frequencypwm
I have a PWM output from a micro controller. I want to turn this into a sine wave.
The typical way to do this is a low pass filter. Unfortunately, I want to change the PWM frequency, and the corresponding sine wave frequency, over more than a couple decades (say, 10Hz to 100kHz). Are there any simple ways to do this?
The 31.25kHz is 625x your 50Hz. So all you have to do is build a 625 entries sine lookup table, and at each interrupt set your output compare value to the next table entry. If you have a 32\$\mu\$s interrupt you'll have ran over the full table (=full sine cycle) every 625 x 32\$\mu\$s = 20ms, so that will be your 50Hz.
Any waveform other than a sine at 12.5 MHz will be composed of the fundamental frequency 12.5 MHz, plus harmonics above that. For ideal waveforms with sharp edges, the harmonics extend to infinity.
It sounds like you have added a filter with a cutoff frequency around 12.5 MHz, so this is why your non-sinusoidal outputs don't look like they should -- your filter is stripping off the harmonics which give those waves their non-sinusoidal identity.
To illustrate, here are a few animations from Wikipedia which show various non-sinusoidal waveforms as harmonics are added/removed:
What your filter should be doing is stripping off the harmonics generated by the impulses at each sample output by your DAC. This is called an anti-aliasing filter, and ideally it's a brick-wall low-pass filter with a cutoff equal to half your sampling frequency. Given that true brick-wall filters aren't realizable, the cutoff is usually a bit below half the sampling frequency, and the roll-off is made as steep as feasible. If you can sample significantly more than twice the highest frequency of interest, then the filter design isn't as critical.
It also follows that if you want to synthesize a 12.5 MHz square (or triangle, or sawtooth...) wave, you need a DAC sampling fast enough to also synthesize the higher harmonics in that wave. How much higher depends on how closely you want to approach an ideal square/triangle/sawtooth wave.
Best Answer
Depends on what hardware you have available and how deep into software you want to go.
If you have the processing power and GPIO pins available, you can get away with a very cheap and simple R-2R ladder as your DAC (ditch the PWM). You can then use a sinewave LUT in your MCU's memory + a software defined Low Pass Filter. You can store multiple LPFs in memory/change the characteristics of it on the fly to get your desired frequency response. The output of the LPF can then be fed right to the R-2R ladder to generate your sinewave. This approach is a tradeoff between software complexity/processor power and flexibility/external component size and cost.
If you only have a few frequencies you want or creating a perfect sine-wave is not that important you could toggle between a few different external LPFs.