Electronics > Beginners
DIY Function Generator
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Smokey:
I have that same DDS board from ebay.  It's amazing what you can get for 6 bucks USD.  The chip itself in 1s is like 15bucks USD.  I already built a motherboard for it with uC, control pots, and +/-12V regulator from +5V input.  Going to level shift the DDS output so it's centered around 0V and amplitude scale from there.  Still need to write the code though.  Should be a killer little USB controlled function gen.

Those DDS chips are pretty much just a DAC, but the magic happens with the internal filtering and PLL stuff from what I can tell.  The data sheets are super dense with info.  Generating a nice sine wave is way harder than it appears.
Rerouter:

--- Quote from: jerry507 on September 27, 2012, 09:10:14 pm ---Unless the frequency you want is really low

--- End quote ---

so what would be the solution for 0-20khz for sine and square if not a conventional dds chip but still digitally controllable?
Smokey:
They make these DDSs for different applications.  Check out the part matrix at the bottom of the page:
http://www.analog.com/en/rfif-components/direct-digital-synthesis-dds/products/index.html
The really fast ones are usually for clock generation, so they have parts that go up into the GHz.  There are other ones specifically for function gen applications, like the AD9833, that has a frequency output from 0MHz to 12.5Mhz at 0.1Hz steps and outputs sine, square, and triangle wave via software settings.  They really are versatile chips.

Here is a really interesting use of the AD9833 DDS.
http://electronicdesign.com/article/analog-and-mixed-signal/turn-dds-chip-into-low-frequency-arbitrary-signal-generator

(edit to add)
For only 20khz you can just hook a DAC up to a uC and step through a sine table.  20khz square wave is easy.
ejeffrey:
Those analog devices function generator chips are nice, but they aren't very flexible.  They can definitely do your basic sin, triangle, square wave, but if you want a nice set of modulation, sweep, burst, and triggering options you have to implement that by manually adjusting the tuning word. 

If I were designing a lab function generator I would use an FPGA with an external DAC.
grenert:
If you're interested in going the one-chip route, Jameco sells a kit (no enclosure) based on the XR2206:
https://www.jameco.com/webapp/wcs/stores/servlet/Product_10001_10001_20685_-1

They also have the bare chip for $6.49:
https://www.jameco.com/webapp/wcs/stores/servlet/Product_10001_10001_34972_-1

Velleman makes a complete kit that is based on a PIC:
https://www.jameco.com/webapp/wcs/stores/servlet/Product_10001_10001_2137262_-1
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