Analog Input and output

[fdfdfdf]

Hi Everyone:

I got a question for the example of Analog Input and Output that comes with 5640r.

I try to use the single Tone mode to generate sine waveform, it works really fine when I use scope to catch it.

However, the problem is when I change the sine waveform to square or triangle waveform, I can not get those waveform correctly.

For some reason, it still gives me sine waveform.

I am new to Labview, so sorry to ask this fundamental question.

Thanks for your help.

Austin

[Chris_W.]

The reason that you are not seeing a square wave output is because of the way that the DAC is configured during this "quadrature modulation mode," which is the default mode for the DAC. If you take a look at the data sheet for the AD9857 DAC that is used on the IF-RIO, you'll notice that built into the DAC is a quadrature modulator which takes the two I and Q bitstreams and mixes them with cos(x) and sin(x), which is your LO signal and your LO signal offset by 90º, respectively. During this mixing phase is where you get back the sine waveform that you're seeing when you provide the DAC a square or triangle wave. Basically you're multiplying a 1 or 0 with a cos(wt) (or cos(wt+90) for the Q path), which still produces a cosine waveform. The values that you're passing in as your sine/square/triangle waveforms are really just, on a point by point basis, controlling the amplitude of the carrier wave (LO) in the I and Q paths. The end result of this is still Icos(wt)-Qsin(wt) = Acos(wt+ø), when the two paths of I and Q are added (subtracted) and sent to the 14 bit DAC. 

So, after all that, essentially the signals that are being passed into the DAC are being used to modulate the carrier signal (LO) on the Digital Upconverter of the AD9857 DAC. I hope this helps. 

Chris W