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From: Bill Sloman <bill.sloman@ieee.org>
Newsgroups: sci.electronics.design
Subject: Re: DDS filters
Date: Thu, 19 Sep 2024 13:49:27 +1000
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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