What is waterfall frequency spacing?

Let us try to understand what waterfall frequency spacing is. Waterfall frequency spacing is the gap between spectral lines in an FFT plot.

For example, if you had an analysis frequency of 0Hz to 100Hz and 100 spectral lines, then Frequency Spacing is 1Hz.

So why is there a ‘Requested Frequency Spacing’ and an ‘Actual Frequency Spacing’?

First, all block sizes are always a power of 2; 1,2,4,8,16,32,64,128,256,512,1024,2048,4096,8192,16384, 32768 and so on.

In DATS the user enters the desired waterfall frequency spacing (called Requested Frequency Spacing).

For example, if the user entered a value of 2Hz as the requested frequency spacing, and assuming the signal has a 20kHz sample rate, then the formula to find frequency spacing is,

Sample Rate / Resolution = FFT Block size

20,000Hz / 2Hz = 10,000

Therefore we would use an FFT block size of 10,000. But this is not possible as FFT’s use a block size that is a powers of 2. Therefore, DATS automatically selects the next highest block size. In this case 16,384Hz. So the Frequency Spacing is not actually 2Hz, but better resolution,

Rearranging the formula above gives us

Sample Rate / FFT Block size = Resolution

20,000Hz / 16,384 = 1.220703125Hz

Therefore the Actual Resolution is 1.22Hz

Please also note,

The sample rate has to be at least as big as the block size or higher.

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James Wren

Former Sales & Marketing Manager at Prosig

James Wren was Sales & Marketing Manager for Prosig Ltd until 2019. James graduated from Portsmouth University in 2001, with a Masters degree in Electronic Engineering. He is a Chartered Engineer and a registered Eur Ing. He has been involved with motorsport from a very early age with a special interest in data acquisition. James is a founder member of the Dalmeny Racing team.