[time-nuts] Symmetricom TimeSource 2500

Attila Kinali attila at kinali.ch
Mon Jan 22 13:55:26 EST 2018

On Mon, 22 Jan 2018 13:17:40 -0500
Bruce Hunter via time-nuts <time-nuts at febo.com> wrote:

> A disappointment of the 2500's architecture is that it burns an SRI PRS-10 in 
> the background as a backup in case of the loss of GPS.  This seems wasteful 
> to me as the operating life of the PRS-10 units is not unlimited.  I suppose 
> the PRS-10 could be disabled, but this would require some simulation 
> circuitry as the 2500 monitors the PRS-10 and goes into a fault condition if 
> the expected PRS-10 presence is absent.  Certainly others have utilized these 
> units for such  use.  Has anyone performed a detailed evaluation of the 2500 
> units in intermittent service?

The PRS-10 is in there because someone needed the additional stability
performance. And to achieve that you need to run it continuously,
because you need to measure its frequency and correct it as it slowly
drifts. As with crystal oscillators, Rb have a retrace as well, so
just measuring the frequency and shutting it off, will not do.
So, what you want is bascially switching off the main component
of the system.

Besides, switching an Rb on and off is not a good thing for its lifespan.
It's like a incandescent light bulb: They can run for decades
if continuously under power, but if you switch them on and off once
a day, they die within a few months.

Also, Rb are spec'ed for a lifespan in the order of 20 years
(the PRS10 has a lifespan of 27 years). Nobody complains if a piece
of equipment needs to be replaced/repaired every 20 to 30 years.
Most likely the power supply will die well before you get even to
the Rb failing.

			Attila Kinali

It is upon moral qualities that a society is ultimately founded. All 
the prosperity and technological sophistication in the world is of no 
use without that foundation.
                 -- Miss Matheson, The Diamond Age, Neil Stephenson

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