By convention, the first harmonic IS the fundamental, 220 Hz. The second harmonic is 440 Hz, etc.
ok, I expressed myself wrong: the first harmonic is the fundamental, indicated with f (or f1), then the second harmonic f2 etc .. (always with multiple frequency)
Those coefficients are correct (after scaling) for a square wave but may not be anywhere close for some other waveform. Here is a video discussing the FFT of a sawtooth. Note that it has both even and odd terms for the sin() while the cos() terms drop out.
the video is terribly into math
![Phew :phew:](https://www.eevblog.com/forum/Smileys/default/phew.gif.pagespeed.ce.n2uLOVFYMP.gif)
But I understand, the operation f / 2- f / 3 etc .. is valid for the square wave, it may not be accurate for other waveforms ..
But let's see the thing in general, and therefore I can say: in the FFT graph, the harmonics to the right of the fundamental will have an amplitude that from time to time will become less and less ..
Returning to the square wave of my example, the reference value for the amplitude is db .. but it can be said that if the fundamental harmonic has an amplitude of 2v, the second harmonic will have an amplitude of 1v, the third of 666mv etc ...
Engineers are a lazy bunch and they don't like carrying a lot of digits. Assuming a voltage ratio, the dB value is 20 log (voltage divided by reference). Let's assume a reference of 1V. In this case, if the voltage is 10V, the db is 20 log 10 or 20 dB. If the voltage is 100 the dB is 20 log 100 or 40 dB. 200 dB is a HUGE number: 200 = 20 log (V) so V is 10 to the 10th or 10_000_000_000 or 10 billion. I'm not sure what the scale is all about on that FFT but it probably isn't voltage. A sound level makes more sense. We can get 175 dB from a gunshot of a rifle or pistol.
Here is a table of sound levels. I jet engine at 25 meters is about 150 db
Logarithms are also used because many of our physical responses are non-linear. By log(), I mean log base 10 of some value. The 'common' logarithm, not the natural logarithm which is base e.
ok, so the Y db scale would be the noise level of the signal; obvious that more noise = more signal amplitude. So I forget in the FFT chart to think in volts, and think about the amplitude in db.
Positive noise levels are already very high: for example when I listen to my receiver, I normally set the volume to -40db, if I want to exaggerate I go to -15; but I would never dream of going to positive decibel values ..
Since the spike on the FFT shows the amplitude at a frequency, I'm not sure they can be negative. I certainly haven't see a series that has that result but it would take more math than I'm up for to prove it one way or another.
is that maybe I get confused by the graph in the time scale, where the signal has high and low peaks if alternated; but the FFT graph in the frequency scale must be seen in another way, the peaks represent the Vpp amplitude of the signal, it is right that the peaks are only upwards to have a clear view of the signal.
thanks
![Azn ^-^](https://www.eevblog.com/forum/Smileys/default/azn.gif.pagespeed.ce.7t5xC2ZH_H.gif)