Author Topic: Lars DIY GPSDO with Arduino and 1ns resolution TIC  (Read 89739 times)

ltvs88 and 2 Guests are viewing this topic.

Offline thinkfat

  • Supporter
  • ****
  • Posts: 1349
  • Country: de
    • Matthias' Hackerstübchen
Re: Lars DIY GPSDO with Arduino and 1ns resolution TIC
« Reply #800 on: November 21, 2020, 06:26:38 pm »
A linear regression. Plot DAC output vs. temperature, feed it into a linear regression solver, this gives you the coefficient. make sure your correlation "r" is high enough, > 0.9 would be perfect.
Everybody likes gadgets. Until they try to make them.
 
The following users thanked this post: Mike99

Offline Mike99

  • Regular Contributor
  • *
  • Posts: 70
  • Country: gb
Re: Lars DIY GPSDO with Arduino and 1ns resolution TIC
« Reply #801 on: November 22, 2020, 11:25:50 am »
A linear regression. Plot DAC output vs. temperature, feed it into a linear regression solver, this gives you the coefficient. make sure your correlation "r" is high enough, > 0.9 would be perfect.

Perfect, thank you  :-+.

Since I optimised the time constant (now 300 seconds) the DAC is much better behaved which should make it easier.

Mike
 

Offline MIS42N

  • Contributor
  • Posts: 6
  • Country: au
Re: Lars DIY GPSDO with Arduino and 1ns resolution TIC
« Reply #802 on: November 24, 2020, 11:41:58 am »
Choosing the optimum time constant depends entirely on the stability of the oscillator's behaviour. You might expect an OCXO to be pretty immune to external temperature variations but once you start chasing after parts per trillion accuracy, you soon discover the effect of temperature on the humble OCXO leaving you hankering after a high quality DOCXO upgrade or even a suitable RFS. ::)

 You mentioned the expense of a good voltage reference (MAX6350CPA+) so, despite the fact that you've already got one on order, I think it's worth considering the temperature stabilised voltage reference available from the better quality OCXOs which provide this on their Vref pin. Even if this comes from a humble LDO supplying the oscillator and the oven controller, it's worth considering that, once warmed up to temperature, it's supplying a constant current and its internal reference is temperature stabilised making it only second in stability to that of an expensive lab standard (at least for use with a low constant current external load such as the 1mA of my reference voltage supply).

By going down the non PLL route, there is no optimum time constant. The processor collects noisy error information every 1ppS, averages it, and determines if the error is 'big enough' to require a correction. 'Big enough' starts off large (my current version starts at around 1Hz error from 5 samples) and becomes smaller as more samples are collected (down to .0003Hz from 2000 samples). This is consistent with applying statistics to the short term jitter of the 1ppS from a cheap GPS module. Any misbehaving by the oscillator is caught in the process.

As I mentioned in previous post, I will be using a used Morion MV89 DOCXO in the next version. It was used in my first attempt at a GPSDO. Its performance is way better than OSC5A2B02 as you would expect. The whole point of the circuit I published was to show how few low cost components could still make a working GPSDO. I have no real interest in using a GPSDO, I like the technical challenge of trying different strategies to see what can be achieved.

I couldn't find any details of the stability of the Vref pin on the Morion. The MAX6350CPA+ claims "Ultra Low, 1ppm/°C Max Tempco". The sensitivity of the OSC5A2B02 is determined as approx 0.1V/Hz with a lock voltage of around 2V. The Morion is advertised as 1V/Hz and its lock voltage was around 2V also. So the MAX6350CPA+/MV89 combo should have a sensitivity of around 2*10^-13/°C which may be good enough. I don't have the equipment to determine if this is better or worse than using the Vref.
 


Share me

Digg  Facebook  SlashDot  Delicious  Technorati  Twitter  Google  Yahoo
Smf