RFSA - Uncommon Behavior - NI-5660

[Aplnx]

       Hello Friends, 

       I'm using PXI-modules to transmit and receive signal. So, I found a strange behavior in reception side. I using PXI-5600 and PXI-5441 to generate a sinusoidal signal being modulated by carrier at the transmitter. I did this using a sample "Arbitrary Waveform Generator.vi". The configuration is showed in next figure.

   

Well, looking at the oscilloscopy I can see the waveform envelope. Like showed by Frequency Offset the modulated wave have exactly 10kHz.

Later, I used a simple .vi to get IQ waveform and show in a graph window. As expected I receive a sinusoidal 10kHz waveform. The next figure show a piece of labview code.

 Like me, you now expecting a constant sinusoidal 10kHz waveform. So, when I run .vi the amplitude of signal shoed in the graph window is oscillating... What is happening? Someone know what is the problem?

[Schneiater]

Hi Aplnx,

It can be a problem in your code. But it can be a problem in TX or RX side.

Even when you can see the envelope in the osciloscope, if you didn't see the spectrum, we can't guarantee the modulation.

Could you detail what is done in both TX and RX codes? Or post the section that do that?

Thanks,

Regards,

[Aplnx]

I have partialy solved the trouble. I link a cable at 10MHz clock Ref in the transmitter to ref IN of the receiver. In the Labview code I put blocks to get Ref IN clock. After this correction the system starts to work better. This make me think about LO frequency difference between transmitter and receiver.

After this I decided start coding of QAM mod-demod. According to lately discussed issue the system present a phase error. See the description below:

-TRANSMITTER:

The code is produced to generate a QAM signal. In this situation I user only the "I" component. The component "Q" is zero. Moreover, the component "I" carry a sinusoidal signal.

-RECEIVER:

As expected the component "I" would be a sinusoidal signal and "Q" zero. But this doesn't happen. On every start run button press we see both "Q" and "I" component with sinusoidal signal.

Can you give me a tip?

[Schneiater]

Hi Aplnx, Could you send us some pictures , printscreen or the code? You said that in the Receiver you get sinusoidal signals for both I and Q components. Are the values similar? Isn't the Q signal small amplitude (check the scale at the graph or chart)? Also, if there is no problem in the code, we should check the connections. You can be at a scenario that inputs distortion in the signal. Regards,

[Aplnx]

ok... I will take photos of device conections and take some screenshots of code.