FFT Analyzer Basic FAQ

PSD (P ower S pectral D The density function (power spectral density function) is the frequency resolution Δ of the FFT calculation. f A spectral function that expresses power values per unit frequency width (1 Hz width) in a way that is independent of the signal, is often used to evaluate non-periodic signals that have a continuous spectrum, i.e., irregular signals (random signals). Its unit is the signal x (t If the physical unit of ) is EU, then EU ² It will be /Hz. Specifically, if the vertical axis is voltage (V), then V ² /Hz or its square root V/ √Hz Therefore, it is often used as an evaluation value for noise in amplifiers and other devices. Also, in the field of random vibration testing, the vertical axis is vibration acceleration (its unit is m/s²). ² Therefore, ASD (A Acceleration S pectral D Also known as ensity (acceleration spectral density), its unit is (m/s²). ²) ² /Hz, or m ² /s ³ This is the result. Please refer to the related items below for the calculation method.

ESD (E nergy S pectral D The energy spectral density function (Essence Function) is a function of time signal x (t This is a function that represents the frequency distribution of the energy of a transient signal, and is used for spectral evaluation of transient signals such as impulse signals. The method for obtaining it is spectral-related. FAQ: "What is signal power?" In this section, the averaging operation is omitted, so the PSD value is equal to the FFT time window length T It can be calculated by multiplying by [a certain factor].

Table 1 Comparison of three types of spectra

Types of spectra	Physical meaning	Target signal
Power Spectrum	Power distribution by frequency band	periodic signal
PSD	Power distribution per unit frequency	Continuous random signal
ESD	Energy distribution per unit frequency	transient signal

(Note) PSD and ESD are mainly used for vibration analysis and are not commonly used for noise analysis.

 

	Power spectral density (FFT basic terminology glossary) Energy spectral density (FFT basic terminology glossary) About power spectral density and energy spectral density (FFT analyzer FAQ)