# What transform can I use to “zoom” in a specific portion of a signal's spectrum?

If I recall correctly, there is a variation of the DFT that can be used to analyze a specific band of the spectrum of a signal. How is it called?

• It is called the Chirp-Z-Transform. – Spacey Mar 4 '13 at 20:06
• @Mohammad: "One common question is : Is the zoom FFT the same as the chirp z-transform. The answer is : Absolutely not." numerix-dsp.com/zoomfft.html Would be a good answer to explain the relationship. – endolith Mar 4 '13 at 21:25
• @endolith Not the same sure, but they are different ways of arriving at the same end result. The 'Zoom-FFT' involves downsampling / BPF / FFT, whereas the Chirp-Z Transform evaluates the Z-transform on the band you want. I have heard people refer to 'Chirp-Z' as 'Zoom-FFT' as in its particular application. – Spacey Mar 4 '13 at 22:07

• Decimate the signal by a factor $D$, such that the resulting sample rate still meets the Nyquist criterion for the filter's passband width. Depending upon where the band's center frequency was, this process might also involve frequency-translating the signal to baseband.
• Perform a DFT on the signal. In order to get the same frequency resolution at the output, the "zoomed" transform only requires a transform length that is $\frac{1}{D}$-th of what you would need to use if you used the original, unfiltered signal for your analysis.