I'd also be interested in any anecdotes out there with regard to this subject. Here's my 2 cents...
Start/Stop -- Either way works and gives the same data. Both ways make your brain spin when you have to figure the sign(s) of the phase, frequency, and drift. DUT -- Note sometimes it's hard to know what "DUT" is. If you're comparing a surplus GPSDO you just picked up with a cesium that you already trust, which is the DUT? In the short-term the Cs is the reference, making the GPSDO the DUT. But in the long-term, if the GPSDO works well, it will be a better reference and your cesium becomes the DUT. So there are cases where the DUT/start/stop issue is ambiguous. Zero -- The one thing that's a pain is sign wrap in time interval data. One solution is to offset one or the other by some amount prior to the experiment. I keep one of my standards ahead (or is it behind) by about 10 us to virtually guarantee all positive time-interval readings. Clearly then the earlier standard, which ever it is, gets the start channel. Drift -- Another consideration is when you know if the DUT is drifting forward or backward in time. In this case arrange the start/stop channels so that, over time, you always diverge from zero rather than approach and cross zero. This argues for putting the device with higher frequency as the start channel (lower frequency as the stop channel). Or is it the other way around. 1PPS -- Lastly, there is one convention I found handy with 1 PPS sources, specifically those GPS boards that are designed to suppress the 1PPS signal when they loose lock. In this scenario make the reliable 1PPS ref the start channel and the GPS 1PPS the stop channel. If lock is ever lost -- your TI readings will reveal the number of missing pulses. For example: (REF=start, GPS DUT=stop) 8.9755 us 8.9368 us 8.8986 us 8.9214 us 8.8829 us 34.0000088063 s 8.8310 us 8.7332 us If you wire the TIC the other way around, then those missing 34 pulses won't even show up in your data. For example: (GPS DUT=start, REF=stop) 8.9755 us 8.9368 us 8.8986 us 8.9214 us 8.8829 us [followed by 34 seconds of silence] 8.8063 us 8.8310 us 8.7332 us In summary, there's isn't a right way and a wrong way. There may be, in some cases, a convenience of one over the other. /tvb ----- Original Message ----- From: "John Ackermann N8UR" <[EMAIL PROTECTED]> To: "Discussion of precise time and frequency measurement" <time-nuts@febo.com> Sent: Sunday, July 30, 2006 18:33 Subject: Re: [time-nuts] How do I know my GPS stabilizedoscillator is working? > My understanding is that the convention is to put the DUT on the start > and the reference on the stop (reversing the two will obviously reverse > the sign of the offset and drift) but I am not sure how widely accepted > that convention is. > > I'd be really happy to have some confirmation that in fact I'm doing it > the right way. > > John > ---- > > Glenn said the following on 07/30/2006 04:28 PM: > > Just curious, why do you use GPS for the stop input instead of the other > > way 'round? > > -glenn > > > > > >>John Ackermann N8UR wrote: > >> > >> > >> > >>>Hi Faisal -- > >>> > >>>I use a time interval counter to compare 1pps signals -- the DUT goes to > >>>the start input, and the reference (here, GPS) to the stop input. > >>> _______________________________________________ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts