Re: DDS filters

On 19/09/2024 4:04 am, john larkin wrote:

On Wed, 18 Sep 2024 16:48:36 +1000, Bill Sloman <bill.sloman@ieee.org>
wrote:
 

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.

 It's interesting that there is a class of people who want to do
totally impractical expensive things on circuit boards. People with no
common sense. The name for such people is "fired."

It's depressing that there is a class of people who suffer from "not invented here" and complain that anything that they didn't think of is impractical and expensive.

 Also, a DDS lowpass filter can have ghasty passband response.

If cobbled together by the likes of John Larkin. The sort of people who can get ghastly jitter out of an ECL-to-TTL converter chip.

What matters is stopband rejection. All the classic filter responses try to
optimize passband flatness.

So John doesn't know what he is talking about.

The jitter of a DDS at low frequencies is domnated by the number of
MSB bits that we pick from the phase accumulator. It's usually better
to synthesize a clean octave and divide down as needed.

And doubles down on being ill-informed.
--
Bill Sloman, Sydney