I'm having a hard time trying to understand signal modulation graphics and how can I calculate/find the properties of the signal. The properties I'm talking are:

  • Max. amplitude
  • Carrier frequency
  • the time it takes to code a bit

Can I do it just observing this two graphics? If not, can I do it using matlab? I'm a little confused since I'm used to see amplitude and frequency graphics but with bars and discrete values, not with continuous functions.

This is one of the examples I was talking and what I believe is a FSK modulation: Modulation and frequency spectrum grapchics

up vote 1 down vote accepted

Establishing the system's parameters from time/frequency plots with absolute certainty is impossible, but you can make educated guesses.

In your example, the signal seems to be binary FSK; it seems to have continuous phase. Assuming that is correct, then you can try guessing the parameters you're interested in:

  • Maximum amplitude: from the time plot, you can see that the amplitude is 2. The units are not specified.

  • Carrier frequency. The carrier could be said to be the frequency at the middle of the two frequency peaks (seems to be around 350 Hz).

  • Bit interval. It is the time interval for which the frequency stays constant. If we assume that the bits transmitted are 1,0,1,0,1, then it seems like the bit interval is 0.015 seconds or so. Note that the bits transmitted could be 1,1,0,0,1,1,0,0,1,1 with a bit interval of 0.015/2 seconds. This is a case where complete certainty is impossible without more information about the system.

  • Thanks! Could you care to explain why/how the carrier frequency is the around 350 Hz. What if instead of two peaks I only had one? – Fabio Antunes Nov 23 '14 at 17:40
  • You're welcome! If you only had one peak, then the signal probably wouldn't be FSK. In FSK-type signals, it is typical to have one peak per each frequency used. You could think of binary FSK as a modulation that maps a bit 0 to $\cos(2\pi (f_c-\Delta_f)t)$ and a bit 1 to $\cos(2\pi (f_c+\Delta_f)t)$, where $f_c$ is the carrier frequency (and the cosines are different from 0 only for a bit time). That is why you can assume that the carrier is the frequency between the two peaks. – MBaz Nov 23 '14 at 19:44

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