by Michael D. McKinley and Kate A. Remley, National Inst. of Standards and Technology, Boulder, CO; Maciej Myslinski, Katholieke Universiteit, Leuven, Belgium; J. Stevenson Kenney, Georgia Institute of Technology, Atlanta, GA
The vector signal analyzer (VSA) has several measurement advantages over a spectrum analyzer in the acquisition of bandpass RF signals, including its time-domain capture, which enables measurement of both magnitude and phase information, and its ability to display data in the time and frequency domains.1,2 The highly sampled, down-converted waveform gives a good amount of spectral detail around the carrier frequency. However, the resolution of the frequency spectrum may be affected by the relation between the length of the time capture and the bandpass signal envelope period for periodic signals such as multisines.3–5
A procedure for optimizing VSA measurements of periodic signals to minimize spectral leakage is presented. The underlying principle of this method has been known for years,5 but it finds new application with the recent emphasis on the use of periodic well-behaved signals to characterize complicated wireless devices, systems and channels. In these situations, multisine test signals, consisting of a collection of sine waves at frequencies that are slightly offset from each other to emulate digital test signals, are often used. In these test environments, complete knowledge of the stimulus is obtained and it becomes practical to use this type of measurement method. This procedure determines the proper settings such that the VSA will obtain an integer multiple of the envelope period of the measured signal, as illustrated in Figure 1. Specifying an integer multiple of periods preserves an undistorted time-domain signal. Thus, the FFT used by the VSA will portray the frequency-domain characteristics of the signal with minimal distortion as well. This method also negates the need for time-domain filtering (windowing) for periodic signals, which is one method for improving the spectrum when a fraction of an envelope period is present at the input.6 Eliminating filtering removes one more potential source of distortion in the measurement and is useful when looking for a weak adjacent tone or distortion product.
Vector Signal Analyzer Settings that Affect Spectral Leakage
FFT Considerations
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