Fourier analyzers

Signals and systems can be characterized equivalently in the time and frequency domains. However, a given parameter is generally more easily revealed in one domain than in the other.

This duality is also found in measuring equipment: the spectrum analyzer is to the frequency domain what the oscilloscope is to the time domain.

Spectrum analyzers can be divided into two categories, depending on the range of frequencies analyzed.

Medium and high frequencies (a few hundred kilohertz to a few tens of gigahertz) are the preferred domain for frequency sweep analyzers. Signal processing here is analog; the most recent devices often feature digital processing – after detection in the resolution filter –, which provides additional ease of use, but does not fundamentally change the device's performance.

In the low-frequency range (DC to 100 or 200 kHz), switched-filter battery analyzers – are reserved for specific applications, such as acoustics, due to their limited performance – and frequency sweep analyzers – expensive, slow – are tending to disappear in favor of digital signal analyzers, which are perfectly suited to the low-frequency range by their very principle.

These devices use – digital processing, performed by a specialized signal processor – of previously sampled and converted signals. On the one hand, this technique improves the performance of spectral signal analysis, and on the other, it offers the user entirely new possibilities for characterizing signals and systems, in both the time and frequency domains.

The quantities generally supplied by digital signal analyzers are :

• spectral amplitude, phase ;

• power, power spectral density ;

• energy spectral density (transients) ;

• autospectra, interspectra, transfer functions, coherence function ;

• temporal representation ;

• auto and intercorrelation functions ;

• impulse responses ;

• in some cases: modal analysis.

Only the specific aspects of this type of signal processing will be considered here. Input stages, for example, and the performance associated with them, are the same as for analog analyzers.