Re: 50 cent DDS synthesizer

From	john larkin <jl@glen--canyon.com>
Newsgroups	sci.electronics.design, comp.arch.fpga
Subject	Re: 50 cent DDS synthesizer
Date	2025-10-26 10:08 -0700
Organization	A noiseless patient Spider
Message-ID	<12lsfk5mffqm7s7iq6gtvqv3bne5htk6ur@4ax.com> (permalink)
References	(6 earlier) <10df7vg$2dvoi$1@dont-email.me> <10djh8v$2h1af$1@paganini.bofh.team> <10dkl12$3sl8j$1@dont-email.me> <ufdsfkpcgu0imeavsc0463actaaf6q51he@4ax.com> <10dlhn7$4cr0$1@dont-email.me>

Cross-posted to 2 groups.

Show all headers | View raw

On Mon, 27 Oct 2025 03:19:45 +1100, Bill Sloman <bill.sloman@ieee.org>
wrote:

>On 27/10/2025 1:56 am, john larkin wrote:
>> 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.
>
>That's risky. Spice isn't great on finding potential parasitic oscillations.

Differential equations and control theory aren't either.

A little practical experience can supplement a Spice sim. Spice
doesn't punch out gates if the voltage is too high, and Spice
transistor models don't include wirebond inductance.


John Larkin
Highland Tech Glen Canyon Design Center
Lunatic Fringe Electronics

Back to sci.electronics.design | Previous | Next — Previous in thread | Next in thread | Find similar

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

csiph-web