[time-nuts] Linear voltage regulator hints...

dan at irtelemetrics.com dan at irtelemetrics.com
Sun Dec 14 15:38:59 EST 2014

     Yeah, I guess it's all relative. :) I do agree it is a HUGE 
change compared to what the GPS should see in normal operation. The 
same blast of air can be pointed at the GPSDO board for 20 seconds and 
see no change. On high at the oscillator for 60+ seconds with no change 
is apparent. The GPS is what responds. 
  Does this mean anthing? It's well beyond what the GPS should see, so 
I doubt it. Is the GPS the most sensitive part in the system right now? 
Maybe. But, Bob could be correct that the OCXO is the sensitive part. 
  The PWM DAC used to respond like the GPS to temp changes. Using 'heat 
and watch' found it. Addressing it has cut overall temp sensitivity 
down by more than a factor of 10. I consider that a measurable 
change. Any temp sensitivity left is really small, and wonder if it 
can be addressed with the hardware and tools at hand. 
  My gut feeling is you are right about the GPS time base being 
sensitive. It would be fun to hack into this and try clocking it off 
the OCXO, but I'm not there yet! :)   I might try some good old 
'overkill' and put the GPS in it's own heavy aluminum box. It's been a 
thought for a while, and should address the thermal integration and 
time constants you referred to. For this system, it's doubtful that an 
underground bomb proof bunker is needed. ;) 
  I'd like to learn more about what testing could be done and would be 
more than happy to discuss this off line, if anyone is interested. 
 This thread has probably out lived it's useful life. 
  > LOL. A couple of seconds of warm air at 15C above ambient is a HUGE
  > temperature transient for any sensitive electronics, especially
  > anything with an oscillator. I would venture a guess that the lion's
  > share of the drift you see is the GPS time base shooting
  > off-frequency, but there are probably other effects, too (voltage
  > regulators, to name just one). 
  > To me, a "little change" in this context might be blowing one warm
  > breath toward the GPS unit from 18" away and seeing what happens over
  > the next minute or two. 
  > But the GPS temperature sensitivity shouldn't be a big factor in
  > actual use. The GPS should be thermally isolated from anything that
  > changes temperature rapidly, and enclosed such that external
  > temperature changes are integrated over at least tens of
  > minutes. Then, the inside of the enclosure will reach its own
  > thermal equilibrium and any external changes will be slowed enough to
  > be tracked out by the GPS discipline. My recommendation would be to
  > put it in a cast aluminum box (search the archives for "cast aluminum
  > box"), but there are others who think you need to build a two foot
  > cube out of cinderblocks and fire brick against a wall in the deepest
  > external corner of your basement. 
  > OR, if my suspicion is correct that the temperature sensitivity is
  > mostly the GPS time base, figure out a way to kludge the GPS to
  > accept the disciplined OCXO as its time base. 
  > Best regards,
  > Charles

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