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Using Calibration to Optimize Performance in Crucial Measurements
Rohde & Schwarz made the first public demonstration of its breakthrough technology that allows accurate measurements of balanced active RF and microwave devices to be made when the devices are operating under large-signal conditions.
The capability, available via Option ZVx-K6 for the R&S ZVA four-port vector network analyzers and R&S ZVT analyzers with three or more ports, is the first technique to allow RF and microwave power transistors and amplifiers, MMICs, RFICs, filters and single-chip receivers to be accurately evaluated under these signal conditions. The technology is the result of more than six years of development and has resulted in several patents.
Balanced devices, which have differential (dual) inputs rather than single-ended inputs, are becoming more and more widely used in a wide array of wireless systems, including handsets and base stations. Until now, engineers have been forced to measure balanced devices in both their linear small-signal and nonlinear large-signal regions with the same technique.
However, this technique is accurate only in the small-signal region, and almost invariably overstates device performance in the large-signal region. Consequently, manufacturers of these devices have in many cases specified their products inaccurately for lack of a more accurate technique. The ZVx-K6 option for the Rohde & Schwarz vector network analyzers makes “true” differential measurements possible for the first time and performs the measurements in less than 300 ms per sweep. S-parameters obtained using both the conventional and true differential techniques can be displayed with a single push of a button.
The benefits for active device manufacturers and those who employ them are significant. When devices are specified using the true differential technique, gain compression is shown to occur at drive levels lower than when measured using the conventional technique. The result is that amplifiers, for example, will produce unacceptable levels of intermodulation products under conditions that were previously thought not to produce them if earlier characterized with the conventional technique. The ability of the true differential to characterize devices accurately under large-signal (nonlinear) conditions makes it possible for device manufacturers to specify their parts conservatively so that they will meet their rated performance under actual operating conditions.
The true differential method produces results different from the conventional method only when a device is operating in its non-linear region. When driven with lower power input levels, the results will be nearly identical. However, this nonlinear, large-signal region is of vital importance to device and amplifier designers because systems employing the Orthogonal Frequency Division Multiplex (OFDM) modulation technique, such as WiMAX, lightwave communications and cable systems, must deliver extraordinarily high levels of linearity.
In addition, when the Long Term Evolution (LTE) enhancement to UMTS wireless systems is deployed in a few years, it too will require the same level of linearity. As a result, the need to accurately specify active devices used in these systems will become increasingly important.
There is no difference in calibration between the conventional and true differential methods, and no special calibration standards are needed. Calibration appears identical to a standard thru-open-short-match (TOSM or SOLT) type calibration. In addition, the technique works perfectly even with unsymmetrical test cables of different lengths or with on-wafer measurements. Both error-corrected S-parameters and wave quantities can be measured, power calibration can be applied and the user can produce “non-standard” conditions that provide greater insight into device performance by conducting amplitude and phase imbalance sweeps.
The option also allows two signals to be generated, each with 0 deg. of phase shift, to produce common-mode test signals for conventional single-ended VNA measurements. Phase shift does not vary with time and temperature variations, a significant breakthrough in itself, since it is one of the primary reasons why this technique could not be achieved in the past. The sources are controlled with a special algorithm and control circuit that precisely maintains the magnitude/phase relationship.
Option ZVx-K6 for the R&S ZVA four-port vector network analyzers and R&S ZVT analyzers with three or more ports is available now from Rohde & Schwarz.
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