Re: 50 cent DDS synthesizer

From	"Don" <g@crcomp.net>
Newsgroups	sci.electronics.design, comp.arch.fpga
Subject	Re: 50 cent DDS synthesizer
Date	2025-10-23 16:54 +0000
Organization	A noiseless patient Spider
Message-ID	<20251023a@crcomp.net> (permalink)
References	<8fjifkteog2a8s8960dk7lak7kh70lrtta@4ax.com> <10dcmfd$1fcrk$1@dont-email.me> <jgekfklf9s7khf3kqf07a1jdcuegik14bi@4ax.com>

Cross-posted to 2 groups.

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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?

-- 
73, Don, KB7RPU                                           veritas    _|_
                                                          liberabit   |
https://www.qsl.net/kb7rpu                                vos         |

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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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