From jimr@maia.usno.navy.mil Thu Dec 3 09:30:20 EST 1998 Received: (from jimr@localhost) by maia.usno.navy.mil (8.8.6 (PHNE_14041)/8.8.6) id JAA25879 for gpst@maia; Thu, 3 Dec 1998 09:24:50 -0500 (EST) From: Jim Ray (USNO 202-762-1444)Message-Id: <199812031424.JAA25879@maia.usno.navy.mil> Subject: 1 pps monitor problems at AMC2 & USNO To: gpst@maia.usno.navy.mil Date: Thu, 03 Dec 1998 9:24:49 EST X-Mailer: Elm [revision: 212.4] Status: RO [It is not really my place to send this notice. However, since the problem described here has led to considerable misunderstandings among users of the 1 pps monitor data from the AMC2 station in Colorado Springs, which was evident yesterday at the PTTI meeting, this report is being sent even though the information may be incomplete. --Jim] Since the replacement of the TurboRogue SNR-12 receiver in Colorado Springs (AMCT -> AMC2) on 20 October 1998, a persistent problem has been observed in the monitor 1 pps output data. (The 1 pps data are available at the site ftp://tycho.usno.navy.mil/pub/usnodc-amc.gpscp/ in the file "amc1-amct.1pps". Please see the "readme" file there for explanations.) At irregular intervals, several times a week, spikes are recorded that deviate by almost 3 ns from neighboring values. After conferring with Larry Young (JPL), Paul Wheeler (Time Service Dept., USNO) did some checking and found that the problem was caused by the odd shape of the 1 pps output pulse from the TR. He showed me an oscilloscope trace that looked something like: | | | +++++++++++++++ | + | + | + | + + | + + + | + + + |+ + + |+ + + 1V + + +| +| + | + | + | + | + | + | -------------+----------|------------------------ ~3ns The rise is initially steep but it reaches a peak (> 1V) then dips back down to close to the 1V level before rising again more slowly to the pulse plateau. With the trigger level set for 1V there are occasional instances when the mid-rise dip comes down to 1V causing a secondary trigger about 3 ns later. According to Paul, all 3 TRs that we own have this 1 pps pulse shape. Only the receiver newly installed in Colorado Springs seems to have a dip that is sometimes low enough to cause a secondary trigger at 1 V. The other 2 receivers have not been observed to have 1 pps glitches (or at least not often enough to cause concern). Larry suggests that the poor pulse shape appears to be caused by a bad impedance match which produces a reflected signal in a cable about 1.5 ns long. Paul has argued that the TRs have "no proper driving circuit" and a variety of different impendances have not improved the pulse shape. Larry does not observe similar performance in the TRs he has tested. (If anybody has a resolution to this discrepancy, I have not heard about it.) Regardless of the cause, a simple solution is to lower the trigger level to 0.5V, which was done on 12 November 1998 for the receiver here at USNO (after the disruption caused by having to remove and reinstall the antenna). This is just a precaution in this case since we have not seen any problems here. Note that this action will introduce a small bias in the time series of 1 pps measurements for (Master Clock 2 - USNO) compared to before and after the change so we would not be able to calibrate for the clock reset due to the antenna work on 12 November as accurately as before. But this is a one-time problem only. At Colorado Springs, the situation is complicated by the measurement system there which is used to monitor a number of other things. Apparently it is not so easy to change the trigger level for the TR only without adversely affecting other unrelated measurements. Paul is continuing to look into other possible solutions for that receiver. In the meantime, the 1 pps data from AMC2 must be filtered to remove the spikes before being used to interpret the TR receiver performance.