hi, it is a little bit tricky when one is going to use this periodogram function. For example, if your signal has a 1000Hz sampling frequency, you will have to present the time-frequency map produced by periodogram function within [0, 500]Hz according to Nyquist theorem. However, periodogram function gives you a time-frequency representation within [0, 1000]Hz. So, you will have to cut it so as to get what is useful. For details, please check the matlab help file. I could not remember exactly at this time. But I did use this function for a quite long time. Good luck!