Anti-aliasing Filter

Anima

Hey,

I'm building a digital signal generator with a microcontroller and I need an anti-aliasing filter for the DAC. All frequencies are in the audio range as its kind of a synthesiser. If anyone has a schematic handy or can point me in the right direction, that would be great.

Thanks.

Michael Collins

Am I correct in assuming you arn't actually sampling anything and the digital signal is just be generated by the microcontroller?

If this is the case you don't need an anti-aliasing filter. You just need a reconstruction (i.e a lowpass) filter for the DAC. What's your output sampling rate going to be? 44 kHz? The higher it is the easier it will be to reconstruct the audio signal (I'm assuming here that your audio signal will have a maximum frequency of 20 kHz).

Depending on the power of your microcontroller you may be able to digitally filter the signal first, this will relax the constraint on the reconstruction filter considerably, you may even be able to use a simple RC filter at this stage! This is actually how audio CDs work, using a method called "oversampling" (note that this is NOT the same as sampling an analogue signal above the Nyquist rate).

As for the lowpass filter itself there's lots of different methods of designing them. You can build some fairly good filters with just an op-amp and a few capacitors & resistors. Have a search for a Sallen Key filter for example or alternatively an IGMF filter (same components, just different method!).

Let me know if you need any clarification on this...

Anima

Thanks Micheal. You're right the signals are being generated on the uP.

I thought an anti-aliasing filter was the same as a reconstruction filter so thats my bad. Indeed I was looking for a high order 20khz LP filter. But if you say I can digitally filter the signal first, well that sounds like a better alternative.

Cheers.

DublinDilbert

What sort of wave forms are you going to be generating?? if it's just pure sine wave you'll get away with a simple low pass filter.

If you going to be generating more complex wave forms, eg square waves ect the task of filtering the wave form becomes more difficult, as most filters will distort the frequency content of a complex wave form ( eg a square wave ). I call a square wave a complex wave form as it's effectivly made up of sine waves of different frequenices.

If your trying to filter a square wave you have to look at the phase of the filter, you need a filter with a linear phase so as not to alter/distort the waveform. A bessil filter is normally used for this purpose, although it doesn't have a truely linear phase it is often good enough..