In normal FFT processing, digital data is stored in the buffer memory concurrently with FFT arithmetic processing. Real time analysis refers to the spectrum average processing where FFT processing time is shorter than FFT time window length T, which means that FFT arithmetic processing is performed without missing any instantaneous time series data. The highest possible frequency range that allows real time analysis is referred to as a real time frequency range.

In the example of the DS-2104A analyzer, when 800 lines are analyzed under the 40 kHz range, the time window length is 20 ms. Since the DS-2104A analyzer performs spectrum average processing of 4ch in about 20 ms, the real time frequency range will be 40 kHz (4ch, 800 lines).