From: jl@glen--canyon.com   
      
   On Tue, 24 Feb 2026 00:46:22 -0000 (UTC), Phil Hobbs   
    wrote:   
      
   >john larkin  wrote:   
   >> On 23 Feb 2026 22:57:00 GMT, Uwe Bonnes   
   >>  wrote:   
   >>   
   >>> john larkin  wrote:   
   >>> ...   
   >>>> That's about 12 power supplies!   
   >>>>   
   >>>> My new gadget, the PPG (Precision Pulse Generator) only needs about 7.   
   >>>>   
   >>>> Interestingly, the most critical one, for noise and drift, is the 1.2   
   >>>> volt FPGA core supply. 100uV changes the prop delay more than I like.   
   >>>>   
   >>>>   
   >>> Consider some stabilization loop. Some FPGA TDC design use similar   
   >>> approaches.   
   >>   
   >> I'm designing basically an economy DDG, digital delay generator. So   
   >> I'm not investing engineering hours or parts cost on extreme   
   >> performance.   
   >>   
   >> So I'll just make the 1.2v supply as good as we reasonably can and   
   >> sell whatever it does.   
   >>   
   >> Prop delay in the FPGA is about inverse on core power supply voltage.   
   >> If the supply is noisy, I don't think we could compensate for that in   
   >> real time.   
   >>   
   >> We will know the PCB temperature so maybe we'll compensate for that a   
   >> little. That would be a quick test and a bit of code.   
   >>   
   >   
   >Using a backwards PLL to regulate the supply would be pretty amusing,   
   >though, you have to admit.   
   >   
   >Cheers   
   >   
   >Phil Hobbs   
      
   Yes. We couldn't compare the FPGA to an external delay line (they   
   aren't very good) so we'd need a clever (and affordable) way to   
   compare a prop delay to a clock.   
      
   Of course, every time you compile an FPGA design it may change the   
   routing radically.   
      
   Sometimes the optimizer decides that you really don't need to do   
   something at all.   
      
   But amusing is the enemy of cheap and done. That's a real problem in   
   this industry.   
      
      
   John Larkin   
   Highland Tech Glen Canyon Design Center   
   Lunatic Fringe Electronics   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)