The FFT is computed based on the ACQ data. In order to perform the FFT over 1 k points, the ACQ data are decimated (reduced) from 10 kW to 1 kW. At this point the sampling rate is equivalent to 100 kSs (1/10 of 1 MS/s). Therefore, the sampling rate display is turned OFF. For example, to perform an FFT over the data sampled at 1 MS/s when the memory length is 10 kW, the data are decimated from 10 kW down to 1 kW. This means that the sampling rate is equivalent to 100 kS/s at this point. Therefore, to avoid misinterpretation, the sampling rate display is turned OFF. In this case, the actual sampling rate is 1 MS/s, which means that the frequency bandwidth of the time axis waveform is 500 kHz. However, because the sampling rate is reduced to 1/10 as explained above, the frequency bandwidth of the FFT is 50 kHz.