Same frequency but different output

[Jieyi]

Hi,

I'm confuse by the output that I recieve from the Scope.  The frequency that I am sending into the upconverter is 25MHz.

Arb_waveform - I set the '# of points' to 4 and 'Sampling Rate' to 100MHz.  This will result in a normalized frequency of 25MHz.

Function Gen - I set the frequency to be 25MHz.

Why is it that the ouput received from the signal I generated from the Arb_waveform contain noise as compared to the ouput I receieved from the signal I generated using the Function Gen? 

 As for the upconverter, when I try changing the 'IF Carrier Frequency' and  'Bandwidth', the output don't seems to change.

Thanks in advance.

[Drew Creel]

You do not have the digital filter enabled in the Arb mode generation.  The digital filter will interpolate up to 4x at 100MS/s, giving you 16 points and a cleaner looking, less noisy sine wave - the digital filter is automatically enabled in Function Generator mode.  If you re-run the Arb Waveform example and enable the Digital Filter, I think you'll see that the output will more closely match the output in Function Generator mode.  (Enable the Analog Filter too.)

Drew Creel

NI Software Engineer

Signal Generators Group

[Jieyi]

Hi,

I have enable both the filters already.  But I'm still seeing the same output.

Any idea why?

[Drew Creel]

Can you post a screenshot with both filters turned on?  Also, make sure you hit the filter on/off buttons before running the VI.

Drew Creel

NI Software Engineer

Signal Generators Group

[Jieyi]

Hi,

Attached is the two screen shots.

I can only see a small difference.  But this might be due to the time when I do the screen shot.  The output is changing with time.

[JerryAL]

Hi

Can you change the AWG's gain down to 0.5 from 1.0. 

Jerry

[Drew Creel]

I tried to reproduce this on a PXI-5441 and a spectrum analyzer but I saw the exact same results in Function Generator and Arb Mode. 

Are you getting underflow errors or are you padding your data somehow?  Because you should not be able to generate a single 4-sample waveform in arb mode continuously at 100MS/s without underflowing (which would affect the ouptut signal considerably).  Check to make sure that there is only one waveform in the "Waveform Array" in the Arb Waveform example and that you're not missing any errors.  I added some labview code on my block diagram to download 4 periods of the 4 sample waveform - 16 points total - so it would not underflow.

[Jieyi]

Hi,

Yes.  I'm getting some data underflow error everytime I press the 'Refresh' button.  The error is as below

"Possible reason(s):
Driver Status:  (Hex 0xBFFA4AC3) DAQmx Error -200316 occurred:
Device data underflow. The device was not able to move data fast enough to keep up with the sample rate for the active script.

Run the operation at a lower sample rate, or look for the following in the active script: markers might be too close together, waveforms might be too small, waits might be too short, or subsets might be too small. If you are using an external clock, the provided clock might have gone away during your generation.

Device:  PXI1Slot6

Status Code: -200316"

< I added some labview code on my block diagram to download 4 periods of the 4 sample waveform - 16 points total - so it would not underflow.>

What do you mean by this?  Where did u add it?

[Drew Creel]

There are two ways to do it:  you can add three more 4-sample sine waves to the "Waveform Array" on the front panel (which will create a 16-sample, 4 period sine wave on the device) OR you can modify the block diagram.  See attached image for an example of how I modified the block diagram.

Drew Creel

NI Software Engineer

Signal Generators Group

[Jieyi]

Thanks it works.