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 :