GPSDO time constant

• GPSDO time constant
  Posted by Ohm_My on 17 Jun, 2019 01:30
• I have one of these eBay GPSDO units that has the Oscilloquartz 8663-XS oscillator (STAR 4+) and I use it to steer a Rubidium standard. What does the time constant value perform and is their an optimal setting? I believe the range for the time constant is 200 seconds and can be adjusted to a max value of 5,000 seconds, I think.
  
  I'm also using using Lady Heather 6.14 b16 to monitor the progress. I'd like to hear from anyone who has a similar setup or experience with this GPSDO unit.
  
  Thanks.

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• #1 Reply
  Posted by jpb on 17 Jun, 2019 17:04
• I think that 200 seconds is way too short. I have mine set to 5000 seconds and am (slowly) doing some ADEV measurements - unfortunately my setup is only able to give measurements above around 10 seconds. From what I've measured so far a time constant of around a 1000 seconds is probably good.
  
  The GPS signal itself doesn't reach an ADEV of 10^-11 until about 2000 seconds with it being around 8 x 10^-12 at around 4000 secs - see this app note
  https://www.u-blox.com/sites/default/files/products/documents/Timing_AppNote_%28GPS.G6-X-11007%29.pdf
  
  my measurements for one Star 4+ vs another (I have two) with both set to 5000 seconds are all below 10^-11 (and you need to divide by root(2) if they are identical) so I think the OCXO is probably below 10^-11 out to a few thousand hence I argue a setting towards the high end (a few thousand) is probably best.
• #2 Reply
  Posted by Theboel on 17 Jun, 2019 21:52
• Hi Ohm_My,
  
  Sorry I confused with Your post.
  what Time constant You ask for ? is the time constant in internal GPSDO (For OCXO) or time constant for steering RB ?
• #3 Reply
  Posted by Ohm_My on 18 Jun, 2019 02:27
• Quote from: Theboel on 17 Jun, 2019 21:52

  Hi Ohm_My,
  
  Sorry I confused with Your post.
  what Time constant You ask for ? is the time constant in internal GPSDO (For OCXO) or time constant for steering RB ?
  

  
  I'm assuming it is the TC for the GPSDO OCXO. I'm using the TC from the Rb to do the steering.
• #4 Reply
  Posted by David Hess on 18 Jun, 2019 22:45
• The GPSDO time constant to set depends on the stability of the local oscillator; higher stability oscillators can use longer time constants to produce better results.
  
  Ultimately the time constant sets the break point between the noise curves from the local oscillator and GPS signal.  The local oscillator controls the output for time periods faster than the time constant and the GPS signal controls the output for periods longer than the time constant.