This tool calculates the Resolution Bandwidth of a Fast Fourier Transform. It also includes the effect of decimation or further downsampling of the digitized signal.

Enter:

**N**= Length of the FFT_{FFT}**Fs**= Sampling rate (Samples-per-second or kSPS/MSPS/GSPS)**D**= Decimation (=1 when the signal is not down-sampled)

**FFT Resolution Formula**

**R = N _{FFT}/(D*Fs)**

**Example Calculation**

For a sampling rate of 1 GSPS and FFT length 4096, the resolution bandwidth is 244 kHz. If the decimation factor is 16, the RBW drops to 15.2 kHz.

**Background**

The resolution bandwidth is the spacing between consecutive bins in the FFT output spectrum. The smaller the RBW, the easier it is to discern two closely spaced signals.

For a high sampling rate, a larger number of samples (FFT size) are required to get the same RBW as for a lower sampling rate.