Randy Warner wrote: > Paul is right about the temp sensitivity. I am surprised he managed to > keep the receiver balancing on one foot for seven minutes, but if you > try hard enough, I guess anything is possible. > > I thought everyone on this thread might be interested in what is causing > the hanging bridge, so here goes. > > All Oncore receivers (M12+, UT+, GT+, M12M) are basically run by an > internally generated 1KHz clock that is "normally" asynchronous to the > 1PPS epochs. If you could compare this 1KHz to the 1PPS on a scope you > would see the 1KHz slowly walking in comparison to the 1PPS. Under > typical conditions these "time slips" occur about once every 15 seconds > or so. Every time you see a "hanging bridge" you know that a time slip > has occurred. Naturally, as temp is changed the 1KHZ will change > frequency, changing the time between each bridge, but it's usually in > the 10-20 second range. > > I have seen several receivers returned over the years that the customer > returned because of the sawtooth not working. Naturally, when the > receiver is fired up in my lab it works perfectly....... After reading > PHK's post I'll bet even money that the 1KHz became synchronous in the > affected receiver for whatever reason. > > Those of you who worked with the UT+ receiver may remember a problem > with the time slips in one version of firmware. Motorola updated the > firmware to do some minor housecleaning and accidentally messed up the > 100PPS. The 100PPS worked normally until a time slip occurred at which > time the 100PPS pulse train was delayed by 1ms. It immediately corrected > itself on the next second, but having your 1PPS slip 1ms every 15 > seconds or so was a real show-stopper. Luckily, few people use the > 100PPS function, so it was not a critical problem. I sure got REAL TIRED > of reflashing those receivers however..... > > > Randy > ________________________________________________________________________ > ____________________________ > > -----Original Message----- > From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On > Behalf Of Poul-Henning Kamp > Sent: Thursday, December 21, 2006 11:17 AM > To: Tom Van Baak; Discussion of precise time and frequency measurement > Subject: Re: [time-nuts] GPS orthodontics: sawteeth & hanging bridges > -theeffect of time averaging > > In message <[EMAIL PROTECTED]>, "Tom Van Baak" > writes: > > >> 1 second raw samples: 10.40 ns >> then removing small linear frequency offset, >> 1 second samples: 9.36 ns >> 30 second averages: 9.62 ns >> 300 second averages: 10.0 ns >> >> Did I do something wrong? PHK, what do you think about this? >> > > Given what we know, the saw-tooth term has a box distribution from -A to > +B nanoseconds where A and B depends on the clock frequency used in the > receiver. > > What we don't know is if A = B or for that matter if A and B are equal > to the period of the clockfrequency. > > It is not inconceiveable that A might be 1 nanosecond larger than B due > to rounding and timing issues in the firmware and hardware and likewise > it is not inconceiveable that A and B are slightly larger than the clock > period for similar reasons. > > We think we know that the negative sawtooth is a box distribution with > equal probability over the -A...+B interval, although the flanks may not > in fact be vertical, due to the mentioned rounding issues. > > The theoretical average therefore must be the average of A and B > possibly with a minor error by the possibly nonvertical flanks. > > The standard deviation must be approached with caution since it is a box > distribution and not a standard deviation. > > So, based on what we think we know, the theoretical average probably is > zero, but could be a few nanoseconds on either side due to rounding and > timing. > > The final thing we know is that no averaging time is long enough to > guarantee anything about the negative sawtooths actual average. > > If you are unlucky, and your oncore is at a very stable temperature, the > clock generator may just stubbornly decide to run at an integer Hz for > an hour and if by pure bad luck the sawtooth value was 20ns at the start > of that, it will be 20 nsec all the way through. > > In practice, the probability for this is quite low, but I have tried > once to provoke , and it is not only possible, it is disturbingly easy: > > Wrap your oncore in a couple of blankets or similar and tweak the > temperature (I varied the supply voltage so the voltage regulator loss > changed the temperature. It takes a couple of hours to get it just > right. I saw a hanging bridge of slightly over seven minutes this way. > > One interesting result of this is that the PRS10 will run better without > the 256 exponential filter if you feed it from an oncore, if your oncore > is in a good enclosure, but if you leave it out where the temperature > changes, the 256 exponential filter helps. > > Poul-Henning > > Randy
If only one had access to the receiver firmware: the cure for "hanging bridges" etc is surely to add sufficient (rms noise ~ PPS positioning quantisation error) bandlimited gaussian noise to calculated PPS position before quantisation of the PPS position occurs in the hardware. Bruce _______________________________________________ time-nuts mailing list [email protected] https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
