SPECTRUM ANALYZER; Types of signal analyzers

Types of signal analyzers
While we shall concentrate on the swept-tuned, superheterodyne spectrum analyzer in this note, there are sev ral other signal analyzer architectures. An important non-superheterodyne type is the Fourier analyzer, which digitizes the time-domain signal and then uses digital signal processing ( DSP) techniques to perform a fast Fourier transform ( FFT) and display the signal in the frequency domain. One advantage of the FFT approach is its ability to characterize single-shot phenomena. Another is that phase as well as magnitude can be measured. How ver, Fourier analyzers do hav some limitations relative to the superheterodyne spectrum analyzer, particularly in the areas of frequency range, sensitivity, and dynamic range. Fourier analyzers are typically used in baseband signal analysis applications up to 40 MHz.

Vector signal analyzers ( VSAs) also digitize the time domain signal like Fourier analyzers, but extend the capabilities to the RF frequency range using downconvert rs in front of the digitizer. For example, the Agilent 89600 Series VSA offers various models available up to 6 GHz. They off r fast, high-resolution spectrum measurements, demodulation, and advanced
time-domain analysis. They are especially useful for characterizing complex signals such as burst, transient or modulated signals used in communications, video, broadcast, sonar, and ultrasound imaging applications.
While we have defined spectrum analysis and vector signal analysis as distinct types, digital technology and digital signal processing are blurring that distinction. The critical factor is where the signal is digitized. Early on, when digitizers were limited to a f w tens of kilohertz, only the video ( baseband) signal of a spectrum analyzer was digitized. Since the video signal
carried no phase information, only magnitude data could be displayed. But ven this limited use of digital technology yielded significant advances: flick r-free displays of slow sweeps, display mark rs, different types of averaging, and data output to computers and print rs.

Because the signals that people must analyze are becoming more complex, the latest generations of spectrum analyzers include many of the vector signal analysis capabilities previously found only in Fourier and vector signal analyzers. Analyzers may digitize the signal near the instrument s input, after some amplification, or after one or more downconvert r stages. In any of these cases, relative phase as well as magnitude is preserved. In addition to
the benefits noted abov , true vector measurements can be made. Capabilities are then determined by the digital signal processing capability inherent in th analyzer s firmware or available as add-on software running either internally ( measurement personalities) or externally ( vector signal analysis software) on a computer connected to the analyzer. An example of this capability is shown in Figure 1-7. Note that the symbol points of a QPSK ( quadrature phase shift k ying) signal are displayed as clusters, rather than single points,
indicating errors in the modulation of the signal under test.

Figure 1-7. Modulation analysis of a QPSK signal measured with a 

spectrum analyzer

We hope that this application note gives you the insight into your particular spectrum analyzer and enables you to utilize this versatile instrument to its maximum potential.