Technique Determines True Differential Device Performance
By Yassen Mikhailov, Product Manager, Network Analyzers, Rohde & Schwarz

The ability to accurately characterize active devices with a vector network analyzer when the device is operating under large-signal (i.e., nonlinear) conditions has eluded instrument designers for years. As a result, these devices are instead evaluated using the same mathematically-based technique employed to measure them under small-signal conditions. Using this “virtual” technique, the device is stimulated with unbalanced signals and the instrument mathematically transforms the unbalanced wave quantities into balanced S-parameters. While providing accurate characterization under small-signal conditions, this technique tends to deliver highly-optimistic results (e.g., higher power outputs) when the same device is characterized when operating in its nonlinear region. With the introduction of the R&S ZVx-K6 option for its four-port ZVA and ZVT network analyzers, Rohde & Schwarz has become the first network analyzer manufacturer to provide an off-the-shelf solution for accurately characterizing balanced devices under large-signal conditions. It is inherently broadband, precise, and simple to use.

The technique has been verified when measuring a large variety of active devices from various manufacturers, with dramatic results. In short, gain compression typically occurs at either higher or lower output levels than measurements produced by the “virtual” technique, as shown in Figure 2. In this figure, both the “virtual” and what the company calls “true” differential measurements of a 2GHz MMIC amplifier are shown together on the display of an R&S ZVA40 analyzer. In the small-signal region, the results are identical (as they should be), but when gain compression occurs, the difference is significant. The virtual technique measured gain compression beginning about 4 dB earlier than when using the true differential stimulus, and peak gain is about 0.5 dB less.

For manufacturers of amplifiers, the implications of this new technique are significant. For example, RF amplifiers used in mobile phones, smartphones, and data access cards are frequently balanced devices, and they have been almost always measured using the virtual technique, for the lack of a viable alternative. As a result, their rated performance may in reality be less than specified. In addition, if gain compression occurs at drive levels lower than specified, unacceptable levels of intermodulation products will occur under conditions previously thought not to produce them. For manufacturers of handsets (for example) that tested their products using the virtual differential method, the power amplifier within them may need to be backed off from its rated conditions to achieve the required linearity. However, backing off a device or amplifier means it produces less power, which can translate into the need for more power devices to deliver a given output level.

True Differential Measurement
The technique developed by Rohde & Schwarz allows balanced nonlinear RF and microwave balanced components such as amplifiers to be stimulated with actual differential signals for the first time to frequencies as high as 40 GHz (depending on the analyzer). The technique is based on vector-corrected wave quantities and uses a patented technique to control magnitude and phase of the two internal sources. The sources in the R&S ZVA and R&S ZVT generate two signals identical in magnitude and phase-shifted 0 deg. or 180 deg., with phase uncertainty below 1 deg. This differential-mode signal then stimulates the device, and mixed-mode S-parameters are then directly calculated from the ratios of the error-corrected differential or common mode wave quantities.

Challenges
Several obstacles had to be overcome in order to create the true differential technique. First, the 180deg. phase shift between the two internal sources had to be created and precisely controlled to ensure accurate phase and magnitude. In addition, the phase shifts had to be transferred directly to the measurement and calibration planes. The cables employed by network analyzers have different loss and other characteristics that have a negative effect on the symmetry of the signal that results in varying phase shifts.

The calibration technique employed by the instruments appears identical to a standard Thru-Open-Short-Match (TOSM = SOLT) type and provides accurate results even with unsymmetrical test cables of different lengths or with on-wafer measurements. The instrument can also generate two signals, both with 0 deg of phase shift, to produce the common-mode signal. The phase shift does not vary with time and temperature variations, which has been a significant problem in the past. The sources are kept in check with a special algorithm and control circuit that precisely maintains the magnitude/phase relationship.

Other Features
The true differential method also allows amplitude and phase imbalance sweeps to be performed. In the amplitude imbalance sweep, the instrument generates a balanced signal at one of its ports and the amplitude of one signal component is varied based on the user-defined power sweep range. For the phase imbalance sweep, the instrument generates a balanced signal at one of its ports, and the relative phase of the two signal components is varied according to the selected phase range. The benefit of both sweeps lies in their ability to let the user produce varying conditions that provide greater insight into device performance.

The user can switch between the virtual and true differential techniques with a single mouse click and can display the results from each technique in real time in the same plot. The calibration technique for both methods is identical, so there is no need to recalibrate for each one. The instrument’s firmware provides a wizard for measurement of balanced devices that makes configuration simple via a simple, step-by-step process.

The true differential measurement technique requires no hardware and can be implemented in any four-port R&S ZVA and all R&S ZVT analyzers with three or more ports and is available as a firmware upgrade. More information can be obtained by visiting our website below.

Rohde & Schwarz
www.rohde-schwarz.com/us
TXTLINX.COM134
Email this article to a friend!