Please cite SAGA whenever you use it for your publications:

The frequency domain filter works on the discrete Fourier transformation.

Name	Type	Identifier	Constraints
Input
Grid	grid, input	GRID
Output
Filtered Grid	grid, output	FILTERED
Options
Filter	choice	FILTER	Available Choices: [0] range [1] power of distance [2] Hann window [3] Gaussian Default: 2
Inverse	boolean	INVERSE	Default: 0
Range	value range	RANGE	Default: 0; 0.5
Power	floating point number	POWER	Default: 0.5
Band Width	floating point number	SCALE	Minimum: 0.000000 Default: 2