Re: DDS filters

On 18/09/2024 8:48 am, john larkin wrote:

I can use an Efinix FPGA and a bunch of cheap fast DACs to make some
DDS clock sources, specifically four. The pain is the lowpass filter.
 Mini-Circuits and other folks make nice surface-mount lowpass filters,
but they are most all in the GHz range. I want maybe 25 MHz. You'd
think there would be a market for packaged MHz-range lowpsss filters.
 It's worth pushing the DAC rate as high as possible to simplify the
lowpass filter. Stay far away from Nyquist.

That kind of circuit cries out for finite impulse response low pass filter.
You feed the digital signal through a shift register and hang sampling resistors on each tap, and sum the currents fed through the resistors.
You do have to watch out for truncation error - Gibb's oscillations - and use a Hamming window when you calculate the value for each sampling resistor.
The neat thing about it is that it is essentially frequency independent - the cut -off frequency scales with the clock rate.
It's sort of bulky - my 32-stage example need two or three E-96 precision resistors per tap to get the precision you need, but in surface mount that's tolerable.
Shorter shift registers don't cut off as sharply but can still do much better than analog parts.
--
Bill Sloman, Sydney