as follows:
The amplitude imbalance is defined as r dB = 20 log (|ak/al|) dB, where ak and al are the wave quantities of the physical ports with the higher and lower port numbers.
The selected amplitude imbalance is applied to both differential and common mode signals. The analyzer generates both stimulus modes according to what the measurement of the selected quantity requires.
The powers of the two sources are swept symmetrically around the equilibrium state |ak| = |al|, where r dB = 0 dB (see also Wave Quantities and Ratios in True Differential Mode).
At least one logical port must be defined, and the true differential mode must be active to enable the amplitude imbalance sweep. Activating the sweep (Channel – Sweep – Sweep Type: Amplitude Imbalance) opens the following configuration dialog:
Logical Port selects the swept logical port.
Compensate Imbalance of a Waves selects the calculation method for S-parameters, ratios and derived quantities; see background information below.
CW Frequency defines the frequency of all stimulus signals, which is constant over the entire sweep. This setting is identical with the CW frequency for power, time and CW mode sweeps.
Source Power at 0 dB defines a constant power reference, corresponding to the channel power Pch in the tables in section Source Power in True Differential Mode. The swept power range (Channel – Stimulus – Start, Channel – Stimulus – Stop) of the involved single-ended sources is defined relative to the reference value. This setting is identical with the constant channel base power for frequency, time and CW mode sweeps.
The power range for the amplitude imbalance sweep is independent of the power range for power sweeps.
Compensate Imbalance of a Waves
Since the mixed-mode S-parameters of a linear balanced DUT depend only on the DUT itself, they will be independent of the amplitude or phase imbalance of the stimulus signal. So looking for example at Sdd21 of a differential amplifier in phase imbalance sweep will not reveal the amplitude reduction of the differential output signal caused by unequal lengths of the balanced input line conductors. This length asymmetry corresponds to a phase imbalance increasing over frequency.
In order to see the effect of such a phase or amplitude imbalance, modified S-parameters are required. The modification is done in such a way that if e.g. the imbalance of port 1 is swept, the imbalance of the a wave of port 1 is compensated before the S-parameters are calculated. The effect of the compensation is a constant amplitude of the differential or common mode stimulus wave of port 1 over the imbalance sweep range. This reflects the situation of the user applying a stimulus signal of known nominal amplitude to the DUT, but getting at the output only the amplified differential contents in this signal, which depends on the imbalance. Usually, S-parameters of balanced devices measured with active imbalance compensation will exhibit a maximum or minimum at zero imbalance.
The imbalance compensation is not only performed for mixed-mode S-parameters, but also for the imbalance-swept a wave itself when it is selected as a measured quantity, as well as for ratios including that wave.
[SENSe<Ch>:]SWEep:TYPE IAMPlitude SOURce<Ch>:TDIF:IMBalance:AMPLitude:LPORt SOURce<Ch>:TDIF:IMBalance:AMPLitude:STARt SOURce<Ch>:TDIF:IMBalance:AMPLitude:STOP CALCulate<Ch>:TDIF:IMBalance:COMPensation[:STATe]
The phase imbalance sweep is a special application of the true differential mode. The analyzer generates a balanced signal at one of its logical ports, however, the relative phase of the two signal components is varied according to the selected phase range. This variation may have an impact on the measured mixed-mode S-parameters if the DUT is operated in its nonlinear range.
The phase imbalance sweep is performed as follows:
The phase imbalance of the differential mode signal is defined as the difference in phase (in deg) of the wave quantities at the two physical ports of the swept logical port plus 180 deg: Imbalance = arg (ak) – arg (al) – 180 deg, where k > l.
The phase imbalance of the common mode signal is defined as the difference in phase (in deg) of the wave quantities at the two physical ports of the swept logical port: Imbalance = arg (ak) – arg (al), where k > l.
The selected phase imbalance is applied to the differential as well as to the common mode signal. The analyzer generates both stimulus modes according to what the measurement of the selected quantity requires.
At least one logical port must be defined, and the true differential mode must be active to enable the phase imbalance sweep. Activating the sweep (Channel – Sweep – Sweep Type: Phase Imbalance) opens the following configuration dialog:
Compensate Imbalance of a Waves selects the calculation method for S-parameters, ratios and derived quantities; see background information.
Source Power defines the constant power of the balanced source, corresponding to the channel power Pch in the tables in section Source Power in True Differential Mode. This setting is identical with the constant power for frequency, time and CW mode sweeps.
The range of relative phases for the phase imbalance sweep is set via Channel – Stimulus – Start and Channel – Stimulus – Stop.
[SENSe<Ch>:]SWEep:TYPE IPHase SOURce<Ch>:TDIF:IMBalance:PHASe:LPORt SOURce<Ch>:TDIF:IMBalance:PHASe:STARt SOURce<Ch>:TDIF:IMBalance:PHASe:STOP CALCulate<Ch>:TDIF:IMBalance:COMPensation[:STATe]