A real-time spectrum analyzer for measuring time domain data of an input signal includes an ADC, a resampler and a density histogram memory unit. The ADC has a first sample rate and is configured to acquire and digitize the input signal at the first sample rate to provide first samples. The resampler has a second sample rate and is configured to acquire the first samples at the second sample rate to provide second samples grouped in sample sets, where the second sample rate is selected so that each sample set includes an integer number of second samples per an integer number of periods at a frequency of interest of the input signal. The density histogram memory unit is configured to store at least a portion of the second samples output by the resampler for generating a continuous real-time gap-free density histogram corresponding to the time domain data.