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From: john larkin <jl@650pot.com>
Newsgroups: sci.electronics.design
Subject: Re: DDS filters
Date: Wed, 18 Sep 2024 11:04:09 -0700
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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."

Also, a DDS lowpass filter can have ghasty passband response. What
matters is stopband rejection. All the classic filter responses try to
optimize passband flatness.

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.