R&S^®ZVA-K6 True Differential Measurements

Requirements

True differential stimulation requires two phase-locked sources. This option is therefore only available for R&S^®ZVA models with four test ports and for R&S^®ZVT models with three or more test ports (i.e. for analyzer models with two or more sources).

True differential measurements up to 50 GHz with the R&S^®ZVA and the R&S^®ZVT

Up to now, network analyzers have only been able to measure balanced components in a roundabout way. The unbalanced S-parameters of a DUT were measured, and the mixed-mode (differential) S-parameters were derived from these values by way of calculation. Rohde & Schwarz has now introduced a software option that adds true differential measurement capability up to 50 GHz to the R&S^®ZVA and R&S^®ZVT vector network analyzers. This makes it possible for the first time to accurately characterize nonlinear balanced components.

R&S^®ZVA-K6 true differential measurements

This option allows you to accurately control the magnitude and phase of the two sources of a four-port R&S^®ZVA directly at the reference plane. The sources are thus able to generate true differential stimulus signals of identical amplitude and with 180° phase shift (differential mode) or 0° phase shift (common-mode). This allows the accurate characterization of nonlinear balanced devices. For example, the compression characteristics of balanced amplifiers can thus be measured.

Operation of the R&S^®ZVA in the true differential mode is based on a second signal source, which is provided in instrument versions with four test ports. The second source can deliver a signal whose amplitude is equal to and whose phase is opposite to that of the signal of the first source. The R&S^®ZVA can also produce signal pairs with adjustable relative amplitude and phase, thus enabling the simulation of real-world signals that are not perfectly balanced i.e. signals that contain common-mode as well as differential-mode (normal-mode) components, or even purely common-mode components with no differential components at all. The R&S^®ZVA-K6 option also supports true phase imbalance and true amplitude imbalance sweeps of the two stimulus signals.

Going deeper into the details of the new R&S^®ZVA true differential measurements option, it should be noted that the user-defined amplitude and phase relationship of the signals refers to the analyzer´s outgoing single-ended waves, which are applied as incoming waves (“a” waves) for the DUT. This relationship is measured with full-system error correction applied at each point of each sweep. Moreover, the signal amplitude and phase are re-adjusted at each point (to the correct 0° or 180° value for the phase) before the desired quantity is measured at that point. This takes the time-varying DUT reflection into account. The error terms used for wave correction are derived from a conventional full n-port calibration, which also defines the reference plane at which the user-defined amplitude and phase relationship applies. The DUT topology can be defined with a high degree of flexibility, which means that any two single-ended ports can be combined to form a balanced port.

An R&S^®ZVA with the true differential measurements option supports three operating modes:

• Source delivering fully error-corrected differential and common-mode stimulus waves with amplitude and phase defined with respect to a reference plane that is in turn defined during conventional n-port system-error calibration. You can shift the reference plane by defining and activating a transmission-line offset, which may involve a loss of power. You can also perform true phase imbalance and true amplitude imbalance sweeps of the single-ended signals and calibrate the power of each signal.
• Source function as above, plus fully error-corrected measurements of all single-ended and balanced wave quantities defined as part of the DUT topology. After a conventional (single-ended) power calibration, you can even perform calibrated measurements of the amplitude of differential and common-mode wave quantities.
• Source function as above, plus fully error-corrected measurements of all mixed-mode S-parameters defined as part of the DUT topology. For these measurements, you stimulate each balanced port in both the differential and the common-mode, and each single-ended port in either the differential or the common mode.

Switching between virtual and true differential mode at a mouse click

Switching between true and virtual differential mode takes no more than a mouse click. When you have configured two measurement channels, you can even measure a DUT in both modes simultaneously.

The diagram below shows the gain compression of a differential low-noise amplifier (LNA) for WCDMA applications measured in virtual and true differential mode during a power sweep. At small source-signal levels, hardly any difference exists between the two modes. As the source power increases, however, the amplifier reaches 1 dB compression at an input power of about -11 dBm in the true differential mode, compared with about -7.5 dBm in the virtual mode. For other differential devices, you can also observe the opposite behavior, i.e. 1 dB compression is reached at a lower input power in the virtual mode.

The power axis for the true differential stimulus signal (trace) has been shifted by 3 dB with respect to the power axis for the virtual differential stimulus signal (trace). This shift is necessary in order to compare the results obtained in the two modes because the amplitudes of the two stimulus signals must be the same. In the virtual differential mode, the R&S^®ZVA power setting refers to a single-ended source signal, whereas in the true differential mode, the setting refers to a differential source signal. Because half of the power of the single-ended source signal is contained in its unwanted common-mode component, the power of this signal must be increased by 3 dB.

Simultaneous display of S-parameters obtained in virtual and true differential mode