About the AWGs

[mqwhale]

I have read a tutorial "Find the Right Signal Generator",There is a pictrue about the block of AWGs.Before the DAC ,there is a digital filter,I 'd like to know what the digital filter is.and is it achieved through software programmed on PC or sth visible on the board of AWGs; After the DAC,there is a analog filter , and is it sth visible on board of AWGs?

Thanks!

[MicrowaveDave]

Hey,

Great questions!  As you saw in the diagram, there is a digital interpolating filter before the DAC, and a low-pass analog filter after it.  Each of these serve a different function.  For a more thorough overview of what each filter does, check out: Interpolation and Filtering to Improve Spectral Purity as well. 

On a high level, the digital filter is a FIR onboard the DAC chip itself which performs 2x, 4x, or 8x interpolation.  This is helpful for sinusoidal or baseband signals, becuase it can be used to increase the effective sample rate to 400 MS/s.  As you may know, when generating any signal with a sample-and-hold DAC, you will observe images of the signal around the sample rate of the device.  Using interpolation, you can increas the effective sample rate and move the images to a higher frequency.  For NI AWG's, this is selected in software with a property node (interpolation factor) and is implemented in hardware on the DAC.

The analog filter is a 154 MHz 7th order elliptic filter which is used to attenuate high frequency images.  Again, when a digitial filter used first, all images can be moved to higher frequencies...and then be attenuated by the analog filter.  This filter is also selected in software with a property node (enable analog filter), but is implemented in hardware after the DAC.

I hope this answers your questions!  For a more thorough explaination, check out: http://zone.ni.com/devzone/cda/tut/p/id/5515.  I hope this helps!

Thanks,

David Hall | Product Engineer

[mqwhale]

Hey,

Great answers!Here another questions about the update rate of the DAC.

See PXI-5431,its max update rate is 40MS/s,it can achieve upto 80MS/s after using FIR,I'd like to know if the 5431 always works at a stable update rate(80MS/s) when generating

sinusoid waveform.If so,that will mean when generating waveform ,the less frequency ,the more points in a period,and the better quality of the waveform.

If not,that will mean 5431 generates sinusoid waveform with a fixed numbers of a period ,and the update rate will change accordingly.

Thanks a lot!