Simple input/output problem

[florian.blaser]

Hi everyone !

I'm currently trying to do some signal processing with a PCI-5640R board, and there are some basic things that I'm not able to understand.

I wrote a simple program that reads I/Q values from the input, write them without processing to the output (interleaved). The modulation and demodulation frequencies are set at the same value.

I'm expecting this program to work as a bandpass filter, the width of the bandpass being calculable from the decimation rate.

Sending white noise in and looking at a spectrum analyzer, the output looks fine. So I think I'm not too far away from understanding what's happenning

However, when I use a single frequency signal (a sine wave) and look at the output with an oscilloscope, I see that the output is following the input, but is amplitude modulated with a really slow frequency (sub-hertz) : I see the signal fading out, then reappearing, then disappearing again... 

Looking at the examples, I found the "Frequency translation example". By setting the "Receive frequency" equal to the "Transmit frequency", this example should basically do the same as my program, and it actually also outputs an amplitude modulated sine wave. The problem is still there.

My config : Labview 2009,Version 1.4 of the driver, all test signals come from an external function generator (ie, I'm not using the "test signal" from the DAC 0 in the example).

Does anybody knows where this modulation could come from ?

Thanks a lot for your answers, any tip will be greatly appreciated.

Best regards from Vienna,

Florian

[Lars.B]

Hi Florian,

just as an idea: is the clock rate of DAC twice that of the ADC?

I ask because of "I wrote a simple program that reads I/Q values from the input, write them without processing to the output (interleaved)"...

If not: there you have it.

Greets,

Lars

[florian.blaser]

Hi Lars !

Many thanks for your answer, and sorry for not replying sooner : I was attending a conference and couldn't make tests lately.

My clocks are set as in the "Frequency translation" example and are actually set at 100 MHz for the DAC and 25 MHz for the ADC. From the comments in the example, I understand that this fits to the choosen decimation and interpolation factors.

Let me reformulate my question such that it cannot involve programming errors on my side :

1) I run the "frequency translation" example with the following parameters : Receive frequency = Transmit frequency = 1MHz, Test signal = 1.1MHz

2) I connect  the DAC0 to the ADC0 (ie, send the test signal to the input)

3) I look at both outputs on an scope

The test signal (DAC0) looks fine, and the transmitted signal (DAC1) follows at the right frequency, but it is also amplitude modulated at a low frequency (below 1 Hz): The signal grows, shrinks to a flat line, grows again (inverted), shrinks, etc...

My question is then : Is this behaviour normal ? If yes, could someone tell me where the amplitude modulation comes from and if there is a way to get rid of it ?

Many thanks in advance for your help !

Cheers,

Florian