[time-nuts] Best Rubidium Frequency Standard

[timenut at metachaos.net]

Bert, Bob, Charles, Corby, Eric, Luciano, Mark, Rob, . . .

Thanks for the excellent advice. It has been very helpful. Let me see if I can
summarize what different people are saying.

   1. The absolute best rubidium standard is the HP 5065A. It has excellent
      short term stability. The only disadvantages are - a) the price, B) it
      is harder to use for disciplining via GPS, c) it is sufficiently old
      that life span could be an issue and d) has poor temperature control and
      suffers from pressure sensitivity.

   2. The second best is the PRS-10. Provided it is working. Its phase noise
      is generally very good, and above 100Hz may even be better than the HP
      5065A but is poor under 1Hz. It is difficult to repair, but that problem
      appears to be shared by everything except the HP 5065A which is still
      through-hole. Other than its performance, it has some advantages - a) it
      is still in production and so can be repaired by SRS, B) it has a
      built-in 1 pps locking circuit that can be used to lock to GPS and c)
      given (a) and the 22 year design, life span issues are not a significant
      problem. It also has several disadvantages - a) that same locking
      circuit may not always be present and B) that locking circuit does not
      perform as well as one would wish for GPS 1 pps signals (although
      pre-applying the sawtooth correction apparently helps a lot as does
      pre-filtering the GPS signal for an effectively larger time constant).

   3. The Efratom M-100 and FRM rubidium standards are also highly
      recommended. Their main disadvantages are a) the same repair issues that
      most of these units have, B) the potential life span limitations and c)
      they are smaller units and not as good as either the PRS-10 or HP 5065A.
      They do have the service manual available. I don't know about
      schematics.

   4. The LPRO-101 is far from the worst of the lot. As far as repair, there
      is a repair guide that people have put together but no official service
      manual with schematics.

   5. The FE-5650A / FE-5680A may also be acceptable, with the same
      disadvantages of life span and difficultly repairing but have very
      high noise levels.

So, as I interpret the above, if I can score a working HP 5065A for an
affordable cost then it would be the way to go. While possible, that is not
likely. Otherwise, a working PRS-10 with a functioning 1 pps input is very
good, again assuming affordable cost which is much more likely.

The ability to personally repair either may be limited. For the HP 5065A there
are no parts available - especially lamps - and the only way to fix one is to
use a parts unit. That gets very expensive and could take many months finding
an appropriate parts unit. For the PRS-10, firmware issues and the heavy use
of SMD would make repair more difficult. But, for it, at least factory repair
is available for a reasonable cost.

It may be possible to get some of the other rubidiums somewhat cheaper, but
all have their disadvantages. Most share the repair issue and life span issues
of the HP 5065A and the SMD construction of the PRS-10. Most of them would
also need to have their output cleaned up when used in an application
requiring low phase noise. Buying some of them in lots may enable repair via
parts units for a reasonable cost. That might be thing to do just to have some
as comparisons.

I plan on more research, especially with respect to the PRS-10, but with
respect to what is currently known, have I missed anything? Any additional
feedback? Other units that are worthy of consideration (and actually
available, of course)?

I have asked SRS about some of the PRS-10 issues. I will everybody know
what they say (assuming that they respond).


Mike