FSK using baseband modulation

Hello,

I want to create a FSK modulator using the "*M-FSK Modulator Baseband*" block, the input is Bernoulli Binary Generator (0, 1) , but i cant get the good output ( 2 different frequencies ) on the scope in real time !!??

Block diagram : http://imageshack.us/photo/my-images/854/blocks.jpg/

Good Output signal : http://imageshack.us/photo/my-images/820/outputu.jpg/

Bad Output signal : http://imageshack.us/photo/my-images/825/signalsk.jpg/

P.S : the output shown is bad because the frequency is the same whatever the input is 0 or 1. !!!

Thanks a lot,

Bye

[Rick Rosson]

Can you please post a screen shot showing the mask parameters (dialog box settings) of the 2-FSK modulator block? Thanks!

[al-comm]

Hello Rick,

Thanks for your comment and sorry for late, I’ve not received any notification for your comment,

Anyway, here are the mask parameters:

http://imageshack.us/photo/my-images/225/setupx.jpg/

P.S: Even if I choose Binary or Gray the result is the same, and the sampling by symbol parameter does not give a good signal too …

This part is bugging my project,

Waiting for answer please,

Thanks again,

Answer 1

[Rick Rosson]

The output of the Simulink model is correct. The reason that it is not the same as the "Good Output signal" is that the Simulink signal is the complex baseband equivalent of the desired FSK signal, whereas the "Good Output signal" is the real-valued passband signal. Although these two signals are not the same, they are related.

In fact, you can generate the real-valued passband signal in the Simulink model by adding a handful of blocks that represent the passband modulation. To do so, multiply the output of the baseband modulator by a complex exponential at the desired passband frequency, and then view the real- and imaginary- parts on the scope.

HTH.

[al-comm]

Thanks for your helpful answer Rick,

But please can I have more detail,

Thanks again,

Be good.

[Rick Rosson]

What are your specific questions?

Please let me know.

Thanks!

[al-comm]

Hello,

I see the difference between Baseband and Passeband, I’ve multiplied the baseband output modulator by complex exponential, but I still can’t see the good output...

My schematic:

http://imageshack.us/photo/my-images/18/expt.jpg/

All parameters are kept the same, and I’ve tested the complex exponential with different frequencies from 10 Hz to 10 KHz…

Thanks a lot.

Answer 2

[Rick Rosson]

You are getting closer to the right answer, but still not quite there yet. In your schematic, you are feeding the output of a real-valued sine wave to the input of the complex exponential function. So the output of the complex exponential block is:

   exp(j*sin(2*pi*Fc*t))

where Fc represents the carrier frequency (in hertz). This modulation signal is not quite correct.

The correct output of the complex exponential should be:

   exp(j*2*pi*Fc*t)

That means the input to the complex exponential function should be the simulation clock t feeding a gain block with a value of 2*pi*Fc.

Please give that a try.

Alternatively, I believe that you can generate a complex exponential output from the Sine Wave block. Please check the dialog box.

HTH.

Rick

[al-comm]

Hello,

I got it Rick, thank you, and as you said I generated the complex signal from the sine wave …

Thanks a lot for your help.

Answer 3

[Derek Eccles]

I am using a Math function block with the exp. function selected and the output data type as complex, to generate the carrier.

I then feed this to a complex to re-im block thinking this would get me the cosine and sine carriers which are required for 2_FSK.

The Im signal is a constant flat line, while the re signal is an exponential. Nothing like cosine or sine which I was hoping for ?