Plot sin or cosine with Simulink scope

Hello all,

I would like to do some simulate some home-made modulation with Simulink, but first things first; so I was trying to plot the output ( a sin wave) from a signal generator into a Simulink scope, however I am coming across a issue when depicting the output into the scope, since I don´t get my sin signal but I got some crappy signal instead as it can be seen below, and looking at its PSD I think I am loosing samples or something with the simulation step.

I have found some aid in this Matlab blog, however, I cannot get the plot that I would like to, my signal input is a sin with f=2.1e6.

Is there something I can do to get my sin signal in the scope?

Thanks in advance,

Regards!

EDIT: I am using matlab2014a

Answer 1

[Javier]

Finally I solved it with the advices of Sebastian and making use of Mathworks blog.

Thus my solution was:

fo(carrier)= 2e6Hz

Be compliant with Nyquist: fs>=2*fo or Ts<=(To/2) where To=1/(2*fo)=2.5e-7s => Ts = 2.5e-7s

 Model parameters
 >> solver >> Max step size = Ts = 2.5e-7
 >> solver >> Stop time = 0.05
 >> Data Import/Export >> Output options : Refine Output >> Refine Factor = 100

Thus I got this signal

Answer 2

[Sebastian Castro]

Javier,

By default, if no other blocks drive it, Simulink will choose a sample time of 1/50 the total time -- so, in your case, 2000/50 = 40 seconds. This is definitely too large for a 2.1 MHz sine wave.

To fix this,

1. Go to Simulation > Model Configuration Parameters and select the "Solver" pane on the left.
2. Once you're there, you can change the "Max time step" parameter to something acceptable -- i.e., anything below 1/(2*fs), and probably an integer divisor of (1/fs).
3. If you don't think you need the default variable-step solver in Simulink, you can change to a fixed-step solver with a sample time similar to what I described above.

Does this solve your problem?

- Sebastian

[Javier]

Hi Sebastian,

I don't understand enterely what you explain in point 2, what is fs for you?. I can now understand that simulink is taking the 40s as Ts, therefore if I want to avoid the aliasing I have to be compliant with

fs>=2*fo or Ts<(To/2) where To=1/(2*fo)=2.5e-7s with fo=2e6

I have set as Max time step To=2.5e-7 and now the simulation seems to take forever, and I am not able to see any signal

What am I doing again wrong?

Thanks in advance,

regards

[Sebastian Castro]

You interpreted my point correctly. I used "fs" to mean the sampling frequency -- so yes, you choose a sample time such that aliasing is prevented.

Also, I think it's expected for the simulation to take forever. If you're running a 2000-second simulation with a sample time of 2.5e-7, there are a lot of data points to process.

One thing you can do is change from sample-based (one sample at a time) processing to frame-based processing (using buffers). Look at the following link:

http://www.mathworks.com/help/dsp/ug/buffering-and-frame-based-processing.html

- Sebastian

[Javier]

Thanks for the advices Sebastian, thanks to then I have managed it but without using the buffers, just reducing the simulation time to 0.05s, a Ts=2..5e-6s(Max time step) and making use of the link I mention in my original post;that's to say, I have use the redefin option under "Data export" and I have set it at 100, then I got a good view of my signal.