How to demodulate AM-DSB signals ?

[Danan]

Hi,

I am new to study Labview and ni-USRP,and I have one ni-USRP 2930..

I have two programs,one for transmitting and another for receiving through TX1 and RX1.

In transmintting vi,I want to set two different frequencies sine and plus them,then AM-DSB modulates the plused signal, and transmit it.

In receive vi,I want to demodulate the received signal. and detecte thethe frequency,amplitude and phase of the  two frequencies.

the results are as follows:

Now,I have some questions :

1. In the transmintting vi, the  AM-DSB modulated signal is same with it be AM-DSB modulated before,why?

2. In the receive vi,the  signal that is demodultated is not recovered as it is before modulated, did i missed something? and why?

3. In the receive vi, there are some other frequencies except the two frequencies I set in  In transmintting vi , how are the unwished frequencies appeared? 

Files are attached . 

Any suggestion will be appreciated,thanks.

[Anthony_F]

Hi Danan,

There is an input to MT Modulate AM called "peak message amplitude" that you should connect. Use an Array Max & Min function and wire the Max output to the peak message amplitude so that your output signal will be normalized correctly (i.e. bounded to +/- 1)

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@Danan wrote:

2. In the receive vi,the  signal that is demodultated is not recovered as it is before modulated, did i missed something? and why?

3. In the receive vi, there are some other frequencies except the two frequencies I set in  In transmintting vi , how are the unwished frequencies appeared? 

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Since the output from MT Modulate AM goes beyond +/- 1, your are clipping the DAC on the USRP and thus have some harmonics in your generated signal.

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@Danan wrote:

1. In the transmintting vi, the  AM-DSB modulated signal is same with it be AM-DSB modulated before,why?

---

Check out the Detailed Help for the MT Modulate AM function. At the end of the help page you will see the forumulas used for DSB and DSB-SC. With your current configuration output = input because you have't correctly specified the peak message amplitude (default = 1).

[Danan]

Hi, Anthony F.

Thanks for your reply.

I modified my transmitting vi code as you said. But I still cannot resolve the problems about the receive vi, and  the results of detected frequencies are as follows:

instead of  60Hz and 90Hz that I wished.

You said that I clipped the DAC on the USRP and thus have some harmonics in my generated signal. But I think it doesn't need the DAC because the received signals are anolog signals. Maybe I misunderstand the niUSRP.

Could you tell why there are not 60Hz and 90Hz in the  the detected frequencies?

Or ,could you give me some examples about demodulate AM-DSB signals?

Thanks again.

[Danan]

Hi, Anthony F.

Thanks for your reply.

I modified my transmitting vi code as you said. But I still cannot resolve the problems about the receive vi, and  the results of detected frequencies are as follows:

As the above picture displayed, the detected frequencies are 30 Hz ,150 Hz ,300 Hz ,450 Hz ,600 Hz ,instead of  60Hz and 90Hz that I wished.

You said that I clipped the DAC on the USRP and thus have some harmonics in my generated signal. But I think it doesn't need the DAC because the received signals are anolog signals. Maybe I misunderstand the niUSRP.

Could you tell why there are not 60Hz and 90Hz in the  the detected frequencies?

Or ,could you give me some examples about demodulate AM-DSB signals?

Thanks again.

[aneengineer]

Danan i have a couple of questions to you,

1. How did you added 'Array max&min ' to your code? İf possible, could you share the last shape of your vi?

2. As much as i know USRP-2930 has a frequency range from 50MHz to 2.2 Ghz however in your code frequency of the Tx signal is 60 hz how is it possible? ( can it be related to carrier frequency in the block of 'niUSRP Configurate Signal' )

thanks

[Danan]

Hi,aneengineer. to your question 1:you can see the Extract Multiple Tone Information.vi to your question 2: USRP-2930 has a frequency range from 50MHz to 2.2 Ghz that refers to the range of carrier frequency

[aneengineer]

Thank you,  Have you finished? Does it worked? if it is possible could you share last shape of your code?

[Kyosake]

Did you get it to work?