Asked by mona
on 17 Nov 2012

I am currently doing amplitude modulation using three very high frequency sine waves which I getting perfectly after adding all the three due to already set parameters in the simulation parameter box.

The carrier frequency is also very high, up to 3000 Hz, but the scope is displaying is correctly. But after the message signal and carrier are passing through the product, I am not getting amplitude modulated signal. Instead, I am getting a completely zero straight line. My sample time in the Product is 0.0001.

I am pretty sure it has to do something with the product parameters or simulation parameters but I so not sure exactly what it is?

Answer by Azzi Abdelmalek
on 18 Nov 2012

Edited by Azzi Abdelmalek
on 18 Nov 2012

Accepted answer

If the carrier frequency is 20 Khz, then your fixed step size must be less then 0.00005. I will set **fixed step size=0.000005**

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## 4 Comments

## Azzi Abdelmalek (view profile)

Direct link to this comment:http://www.mathworks.com/matlabcentral/answers/53983#comment_111643

Provide more details about your simulink model

## mona (view profile)

Direct link to this comment:http://www.mathworks.com/matlabcentral/answers/53983#comment_111694

Message signal is composed of three sine waves, 500 Hz, 1.5kHz and 2.5kHz with different amplitudes (1, 0.5, 0.2) as well. These were distorted but now the step size is reduced in the configuration parameters. Hence I'm getting perfect sine waves. Regarding the carrier, its 20kHz with Amplitude 1. The sine wave which I'm getting in the time scope connected to the carrier is also fine but a bit distorted. But as soon as I am multiplying message signal (sum all sine waves after passing through adder) with carrier, I'm getting a straight line along zero on the x-axis. My product's sample time is 0.0001. Now from this, where could you detect the problem?

## Arsalan (view profile)

Direct link to this comment:http://www.mathworks.com/matlabcentral/answers/53983#comment_111726

Make sure you have the correct sampling rate again, both at transmitters and receiver. so you sampling rate will be 2.5*(max frequency component) i.e. 3Khz. make sure your are adding the three message signals and then multiplying the sum with the carrier wave. if your scope is working based on a FFT alogorithem then check the FFT paramaters

## mona (view profile)

Direct link to this comment:http://www.mathworks.com/matlabcentral/answers/53983#comment_111736

What do you mean when you say sampling rate? I am using signal generator basically along with simple time scope and not the one based on FFT.