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Re: 50 cent DDS synthesizer

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On Thu, 23 Oct 2025 16:54:31 -0000 (UTC), "Don" <g@crcomp.net> wrote:

>john larkin wrote:
>> Jan Panteltje wrote:
>>>>john larkin wrote:
>>>>>
>>>>We have a little proto board with a RP2040 Rpi cpu and an Efinix T20
>>>>FPGA. The Pi configures the FPGA at powerup and then talks SPI to it.
>>>>
>>>>We often need programmable clocks so I added a few parts to make a DDS
>>>>frequency synthesizer. We use a PLL inside the FPGA to spin up a 204
>>>>MHz clock for the 32-bit phase accumulator. The sine lookup is 4K
>>>>points and the "dac" is six resistors.
>>>>
>>>>This lowpass filter is tacky, but it all works. At 50 MHz, the jitter
>>>>is a few hundred ps p-p, and that looks mostly like amplitude noise to
>>>>me.
>>>>
>>>>https://www.dropbox.com/scl/fo/336qn643xyilkqz5rc9lp/ADC4ThBQa3B-L5auaTDh8fQ?rlkey=q2qrsbdow816id6wgc4eypuas&dl=0
>>>>
>>>>The FPGA 1.2v core supply uses about 30 mA total when this is running
>>>>at 50 MHz out.
>>>
>>>Nice, 250 Ohm is a big load, good the FPGA can handle that.
>>
>> The MSB actually sees 500 ohms.
>>
>>>
>>>I was using R2R for video:
>>> https://www.electronics-tutorials.ws/combination/r-2r-dac.html
>>
>> One can buy a real r-2r network cheap, but it's easier to use 0603
>> resistors that we have in stock.
>
>Also, the ability to adjust assembled resistor values works better for 
>me as a DIYer. Here's mouser's offering for the geniuses in our midst, 
>with the foresight to always choose the correct component:
>
><https://www.mouser.com/c/passive-components/resistors/?circuit%20type=R%2F2R%20Ladder>
>
>Jan's cited tutorial says:
>
>    Compared to the R-2R DAC, the binary weighted digital-to-analogue 
>    converter has an analogue output voltage which is the weighted 
>    sum of the individual inputs. Thus it requires a large range of 
>    precision resistors within its ladder network, making its design 
>    both expensive and impractical for most DAC's requiring lower 
>    levels of resolution.
>    
>Discussions in this group left me with the impression precision resistor
>fabrication became trivial thanks to the trimming technology of modern 
>lasers? Is the price of precision resistors still a factor?

I figured that using 1% resistors, this sort of dac would be about
monotonic at 6 bits. Six bits makes a pretty decent looking sine wave,
which filters/interpolates into a low jitter clock.

https://www.dropbox.com/scl/fi/n12quxajp59egahv1b48i/DDS_1KHz.jpg?rlkey=8w0tlr3h1rxeif3byepzub947&raw=1

R-2R made from discretes would use twice as many resistors.

The 204 MHz phase accumulator clock has 5 ns p-p jitter. Adding this
stuff improves jitter about 20:1.

It will need a fast comparator to make the sine into a cmos level
clock to feed back into the FPGA. An LVDS line receiver can do that.

Just a fun little diversion.


John Larkin
Highland Tech Glen Canyon Design Center
Lunatic Fringe Electronics

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Thread

50 cent DDS synthesizer john larkin <jl@glen--canyon.com> - 2025-10-22 14:49 -0700
  Re: 50 cent DDS synthesizer Jan Panteltje <alien@comet.invalid> - 2025-10-23 07:45 +0000
    Re: 50 cent DDS synthesizer john larkin <jl@glen--canyon.com> - 2025-10-23 07:36 -0700
      Re: 50 cent DDS synthesizer Jan Panteltje <alien@comet.invalid> - 2025-10-23 15:16 +0000
      Re: 50 cent DDS synthesizer "Don" <g@crcomp.net> - 2025-10-23 16:54 +0000
        Re: 50 cent DDS synthesizer Jan Panteltje <alien@comet.invalid> - 2025-10-23 17:20 +0000
          Re: 50 cent DDS synthesizer Martin Brown <'''newspam'''@nonad.co.uk> - 2025-10-23 19:33 +0100
          Re: 50 cent DDS synthesizer Chris Jones <lugnut808@spam.yahoo.com> - 2025-10-24 10:12 +1100
            Re: 50 cent DDS synthesizer Jan Panteltje <alien@comet.invalid> - 2025-10-24 06:56 +0000
              Re: 50 cent DDS synthesizer Martin Brown <'''newspam'''@nonad.co.uk> - 2025-10-24 09:53 +0100
              Re: 50 cent DDS synthesizer Lasse Langwadt <llc@fonz.dk> - 2025-10-24 13:40 +0200
                Re: 50 cent DDS synthesizer john larkin <jl@glen--canyon.com> - 2025-10-24 07:09 -0700
                Re: 50 cent DDS synthesizer Jan Panteltje <alien@comet.invalid> - 2025-10-24 17:17 +0000
              Re: 50 cent DDS synthesizer antispam@fricas.org (Waldek Hebisch) - 2025-10-25 22:00 +0000
                Re: 50 cent DDS synthesizer Jan Panteltje <alien@comet.invalid> - 2025-10-26 08:10 +0000
                Re: 50 cent DDS synthesizer john larkin <jl@glen--canyon.com> - 2025-10-26 07:56 -0700
                Re: 50 cent DDS synthesizer Bill Sloman <bill.sloman@ieee.org> - 2025-10-27 03:19 +1100
                Re: 50 cent DDS synthesizer john larkin <jl@glen--canyon.com> - 2025-10-26 10:08 -0700
                Re: 50 cent DDS synthesizer Bill Sloman <bill.sloman@ieee.org> - 2025-10-27 16:02 +1100
                Re: 50 cent DDS synthesizer antispam@fricas.org (Waldek Hebisch) - 2025-10-27 00:10 +0000
                Re: 50 cent DDS synthesizer Jan Panteltje <alien@comet.invalid> - 2025-10-27 09:52 +0000
                Re: 50 cent DDS synthesizer antispam@fricas.org (Waldek Hebisch) - 2025-10-28 16:31 +0000
                Re: 50 cent DDS synthesizer Jan Panteltje <alien@comet.invalid> - 2025-10-28 17:41 +0000
                Re: 50 cent DDS synthesizer john larkin <jl@glen--canyon.com> - 2025-10-27 07:58 -0700
        Re: 50 cent DDS synthesizer john larkin <jl@glen--canyon.com> - 2025-10-23 11:46 -0700
          Re: 50 cent DDS synthesizer john larkin <jl@glen--canyon.com> - 2025-10-23 18:55 -0700
            Re: 50 cent DDS synthesizer Jan Panteltje <alien@comet.invalid> - 2025-10-24 06:57 +0000
              Re: 50 cent DDS synthesizer Jan Panteltje <alien@comet.invalid> - 2025-10-24 07:38 +0000

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