DSP Tech Brief : The Zoom-FFT

The Zoom-FFT is a process where an input signal is mixed down to baseband and then decimated, prior to passing it into a standard FFT. The advantage is for example that if you have a sample rate of 10 MHz and require at least 10Hz resolution over a small frequency band (say 1 KHz) then you do not need a 1 Mega point FFT, just decimate by a factor of 4096 and use a 256 point FFT which is obviously quicker.

 Advantages of the Zoom FFT are :

• Increased frequency domain resolution
• Reduced hardware cost and complexity
• Wider spectral range

Applications of the Zoom FFT include :

• Ultrasonic blood flow analysis
• R.F. communications
• Mechanical stress analysis
• Doppler radar

The following diagram shows the zoom process :

While the following diagram shows the basic architecture of the Zoom-FFT :

One common question is : Is the zoom FFT the same as the chirp z-transform.

The answer is : Absolutely not. The FFT calculates the FFT at N equally spaced points around the unit circle in the z-plane, the chirp z-transform modifies the locations of these points along a contour that can lie anywhere on the z-plane. In contrast, the zoom-FFT uses digital down conversion techniques to localise the standard FFT to a narrow band of frequencies that are centered on a higher frequency. The chirp z-transform is often used to analyze signals such as speech, that have certain frequency domain charactgeristics. The zoom-FFT is used to reduce the sample rate required when analysing narrowband signals - E.G. in HF communications.

These functions, and more, are available in the SigLib DSP Library.

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