Fundamentals of Modern Spectral Analysis Matthew T. Hunter, Achilleas G. Kourtellis, Christopher D. Ziomek, and Wasfy B. Mikhael D igital Signal Processing (DSP) has revolutionized spectral analysis. Where the swept spectrum an- alyzer dominated the market in the past, the Fast Fourier Transform (FFT) based spectrum analyzer is now gaining acceptance as the method of choice. This is due in part to the prevalence of high speed, high dynamic range An- alog-to-Digital Converters (ADCs) and high speed signal processing devices such as Field Programmable Gate Arrays (FPGAs). Because the FFT-based spectrum analyzer is read- ily implemented with a limited set of generic hardware, it is an attractive technique for Synthetic Instruments (SI), where the goal is to form multiple measurement functions from a limited set of generic hardware modules. In its most basic form, the fundamental task of a spec- trum analyzer is to measure signal power versus frequency. In times past, this was accomplished almost exclusively us- ing the well-known analog Swept Spectrum Analyzer (SSA). However, the availability of high speed, high dynamic range Analog-to-Digital Converters (ADC) coupled with high speed Digital Signal Processing (DSP) has brought about dramatic changes in the architecture of the spectrum an- alyzer. The majority of the SSA specifc signal processing, e.g. resolution bandwidth (RBW) fltering, can now be done digitally, improving performance and reducing calibration requirements. The rise of DSP has not only improved the per- formance of the SSA, but also has led to the development and prevalence of the Fast Fourier Transform (FFT) based spectrum analyzer. FFT-based spectral analysis (FFTSA) has become the method of choice for many implementa- tions [1]-[4]. This is particularly true in the design of fexible or software reconfgurable instrumentation, where spectral analysis is just one of the many functions to be performed by the same set of “generic” hardware modules [5]. This type of system, in which multiple functions are synthesized from a limited set of generic hardware components, is called a Syn- thetic Instrument (SI) [6], [7]. In this paper, we cover several of the fundamental or key design parameters affecting the performance of a spectrum analyzer. We present a high level overview of the spectrum analyzer dynamic range and show how it can be achieved with both FFTSA and SSA. We discuss the effect of instan- taneous bandwidth on system fexibility and measurement speed, we explore the importance of image rejection and anti-aliasing and how the lack thereof can lead to false mea- surements, and we conclude with fnal comments and analysis. Spectrum Analyzer Architecture Both SSAs and FFTSAs can be described by the block diagram in Fig. 1. The input signal is frst applied to the analog down- converter module whose primary function is to translate the input signal to a suitable fxed intermediate frequency (IF) and level for subsequent processing. The local oscillator (LO) mod- ule provides a sinusoidal signal to the downconverter module that is mixed with the input signal, producing the desired fre- quency shift to the IF. In an SSA, the sinusoidal signal is swept linearly over the frequency band or span to be measured. In an FFTSA, the LO module provides a frequency stepped sinu- soidal signal whose step size is determined by the frequency coverage of the FFT. See [8] for a thorough examination of the basic differences between SSAs and FFTSAs. The digitizer module converts the analog output of the downconverter module to the digital domain. It also may contain the complex DSP algorithms used to produce a measurement. This includes operations such as digital down- conversion (DDC) to baseband, FFT, averaging, and triggering. The host computer may perform some, all, or none of the re- quired algorithms depending on the digitizer’s capability. Measurement results and/or raw data are transferred to a host computer for further processing and/or display. The dig- itizer plays a key role in determining the speed of the system. If the digitizer contains a DDC and a resampler [9], the amount of data to be transferred to the host can be minimized often This paper was frst presented at the IEEE AUTOTESTCON 2010 (© 2010 IEEE, Proc. AUTOTESTCON 2010, used with permission, [7]). It has been slightly edited and formatted for the Magazine. 12 IEEE Instrumentation & Measurement Magazine August 2011 1094-6969/11/$25.00©2011IEEE