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This is a bit of a weird question. I have a bunch of IQ data. Its a very big file that i cant upload on here. Im working through a few exercises trying to analyse the data. I have plotted the signal envelope and the instantaneous frequency of the signal over the 25s period. The match the expected graphs that i should be getting. They correspond to Fig.1 and 2. Then i have a Fig.3 and a short description but ill be honest im not sure where to start. This is the description that goes with Fig. 3

Figure 3 shows the frequency domain representation of the doppler shift sequence. The phase noise plus thermal noise output of the frequency discriminator is observable in the output sequence. For a white noise input to the frequency discriminator, the noise output would be a parabolic function of frequency. We are observing the square root of the power spectral density which should be a linear function of frequency. The observed frequency discriminator output noise is approximately linear with frequency. In addition, there is a large spurious sinusoidal spectral component present at 2 MHz, and this indicates that either the transmit or receive local oscillator is being FM modulated with a frequency of 2 MHz. The phase noise increases with increasing frequency until approximately 4 MHz. It should be noted that the magnitudes of both the spurious sinusoidal FM modulated signal and the noise are significantly larger than the magnitudes of the low frequency signals of interest.

My initial approach was to do an fft on the original data to try and get the plot. But i keep getting something that is nowhere near fig.3. If anyone could help me i would really appreciate it.

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  • $\begingroup$ Most frequency discriminators are also sensitive to the magnitude of the envelope - have you hard limited the waveform prior to the frequency discriminator you eliminate this from occurring? Also what did you actually get for figure three? $\endgroup$ – Dan Boschen Jul 9 '20 at 19:05
  • $\begingroup$ Hi there thank you for getting back to me. I did not hard limit the waveform. How would i do that? $\endgroup$ – Sa_as Jul 10 '20 at 2:46
  • $\begingroup$ I added my figure 3 $\endgroup$ – Sa_as Jul 10 '20 at 2:46
  • $\begingroup$ Take the sign of your waveform after bandpass filtering. $\endgroup$ – Dan Boschen Jul 10 '20 at 2:47
  • $\begingroup$ Also plotting your figure 3 with a log vertical scale may show more detail to help explain what is actually happening. $\endgroup$ – Dan Boschen Jul 10 '20 at 2:54

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