Sorry I meant to say I'm confident the measurements are above the noise floor after approx 100 seconds.
------------------------------ On Mon, 3 Dec, 2012 4:56 AM PST Mark Spencer wrote: >Thanks for the comments and suggestions. > >With regards to the equipment used to collect the data, I am using two HP >5370B's. The 1 pps output of the PRS10 is connected to the start input of >each counter, the 10Mhz output of each of the GPSDO's is connected to the stop >input of one of the counters. The 10 Mhz output from the PRS10 is also routed >via a distribution amp to the reference input of both counters. I'm >confident that the measurements are below the noise floor of the counters >beyond approx 100 seconds. The trigger settings on the start input of each >counter are identical so I'm hopeful the measurements are being taken at more >or less the same time. > >With regards to the common mode issues, yes this crossed my mind, and I expect >what I will end up measuring will likely be a function of the longer term >performance of the lesser of the two receivers rather than the true GPS noise >floor. I should probably have made the "sort of" disclaimer in the title of >the thread stronger (: Still I expect this would give me an indication of the >likely long term performance of the two GPS receivers in question which from >my end user perspective is still useful. > >The two receivers do however receive different numbers of satellites so I'm >thinking some GPS system bias may work it's way into the measurements over the >long term. I am curious if at some point I will see the computed Adev between >the two GPSDO's flatten out and trend upwards. > >The comment regarding the differences in the performance of the GPSDO's vs the >GPS system itself also makes sense. > >Regards >Mark Spencer >> Message: 5 >> Date: Mon, 03 Dec 2012 00:17:47 +0100 >> From: Magnus Danielson <mag...@rubidium.dyndns.org> >> To: time-nuts@febo.com >> Subject: Re: [time-nuts] Measuring GPS noise floor (sort >> of) >> Message-ID: <50bbe19b.3040...@rubidium.dyndns.org> >> Content-Type: text/plain; charset=ISO-8859-1; format=flowed >> >> On 12/02/2012 11:37 PM, Mark Spencer wrote: >> > I'm wondering how useful it is to compare two different >> GPS receivers to a single Rb to get a sense of the GPS noise >> floor. I've been collecting some data for >> the last week or so seem be making some headway with this. >> > >> > Not having a cesium or an Hmaser I elected to compare >> two different GPSDO's to a single Rb and then assuming the >> data seemed sane, I thought I would then compute the Adev >> between the two GPSDO's. My assumption is >> that by comparing the two GPSDO's to the Rb it should be >> possible to detect when one GPSDO is mis behaving or >> possibly when the Rb mis behaves. So far other than a >> bit of a glitch at the start of the process things seem to >> be proceeding as I expected. I hope to >> collect another two weeks or so of data if all goes >> according to plan. >> > >> > I'm really only interested in the longer term numbers >> and I wouldn't put to much trust in the shorter term >> results. >> > >> > Comments would be welcome especially if there is a >> fundamental flaw with this process. >> > >> > Looking at the data gave me something to do this >> afternoon during some down time on a business trip (: >> I reazlize this approach is probably not how the pros would >> do this. >> >> You do realize that several issues with the GPSDOs will be >> common mode, >> such as the errors of the ionspheric model and actual >> ionspheric effect >> and that of multipath. The rubidium will to some degree aid >> to >> illustrate these shifts, but it will be accounted for on the >> rubidium >> deviation from the common. If you look at the CRIT paper we >> discussed >> yesterday, they did exactly what you proposes. For most >> stuff the 5 >> recievers they used didn't show much differences. If you >> increase the >> resolution you might get out more. You might have use for >> additional >> local references, in order to separate out other effects. >> For instance, >> with three rubidiums measured, you could single out which of >> them has an >> error, and any common mode effects of the GPSDOs could be >> fairly well >> separated out from the local performance of your rubidiums. >> >> Cheers, >> Magnus >> >> >> > > _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
