The presentation of the square of the amplitudes of the harmonics of a time series as a function of the frequency of the harmonics.

a representation of a signal in the frequency domain

A power spectrum plot shows the relative levels of power at different frequencies. The example power spectrum plot discussed in the section on music and the ear shows that the power around 4 kHz is about 20 dB below (1% of) the power at the lowest frequencies, and power drops another 20 dB for frequencies above 12 kHz.

intensity [= squared amplitude] of the Fourier transform, presented either in the form of an image (= the outcome of the PW operation) or as a profile (= the outcome of averaging the 2D power spectrum azimuthally). [Note that the SPIDER operation PW furnishes the amplitude, not the squared amplitude! This makes it easier to use the distribution in displays and other operations, because of the high dynamic range]. The complete length of the abscissa in such a plot corresponds to the highest resolution represented in the digitization: 1/(2 x sampling distance). For example, if the scanning is done with 20 microns, and the electron optical magnification was 50,000 x, then the highest resolution represented in the digitized image is 5 x 10**4/(2 x 20 x 10**4) A**(-1) = 1/8 A**(-1). This means that the total length of the abscissa corresponds to 1/8 A**(-1) in this case.

The graph of the energy in the component frequencies of a signal.

A plot which shows how the amount of power in a signal is distributed across different frequencies. The power spectrum is calculated using the Fourier transform. For a good discussion of the use of spectral analysis in trapping, see Gittes and Schmidt (1998). For a discussion of the power spectrum in optical trap data, see this page.

The amplitude information of each frequency. Unlike real display, the power spectrum is the average of the second power of a value for smoothing irregularities. The value is obtained by 20logâˆšï1/4ˆR2 + I2).