I'm trying to desing simple DDS waveform generator using DDS IC, but I'm a bit lost about designing output filter for it. My requirements are 1 Hz - 25 MHz sine, square and triangle/ramp wave. Using Analog Device's great online evaluation tool I've came up with a fact that I need to use ~50 MHz lowpass filter, but I'm struggling about how will it behave with square and triangular waves of maximum required frequency. Won't it just try to drop them down to sine waves. If it's true, then do I have to create 3 different filters for different waveforms and switch them when switching waveforms? How this problem is solved in production-quality devices?
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\$\begingroup\$ Square and triangle can use the same filter. If you want to use it for sine too, think about what spectral energy it can exclude, and does that meet your target spec for sinewave? \$\endgroup\$– user16324Feb 4, 2021 at 21:58
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If you are trying to push the Dds chip to its max frequency, then you do need a sharp cutoff LP filter. This can get you nice sine waves. To get square waves, use an analog comparator fed with the sine waves. To get triangles, perhaps use a switched integrator-type circuit, switched with the square waves. (The triangles are more of a challenge.)
Note: some Dds chips come with a comparator.
A commercial instrument would likely not use a Dds chip, as these are set up for sine waves, but would digitally generate the 3 kinds of waves using 3 different look up tables in the Dds ROM.
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\$\begingroup\$ I'm not going to push IC to its max theoretical frequency, I think that I'll use no more that ~30-35% of available frequency for better result. Solution with comparators seems much better than anything that I've came up with, so I'll probably go for it. I don't really understand solution with triangles, but AFAIK my DDS chip could bypass ROM lookup and output almost ideal triangle wave, so I'll just bypass the filter for it. \$\endgroup\$– PoseydonFeb 4, 2021 at 22:16
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\$\begingroup\$ You still need a filter at half the Dds clock frequency, to get rid of the steps in the triangle out of the Dds DAC. \$\endgroup\$ Feb 4, 2021 at 22:23
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\$\begingroup\$ That last thing you mentioned sounds like a DDS. You're saying a commercial instrument would likely not use a DDS chip, it would use DDS instead...? \$\endgroup\$ Feb 5, 2021 at 3:51
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\$\begingroup\$ User 253751 : I mean a commercial instrument that was made to generate multiple wave shapes would likely do their own direct digital synthesis, rather than using a chip from Analog Devices. \$\endgroup\$ Feb 5, 2021 at 4:02