Wireless communication systems continue to progress to wideband modulation formats. In particular, third-generation (3G) wireless and wireless local-area networks (WLANs) present extraordinary increases in channel bandwidth. As a result, designers are confronted with a greater divergence between the sinusoidal and modulated stimulus responses of a device. Traditional scattering (S)-parameter measurement techniques use narrowband, sinusoidal stimulus signals, resulting in the incomplete characterization of active devices. Modulated Vector Network Analysis (MVNA™) allows S-parameter measurements to be performed with complex modulated signals resulting in truer device characterization.

Modulated S-parameter analysis represents the first major advancement in network analysis since the introduction of the automatic vector-network analyzer (VNA) more than 30 years ago. This new capability, along with traditional wireless measurements such as adjacent-channel power ratio (ACPR), noise figure, and traditional sinusoidal S-parameters are contained in the ASL 3000RF measurement system from Credence Systems Corp. (Fremont, CA). This range of RF measurement capabilities combined with mixed-signal instrumentation creates a total wireless-device-characterization solution.

Wireless technologies are proving to hold the key for satisfying increasingly bandwidth-intense content. To meet these demands, increasingly efficient modulation techniques have been developed to deliver the high bandwidth consumers demand, while attempting to preserve the limited amount of spectrum. A consequence of these modulation formats has been an increase in channel bandwidths and nonconstant power-envelope signals. These trends have made amplifier design more difficult, particularly in light of higher linearity requirements coupling with the demand for better efficiency.

Figure 1 illustrates the dynamic signal envelope produced by various complex modulation techniques. The resulting ratio between the peak excursions and the root-mean-square (RMS) signal power is referred to as the peak-to-average ratio. These vary for different types of modulations, but generally speaking, the wider the modulation bandwidth, the higher the peak-to-average ratio. Today, WLAN systems in the form of 802.11 are taking hold in the marketplace, and wireless communication proponents are already discussing fourth-generation (4G) systems with 100-MHz channel bandwidth. Clearly, these difficult design trends will continue. The table highlights peak-to-average ratios for common wireless-communication systems.

S-parameters still present a crucial starting point for active-circuit designers. While many improvements have been made in network analyzers since their widespread introduction in the early 1970s, they still rely on narrowband sinusoidal stimulus signals. S-parameters are well-understood and form the basis for a vast variety of RF and microwave devices from filters to amplifiers. But some phenomena in wideband communications are not well-described with traditional sinusoidal S-parameters.

S-parameters are essentially various ratios of incident, reflected, and transmitted power. While S-parameters can be traced back to their definitions in terms of voltage or current, the difficulty in measuring these quantities at high frequencies results in S-parameters typically being determined from power ratios, as these can be measured with great accuracy, even at microwave frequencies. Figure 2 depicts the typical two-port device model which provides an intuitive understanding of S-parameter definitions.

Equations 1-4 relate the two-port S-parameter ratios for the flowgraph shown in Fig. 2:

VNA equipment measures the a1, b1, a2, and b2 signals with narrowband receivers (Rxs) [10 Hz to 35 kHz] and then performs the required ratioing and error correction to measure S-parameters. Figure 3 illustrates this process for the case of S₂₁ (forward gain). In the case of modulated S-parameters, the basic tenets of network analysis still apply. The two-port model definition is still identical, the ratios are still defined in the same manor, but a complex modulated stimulus is applied to the device under test (DUT) rather than a single-tone sinusoid. Building on the sinusoidal example, Fig. 4 illustrates the ratioing process for modulated S-parameters.

The ratioing process illustrated in Fig. 4 indicates that modulated S-parameters are only calculated where significant signal energy is present. In measuring S-parameters, if ratios are performed outside of the channel, i.e., where only noise exists, then the ratios will not converge. The S-parameter ratioing must be performed with the channel bandwidth in mind. For example, in the case of IS-95 [code-division multiple access (CDMA)] a processing (channel) bandwidth of 1.2288 MHz would be used. Figure 5 demonstrates the signal-processing bandwidth approach used for modulated S-parameters.