Recall from §5.4 that the frequencydomain image of a sinusoid ``through a window'' is the window transform scaled by the sinusoid's amplitude and shifted so that the main lobe is centered about the sinusoid's frequency. A spectrum analysis of two sinusoids summed together is therefore, by linearity of the Fourier transform, the sum of two overlapping window transforms, as shown in Fig.5.12 for the rectangular window. A simple sufficient requirement for resolving two sinusoidal peaks spaced Hz apart is to choose a window length long enough so that the main lobes are clearly separated when the sinusoidal frequencies are separated by Hz. For example, we may require that the main lobes of any BlackmanHarris window meet at the first zero crossings in the worst case (narrowest frequency separation); this is shown in Fig.5.12 for the rectangularwindow.
To obtain the separation shown in Fig.5.12, we must have Hz, where is the mainlobe width in Hz, and is the minimum sinusoidal frequency separation in Hz.
For members of the term BlackmanHarris window family, can be expressed as , as indicated by Table 5.1. In normalized radian frequency units, i.e., radians per sample, we have . For comparison, Table 5.2 lists minimum effective values of for each window (denoted ) given by an empirically verified sharper lower bound on the value needed for accurate peakfrequency measurement [1], as discussed further in §5.5.4 below.

We make the mainlobe width smaller by increasing the window length . Specifically, requiring Hz implies
(6.26) 