From: "Tom Van Baak" <[EMAIL PROTECTED]> Subject: Re: [time-nuts] How do I know my GPS stabilizedoscillator is working? Date: Sun, 30 Jul 2006 19:54:34 -0700 Message-ID: <[EMAIL PROTECTED]>
Tom, > 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. This is a problem for some counters. Others will not have that problem, in particular those with zero dead-time. Then event counters and time counters with interpolators will keep you on track regardless. It also depends on the particular measurement selected. They may vary in particular details, which may bite you. Choose the right one and your post-processing becomes a bliss of simplicity. > 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: We in the telco buissness do this, it is really necessary and the only way to convey a known error. Even when you have a side-channel with more info (like a serial line), you need to be prepared for the lack of "propper signal" so this is a good way, since relying on the side channel may lie to you if you loose signal, and besides, no side channel looks the same and may even be non- existent or otherwise unreasnoble to handle. > (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. Well, in your above measuring case with 34 missing pulses, you *DO* have a problem if you are going to do Allan Deviation (ADEV), Modified Allan Deviation (MDEV) or Time Deviation (TDEV). Replacing the start and stop as you did above will destory that information. The gap will cause a measurement error. For ADEV/MDEV/TDEV measurements you also want to compensate out driftrate before doing the actual calculation. The frequency error is not a problem for the calculation, but the frequency error can be a problem for some, but not all, counters. Cheers, Magnus _______________________________________________ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts