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 :
