[time-nuts] Tale of Two GPStars...

[Charles Steinmetz]

Bruce wrote:

> And I'm still amazed I lucked out with the rubidium module.

Actually, the unit's PN and ADEV performance would almost certainly be 
considerably better with a good OCXO instead of the Rb.

At low tau, the PN and ADEV performance of a GPSDO is determined by the 
local oscillator (generally, an OCXO or Rb).  The OCXO is better with 
respect to PN and ADEV than a Rb out to tau >~1000 seconds, so in the 
very important area of tau <1000 seconds, the Rb unit loses to the OCXO 
unit.  Above tau >~1000, the averaged GPS signal governs PN and ADEV 
performance, so you never really benefit from the Rb oscillator.

Unless holdover performance is the most important criterion (which it 
almost certainly will *not* be for a time nut), a GPSDO with a good OCXO 
is superior at all tau to one with a Rb.

Best regards,

Charles