Re: DDS filters

On Tue, 17 Sep 2024 22:57:57 -0000 (UTC), piglet
<erichpwagner@hotmail.com> wrote:

john larkin <jl@650pot.com> 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.
 
 
 
 

>
Wouldn’t your nu- hertz package create a filter out of a few 0603 parts far
cheaper than a bought in filter?

Sure, but that will take a bunch of parts on the board, times four.
The filters will be bigger than the DACs.

I'm updating an old VME board design, a good seller full of obsolete
parts. It used an Analog Devices DDS chip, and the filter has four
parts, LCLC, which isn't too awful. But some little ceramic LTCC
thingie would be great.

Some people make ferrite-bead sorts of things, 3-terminal emi filters
with caps inside. I'll look into those. Might get lucky.