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Multiport & Balanced Device Measurement Application Note Series Concepts in Balanced Device Measurements Application Note 1373-2 Introduction The use of differential components such as surface acoustic wave (SAW) filters and differential amplifiers is becoming more common in the wireless industry because they have greater performance than their sin- gle-ended counterparts, such as the traditional single- ended three-terminal duplexer filters used in mobile handsets. Accurate measurement of these components is challenging as most vector network analyzers have single-ended RF ports that cannot directly measure dif- ferential parameters. However, there are several alter- native methods of obtaining the differential parameters needed to characterize these devices. This article describes the challenges designers face in measuring the performance of differential components, and describes each of the most widely used techniques. While each technique produces a specific level of accu- racy that depends on the characteristics of the device to be tested, the "calculated mix-mode" method pro- vides the most accurate device characterization and has the fewest drawbacks. Differential devices Two types of differential components, one with a single- ended output and the other with a differential output are shown in figure 1. The differential port of the devices consists of a pair of physical terminals. Differential components are unique in that signals are referenced not only to a common ground but to each other as well. The signals referenced to each other are called "differential mode" and the signals referenced to a common ground are called "common mode." Differential components can have both common mode and differen- tial mode signals. In most cases, the differential mode signals are out of phase because their phase relative to each other is 180 degrees, which creates a virtual ground along the axis of Figure 1: Two types of differential components, showing single-ended and differential symmetry of the device. At the virtual ground, the outputs. potential at the operating frequency does not change with time, regardless of the signal amplitude. Common- mode signals are induced at the terminals of the device with the same phase and amplitude relationship. While a differential component has no performance advantage over a single-ended component when used in common mode, it exhibits significant benefits when used in differential mode, because it will pass differential- mode signals and reject common-mode signals. For example, noise from a power supply will affect both ter- minals of the device equally with the same phase rela- tionship. The device