On Fri, Nov 30, 2018 at 8:56 AM Attila Kinali <[email protected]> wrote:
> On Thu, 29 Nov 2018 21:22:44 -0700 > Mark Goldberg <[email protected]> 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. > I definitely have a bad sky view. The antenna is in a ground floor window of a two story home with a metal covered RV parking area next to it. I can only see maybe 30 degrees of sky. I am in Arizona. The view is basically from about straight up to 60 degrees from the horizon to the south. > > 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. I made the measurements using the setup described at: https://sites.google.com/site/spectrumlabtesting/ and the other sites linked there. There could be some anomalies due to the peak frequency measurement algorithm. > 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. > > Unfortunately, I have what I have for now. I have basically three devices, the TruePosition and Bodnar GPSDOs, and an 8642A generator with the OCXO option. None are optimal I know, I just want to do the best with what I have. Thanks for the suggestions though. Regards, Mark _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.
