Spectrum output from a 2-FSK baseband modulator

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Derek Eccles 2020-3-26

Hi, I have built a very simple simulink model which includes a 2-FSK baseband and spectrum analyzer.

The frequency seperation of the modulator is set at 400 KHz, therefore peaks are expected at + and - 200KHz.

I have read the results from the spectrum analyzer into an excel spreadsheet and compared them with the analogue fourier transform

of such a signal - see attached.

The simulink graph is blue and the analogue FT is red. As you can see although they are very close they are not matching exactly ?

The vertical scale is volts and the horizontal scale is Hz. I have made a number of observations :

1. The main peaks of the simulink graph at +/- 200 KHz is not exactly linear ?

2. The simulink side lobes at +/- 350, 450 and 550 KHz are not linear ?

3. The simulink side lobes at +/- 50KHz are substantially bigger than the analogue FT ?

Wireless Communications Communications Toolbox PHY Components Modulation FSK

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