Choosing Window Length to Resolve Sinusoids

Recall from §1.4 that the frequency-domain 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.1.18 for the rectangular window. A simple sufficient requirement for resolving two sinusoidal peaks spaced $\Delta f$ 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 $\Delta f$ Hz. For example, we may require that the main lobes of any Blackman-Harris window meet at the first zero crossings in the worst case (narrowest frequency separation); this is shown in Fig.1.18 for the rectangular-window.

Figure 1.18: Two length-$M$-rectangular-window transforms displaced by $2\pi \Delta f T=2\Omega _M=4\pi /M$ rad/sample.

To obtain the separation shown in Fig.1.18, we must have $B_w \leq \Delta f$ Hz, where $B_w$ is the main lobe width in Hz, and $\Delta f$ is the minimum sinusoidal frequency separation in Hz.

For members of the $L$-term Blackman-Harris window family, $B_w$ can be expressed as $B_w = 2L f_s/M$, as indicated by Table 1.1. In normalized radian frequency units, i.e., radians per sample, we have $2\pi B_w T = 2L\Omega_M\isdeftext K\Omega_M$. For comparison, Table 1.2 lists minimum effective values of $K$ for each window (denoted $K^\ast$) given by an empirically verified sharper lower bound on the value needed for accurate peak-frequency measurement [1], as discussed further in the next section.

Table 1.2: Main-lobe width-in-bins $K$ for various windows.

Window Type	$K$	$K^\ast$
Rectangular	$2$	$1.44$
Hamming	$4$	$2.22$
Hann	$4$	$2.36$
Generalized Hamming	$4$	--
Blackman	$6$	$2.02$
$L$-term Blackman-Harris	$2L$

Requiring $B_w \leq \Delta f$ Hz implies

$$B_w = K \frac{f_s}{M} \leq \Delta f \quad \Rightarrow \quad M \ge K \frac{f_s}{\Delta f}$$

or

$$\zbox {M \ge K \frac{f_s}{\left\vert f_2-f_1\right\vert}.} \protect$$ (2.8)

Thus, to resolve the frequencies $f_1$ and $f_2$, the window length $M$ must span at least $K$ periods of the difference frequency $f_2-f_1$, measured in samples, where $K$ is the width of the main lobe, measured in side-lobe widths. Let $D\isdeftext \lceil f_s/\vert f_2-f_1\vert\rceil$ denote the difference-frequency period in samples, rounded up to the nearest integer. Then an ``$L$-term'' Blackman-Harris window of length $M\ge KD=2LD$ samples may be said to resolve the sinusoidal frequencies $f_1$ and $f_2$. Using Table 1.2, the minimum resolving window length can be determined using the sharper bound as $M\ge \lceil K^\ast \cdot D\rceil$.