Electronics > Beginners
DIY Function Generator
ejeffrey:
--- Quote from: Rerouter on September 28, 2012, 10:07:21 pm ---so what would be the solution for 0-20khz for sine and square if not a conventional dds chip but still digitally controllable?
--- End quote ---
The problem with using a conventional DDS for low frequency is that you can really see the stair-stepping with a typical 10 bit DAC. If you know you are only going to go to 20 kHz you can just change the output filter cutoff, but it is better to use a high resolution DAC or use an FPGA+DAC and apply dither and noise shaping.
jerry507:
I am always very skeptical of advising people to go with an FPGA for a project if learning to use the FPGA isn't the point. Perhaps this is bias from when I got my butt kicked by an FPGA in college, but it seems like just learning the FPGA can derail the original intent.
That said, wanting to build a function generator or similar test equipment is rarely about building the test equipment and more about learning whatever technology you want to use to build it. So the OP should look at his options:
1. FPGA w/ DAC
2. Microcontroller w/ DAC
3. DDS chip with perhaps some switchable output filters to clean up excessive stairstepping
4. A fully analog solution
Pick a path and then go down that one, so you're focused and have a real goal.
Mechatrommer:
or switchable among all 1-4? as in "audiophoolery" anything other than 4 is not tolerated? stairstepping will produce highly unwanted harmonics noise.
perfect_disturbance:
OK so I m interested in the AVR DDS I was looking at some of the schematics and had a couple questions.
first I noticed in the power supply that after each of the regulators there are 2 caps a 100 and a .01 (I'm assuming uF). In most reference designs I'v seen there is only a .1 cap after the regulator what is the 100 in this on for? Filtering lower frequency noise? Adapting to rapid changes in current drawn buy the rest of the circuit?
Next question is about the resistor network would I be better off buying resistor network or building my own?
Thanks
poptones:
You should not get bad stairstepping with "conventional dds" if you use linear interpolation. It's not that hard.
If your goal is to make an audio frequency function gen buy a low cost audio i2c DAC and drive it from a microcontroller. Feed it 48ksamples/sec and the built in 8x oversampling feature will take care of a lot of it.
Another way, if you just want sines, is to use a mux as a 3 bit dac and a switched capacitor filter. I built one of these years ago and it works pretty well. And you can get all the parts sampled from maxim.
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