Hello everyone! I want to make in Matlab a spectrogram of a rectangular pulse wave with 25% duty cycle. This pulse looks like in figure 3, and its FFT in figure 1. I have a problem with the spectrogram in figure 4. I think i can say about the spectrogram that it has energy concentrated in baseband, at low frequencies. I don't know if am i right. Also i don't know how to interpret and what frequencies i want to see in the spectrogram, and why there are some large lines with some spectral density in the most part of the figure and then a thin line(in the red circle) between these large lines and then the same story. There are here because of the frequencies of the signal from fft? Thanks in advance!
Dear. Refer to this link: https://lpsa.swarthmore.edu/Fourier/Series/ExFS.html
Your time window may be too small, when your wave is zero, you see it as a blank. (-140 dB). The thin lines are effectively oscillations from the rectangle pulse's fourier transform (Sinc).
Hello, I am trying to reproduce your result, what is the period of your signal?
Fourier transform of the Rectangle is Sinc function that periodically touches zero. So, the lines in red circles are probably due to Sinc. However, the function may not be exactly Sinc because of the multiplication of the analysis window with the rectangular pulse.
Most of the energy of Sinc ( in the frequency domain) is around lower frequencies that can be observed in the Spectrogram. The given implementation of Spectrogram does not take into account negative frequencies otherwise you must have noticed symmetry around 0 Hz.
You are seeing a pattern of yellow strips (parallel to frequency axis) followed by blank spaces because Spectrogram goes to zero in the region signal becomes zero and becomes close to Sinc in a region signal has some amplitude.
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