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I want to build a signal generator using a ATmega 328P on an Arduino. Which way would be better. Using one of the timers (8bit or 16bit) built in to generate a PWM and then low-pass filter it, or using a R2R ladder system?
I care more about harmonics than frequency, but I'm hoping for about 10kHz or above.
Thanks for your advice.
If you are worried about harmonics, you should definitely avoid PWM. Rectangular waves are jam-packed with harmonics that extend as far as pin slew rates permit. Although low-pass filtering will get rid of the higher order stuff easily, the lower order stuff will be harder to remove without some serious filtering. You don't specify how much harmonic distortion you could live with or what frequency range and waveshapes you are trying to generate (10kHz makes me think that you are trying to make an audio waveform generator), but you're fighting a losing battle that is going to end up with you needing external active components anyway - so why not add a DAC?
I'm also going to guess that you don't want to write any code, so a quick Google shows that Adafruit offer a MCP4725 breakout board with a prewritten library and tutorial. The MCP4725 is a 12-bit DAC. Even though its INL leaves something to be desired, you'll be a good order of magnitude better off than even your best efforts with the onboard PWM.
As Leon Heller says, if you really want to generate high quality signals (and over a much broader frequency range) you could go for a DDS solution. Analog Devices do some nice ones with onboard SRAM for arbitrary waveform storage, so you simply set the chip up, tell it to go and you're done - no need for a constant datastream. This is probably beyond your requirements at this stage, however. There are also a bunch of cheap Chinese boards based on the AD9850 all over eBay which might be of interest.
It's hard to say definitively which one is better, as it depends on how you implement either technique. In general I would say PWM is the way to go, as R2R requires a lot of pins, well matched resistors, is rarely used for much more than 8-bits, and still needs filtering.
As mentioned, I'd go for a dedicated DAC if you want good results, but if it's for learning purposes going the timer/PWM module route with a decent filter plus opamp should give half decent results and you will pick a few things up in the process. There are plenty of tutorials on PWM signal generators out there to gt you started.
