I am trying to estimate the average amplitude of some signal with frequency 6 Hz, sampled at ~300 Hz. See figures for a part of the signal and its dft calculated using matlab.
I estimate the average amplitude of the signal by calculating its root mean square,
>> rms(signal) ans = 5.1651
However, when looking at the signal (see first figure), it is very clear the mean amplitude is slightly greater than ~5, and is probably closer to ~8. Summing the peak amplitude in the frequency domain and its first few aliases results in the same value (~5).
What am I doing wrong here? How to properly estimate the signal average amplitude from its dft?