Algorithms - Zoom-FFT

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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 algorithms are included for information and are by no means definitive descriptions. If you would like to add anything to these pages then please feel free to email me and I will add your information, with my thanks.

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If you have any comments or questions please email Numerix : numerix@numerix-dsp.com
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