Re: [time-nuts] pps vs. 10 MHz timing
Matt Ettus skrev: On my Oscilloquartz GPSDO, the 10 MHz output goes low at very close to the same time as the 1 PPS output goes high. On my Fury, the 10 MHz sine wave is just dropping off of its max high voltage as the 1 PPS goes high. The 10 MHz CMOS output goes high just shortly before the 1 PPS. What do other GPSDOs do? You can expect anything. Some buffers the PPS from the GPS engine and then make some more or less elaborate 10 MHz training algorithm. Don't expect rising edges to coinside for all of them. Clearly, any device trying to latch the 1 PPS signal using the 10 MHz clock will need to choose which clock edge to use, depending on which type of GPSDO is used. How is this normally handled? Depends, several approaches is possible. A simple approach is just a dual flip flop approach which is simple and straightforward. There is a risk for it to switch between 10 MHz cycle, but it may be acceptable for some applications. You could use any of many forms interpolating techniques to reduce or remove this risk. Exactly what you can achieve depends not only on the receiver hardware but also the transmitter side. The trouble is that the 10 MHz + PPS is a very weekly defined interface which is not properly standardised. The only standard I know off is an old US MIL-STD document. Also, I noticed that the TADD-1 has some weird distortion of a 10 MHz sine wave. It turns it into a sort of a 3 valued square wave. Essentially, it stops and dwells at the midpoint for a while instead of making a sharp transition. I think this would be bad if that dwell point is close to the threshold for the receiving device. Has anyone else noticed this? Yes, on a signal not properly terminated. It occurs if you T a signal to your scope while the signal goes further on... to an open end. Then the reflex signal will create this type of dwelling. 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.
Re: [time-nuts] time-nuts Digest, Vol 53, Issue 46
On Thu, 2008-12-11 at 14:38 -0800, Brooke Clarke wrote: Hi Mark: I think it's out of date. The current method is to drop an optical corner cube (retro-reflector) in a vacuum and using a laser measure the distance it moves (which requires a reasonably good source of time. http://www.ngs.noaa.gov/GRD/GRAVITY/ABSG.html Have Fun, Brooke Clarke http://www.prc68.com Mark Sims wrote: The quintessential gravity meter (Worden Gravity Meter by Texas Instruments)... still being made after 60 years or so: http://www.mssu.edu/seg-vm/pict0246.html Spring based (relative) gravimeters are NOT yet dead... http://large.stanford.edu/courses/ph210/lee1/ and in local lingo for those so inclined... http://www.lantmateriet.se/templates/LMV_Page.aspx?id=4912 These are not _as_ good, but very mobile http://inertialsensor.com/qa3000.shtml -- Björn ___ 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.
Re: [time-nuts] pps vs. 10 MHz timing
Hello Matt, Magnus, unfortunately there will be a phase shift between the 1PPS and the 10MHz rising edge due to the nature of how we generate the 1PPS pulse from the clean OCXO 10MHz signal. But this trade-off allows us to offer the 1PPS phase-shift option via the SERV:1PPS command. We can simply shift the 1PPS by 16.66ns steps with the software command, which would not be possible if we had to align the signal to the 10MHz, 100ns period. The system works by multiplying the 10MHz signal up to 60MHz using a PLL, then using a 60E6-to-1 divider with arbitrary phase alignment (selected by the user) to generate the clean 1PPS output. This allows a resolution of 16.66ns on where to place the clean 1PPS using the serv:1pps command. As Magnus has suggested, if you need a strict phase alignment, then using FF's to latch the 1PPS rising edge with the 10MHz signal can phase-align the two signals. Due to metastability being possible, metastable-hardened FF's (from NXP etc) should be used. Alternatively an external 10E6-to-1 divider could also be used for this, clocked by the 10MHz signal. There would still be at least a clock-to-output phase shift when using FPLD's etc to implement this divider. BTW: by using the serv:1pps command for the coarse setting, and the GPS antenna-delay command for fine-setting the user can place the 1PPS pulse anywhere with 1ns resolution on the Fury GPSDO. The coarse 16.66ns steps will be instantaneous, the 1ns antenna-delay steps will take some time for the PLL to shift. Hope that helps, bye, Said In a message dated 12/12/2008 12:33:25 Pacific Standard Time, mag...@rubidium.dyndns.org writes: Matt Ettus skrev: On my Oscilloquartz GPSDO, the 10 MHz output goes low at very close to the same time as the 1 PPS output goes high. On my Fury, the 10 MHz sine wave is just dropping off of its max high voltage as the 1 PPS goes high. The 10 MHz CMOS output goes high just shortly before the 1 PPS. What do other GPSDOs do? You can expect anything. Some buffers the PPS from the GPS engine and then make some more or less elaborate 10 MHz training algorithm. Don't expect rising edges to coinside for all of them. Clearly, any device trying to latch the 1 PPS signal using the 10 MHz clock will need to choose which clock edge to use, depending on which type of GPSDO is used. How is this normally handled? ___ 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.
[time-nuts] gravimeters
-Original Message- From: time-nuts-boun...@febo.com [mailto:time-nuts-boun...@febo.com] On Behalf Of Björn Gabrielsson Sent: Friday, December 12, 2008 1:55 PM To: bro...@pacific.net; Discussion of precise time and frequency measurement Subject: Re: [time-nuts] time-nuts Digest, Vol 53, Issue 46 On Thu, 2008-12-11 at 14:38 -0800, Brooke Clarke wrote: Hi Mark: I think it's out of date. The current method is to drop an optical corner cube (retro-reflector) in a vacuum and using a laser measure the distance it moves (which requires a reasonably good source of time. http://www.ngs.noaa.gov/GRD/GRAVITY/ABSG.html Have Fun, Brooke Clarke http://www.prc68.com Mark Sims wrote: The quintessential gravity meter (Worden Gravity Meter by Texas Instruments)... still being made after 60 years or so: http://www.mssu.edu/seg-vm/pict0246.html Spring based (relative) gravimeters are NOT yet dead... http://large.stanford.edu/courses/ph210/lee1/ See, there *is* a need for garage Liquid Helium plants... ___ 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.
Re: [time-nuts] gravimeters
The problem with helium is that it comes out of oil wells. It does not cost much because they were digging up the oil anyway. If what they say about peak oil is true, in 30 years or so there will be no more helium and nowhere to get it, save tiny quantities from nuclear plants. It seems to escape from earth's atmosphere to be lost for all time. We should be doing all the research that needs helium now, because we will not be able to do it in the future. Every time my grand kids play with a helium balloon I feel guilty. Neville mIchie On 13/12/2008, at 9:15 AM, Lux, James P wrote: -Original Message- From: time-nuts-boun...@febo.com [mailto:time-nuts-boun...@febo.com] On Behalf Of Björn Gabrielsson Sent: Friday, December 12, 2008 1:55 PM To: bro...@pacific.net; Discussion of precise time and frequency measurement Subject: Re: [time-nuts] time-nuts Digest, Vol 53, Issue 46 On Thu, 2008-12-11 at 14:38 -0800, Brooke Clarke wrote: Hi Mark: I think it's out of date. The current method is to drop an optical corner cube (retro-reflector) in a vacuum and using a laser measure the distance it moves (which requires a reasonably good source of time. http://www.ngs.noaa.gov/GRD/GRAVITY/ABSG.html Have Fun, Brooke Clarke http://www.prc68.com Mark Sims wrote: The quintessential gravity meter (Worden Gravity Meter by Texas Instruments)... still being made after 60 years or so: http://www.mssu.edu/seg-vm/pict0246.html Spring based (relative) gravimeters are NOT yet dead... http://large.stanford.edu/courses/ph210/lee1/ See, there *is* a need for garage Liquid Helium plants... ___ 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. ___ 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.
Re: [time-nuts] pps vs. 10 MHz timing
saidj...@aol.com skrev: Hello Matt, Magnus, unfortunately there will be a phase shift between the 1PPS and the 10MHz rising edge due to the nature of how we generate the 1PPS pulse from the clean OCXO 10MHz signal. But this trade-off allows us to offer the 1PPS phase-shift option via the SERV:1PPS command. We can simply shift the 1PPS by 16.66ns steps with the software command, which would not be possible if we had to align the signal to the 10MHz, 100ns period. Being able to fine-tune PPS-10 MHz phase relationship on the output is certainly a good thing. Some have very tight requirements for this relationship, but it seems to me that they overspec it since they do not really know how things work and it is easy to verify. On the other hand, for some equipment I haven't seen any reasnoble specs at all on the inputs. Seems they have designed to match what their GPS gives them, whatever that is. The system works by multiplying the 10MHz signal up to 60MHz using a PLL, then using a 60E6-to-1 divider with arbitrary phase alignment (selected by the user) to generate the clean 1PPS output. This allows a resolution of 16.66ns on where to place the clean 1PPS using the serv:1pps command. Sweet. Personally I did it a bit differently, but that's another thing. There are many things to implement the same thing. As Magnus has suggested, if you need a strict phase alignment, then using FF's to latch the 1PPS rising edge with the 10MHz signal can phase-align the two signals. Due to metastability being possible, metastable-hardened FF's (from NXP etc) should be used. Alternatively an external 10E6-to-1 divider could also be used for this, clocked by the 10MHz signal. There would still be at least a clock-to-output phase shift when using FPLD's etc to implement this divider. You can use a FPLD for the counter state, but then use a good DFF to clock the state into a low jitter form. Since both is running of the same clock, just ensuring propper setup and hold times at the DFF input would suffice to ensure it is not be unstable. Just traditional engineering. You really can't sync up some PPSes to the 10 MHz by simple DFFs since the variations may be more than 100 ns during some phases of the training and holdover shifts. The PPS output should always be generated using the 10 MHz such as they have a stable phase-relationship. The generated PPS is then compared to the source PPS and controlled phase adjustments should be done. 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.
Re: [time-nuts] gravimeters
There is just something nice about a 60 year old piece of technology (that is still being made and used daily) that can be schlepped by a trained ape (aka graduate student) to the remote corners of the earth and back and measure something as piddly as gravity to parts per billion resolution... and it is totally mechanical (except for perhaps a light bulb). -- Spring based (relative) gravimeters are NOT yet dead... http://large.stanford.edu/courses/ph210/lee1/ See, there *is* a need for garage Liquid Helium plants... _ Send e-mail anywhere. No map, no compass. http://windowslive.com/Explore/hotmail?ocid=TXT_TAGLM_WL_hotmail_acq_anywhere_122008 ___ 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.
