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Re: 50 cent DDS synthesizer

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On Sun, 26 Oct 2025 08:10:08 GMT, Jan Panteltje <alien@comet.invalid>
wrote:

>>antispam@fricas.org (Waldek Hebisch)wrote:
>>>In sci.electronics.design Jan Panteltje <alien@comet.invalid> wrote:
>>>>Chris Jones <lugnut808@spam.yahoo.com>wrote:
>>>>>On 24/10/2025 4:20 am, Jan Panteltje wrote:
>>>>>> "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?
>>>>> 
>>>>> In the case of R2R you can use resistors from one batch and that may have less spreading in values
>>>>> No precision resistors needed.
>>>>> (2R is then 2 1R resistors in series).
>>>>> 
>>>>
>>>>You don't want them the same though: The MOSFETS of the CMOS logic 
>>>>outputs have some on-resistance (maybe 30-50 Ohms) and so the resistors 
>>>>in series with each of these outputs should be decreased by whatever you 
>>>>measure that output resistance to be, otherwise the resistance ratio 
>>>>isn't 1:2 in the R-2R network, and the DNL will be poor.
>>>>
>>>>You could use the average of the on-resistance of the NMOS and the PMOS 
>>>>(which are not quite the same), measured on a typical chip, at room 
>>>>temperature, so that it is usually at least roughly correct, rather than 
>>>>systematically and always wrong.
>>>>
>>>>The MOSFET on-resistance is usually a more important effect than the 
>>>>error of 1% resistor tolerances, so it makes sense to fix that before 
>>>>worrying about selecting resistors from the same batch.
>>>>
>>>>It is worth fixing it for R-2R and also for at least a few msbs of 
>>>>binary weighted resistor DACs.
>>> 
>>> How would you go about measuring MOSFET on resistance?
>>
>>For logic MOSFETS in IC-s I just used a multimeter.  For power
>>MOSFETS I used "two meter" method: resitor to limit current to
>>reasonable range, ammeter to measure current, voltmeter to
>>measure voltage drop on the MOSFET.  Actually, I had extrea
>>voltmeter to measure gate voltage.
>
>Yes, that should work.
>But in the case of FPGA output there is pull up, pull down, linearity and likely some effects in-between.
>But as a simple rule it should work.
>Now people here were talking about that IBIS model.
>If I ever get spice working on my computers (Raspberries these days) ....
>Old spice version could still be running in wine in Linux on old PC upstairs.
>For some reason I do not need Spice except in food.

Spice is great for playing with ideas. We are developing a series of
resistor-inductor simulators, RL dummy loads, that were invented by
fiddling with Spice. They work but I still don't understand them.


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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