[time-nuts] Trueposition Antenna Location

[Attila Kinali]

On Thu, 29 Nov 2018 21:22:44 -0700
Mark Goldberg <marklgoldberg at gmail.com> wrote:

> How much could I expect ADEV to improve if I move the antenna to a better
> location with clear view of the sky? The things I am testing need to be
> better than 8e-10, so this at just under 3e-10 may be "good enough". Moving
> the antenna would involve drilling a hole in the outside wall, so it is not


As Bob already wrote, antenna position is only one thing. But if your
receiver is going into holdover at all, then you are already in a place
where you will not have good enough reception to get down to your requirement.
Antenna position, ie sky view and multipath, are the two things that
kill most of the performance of any GPSDO. Bad sky view can result
in several µs of phase variation and multipath in several 100ns.

After that, you will need a local oscillator with sufficient stability
to average over a long period. To get below 1e-10 you will need to average
over at least 100s (given good receiver and perfect antenna position),
better make it 1000s to 10ks. This means, the local oscillator has to be
stable enough to stay below 1e-10 over that period. Which means you
need to use a good OCXO at the very least, in a temperature stable
environment. No-one of the GPSDOs you mentioned fullfills that requirement.
(The NV47* of the TruePosition is decent, but falls slightly short.
Leo's GPSDO uses a TCXO)

I would recommend you to get either a Thunderbolt (first choice)
or a Star4 (second choice) from ebay. Alternatively, get one
of Nick Sayer's GPSDO, modify it to use a good OCXO and crank
up its loop time constant.
Do not get one of the Trimble UCCM, they have quite horrible
frequency jumps.



			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, Neal Stephenson