Hi

Ok let's toss some numbers into the mix.

The counter time base one day after calibration is in the 0.5 to 1.5 ppb range.

The LPRO ten years after it left the factory is in the 0.5 to 1.5 ppb range.

The GPSDO when running properly should be in the 0.005 to 0.02 ppb range at one second.

One ppb at 10 MHz is 0.01 Hz. It also is a the resolution limit on a 5335 at 1 second.

Bob

Bob:
With a GPSDO as a general rule is there much variation of the 0.005 to 0.02 ppb range. Would there me much if only 2 satellites were visible but the unit was no in holdover vs say 6 satellites. I assume there is probably a lot of variables such as quality of GPSDO and OCXO, multi path interference, antenna location, etc, but worse case what would a user expect assuming the unit isn't in holdover.

-=Bryan=-

Bryan wrote:

There are lots of ADEV plots of GPSDOs on the web, if you search for
them.  Tom has the attached plot posted on his web site -- it includes
one decent (but not stellar) GPSDO, the HP Z3801.  That is about what
you can expect without heroic tuning efforts (on a GPSDO that supports
user fiddling, such as the Trimble Thunderbolt).

Since many counters gate for one second (or gate faster, then average),
the ADEV at tau=1 second is a reasonable estimate of the real-world
performance of a good counter driven by a GPSDO.  Note that, since the
ADEV at 1 second depends largely on the OCXO, the heroic tuning efforts
mentioned above won't change the ADEV at tau=1 second hardly at all.
That is attributable strictly to the OCXO (unless the GPSDO is very
poorly designed).  Building a GPSDO using the best OCXO available can
improve things.  For example, look at the "BVA" plot -- at 1 second, its
ADEV is nearly a full decade better than the Z3801, better even than the
hydrogen masers.  (While I referred to the 3801 above as "decent but not
stellar," that is based on its performance at tau from 100-100k seconds.
Its OCXO -- and, thus, its ADEV -- is better than most others at 1
second.)

Best regards,

Charles

Hi

I do not know of any commercial GPSDO's that do not have a holdover
feature. There are a lot of variables. With a large package OCXO and a couple days
operation, the numbers I gave should be good 90 to 99% of the time.(99 seconds out
of 100).

If the unit has such a poor antenna location that (unlike the original post) it never
completes survey, then all bets are off... The basic assumption is "locked to > 4 sats
at all times. That generally translates to 6 or more most of the time.

Bob

Sent from my iPad

Hi

At the most basic level, ADEV is a standard deviation. When people read an instrument they usually want a "max error" sort of number. ADEV operates on delta between readings so a simple "three sigma" mental math conversion is not all you need. It is the right measure. It can take a bit of work to convert it to useful units .

Bob