Re: [time-nuts] Sneaky Errors
It would be easy to modify my GPSMonitor project to drive a pin with an alarm signal if the GPS has lost lock. What you do with that signal is up to you. See the project page at http://www.ko4bb.com/Timing/GPSMonitor/ Didier KO4BB Sent from my Verizon Wireless BlackBerry -Original Message- From: David VanHorn d.vanh...@elec-solutions.com Sender: time-nuts-boun...@febo.com Date: Fri, 21 Oct 2011 10:43:28 To: Discussion of precise time and frequency measurementtime-nuts@febo.com Reply-To: Discussion of precise time and frequency measurement time-nuts@febo.com Subject: Re: [time-nuts] Sneaky Errors Playing with my A unit Timex (It takes a licking and keeps on ticking) I'm not getting a reliable unlock even at + or - 5V Once unlocked, it seems to need to be inside 1.2V before locking. That results in about 4 hz error, which would be ok for me. Is there a way to automatically turn off the output if the GPS is unlocked? From: time-nuts-boun...@febo.com [time-nuts-boun...@febo.com] On Behalf Of Mark Sims [hol...@hotmail.com] Sent: Thursday, October 20, 2011 9:39 PM To: time-nuts@febo.com Subject: [time-nuts] Sneaky Errors 1 ppm at 10 MHz is 10 Hz. It's been quite a while since I tested it, but if I remember correctly, if the Tbolt OCXO is off that far the GPS won't lock and the failure will be rather obvious. The typical Tbolt oscillator has a DAC gain of 3.5 Hz/V. 10 Hz would be 3V of DAC offset. Lady Heather can determine your actual oscillator gain constant using the oscillator autotune function ( A from the keyboard). From that you can calculate the DAC offset required to get you 10 Hz of shift. Put the unit into manual DAC control mode and set the DAC voltage to what is needed to shift the freq that much. See if it locks... ___ 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. ___ 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] Sneaky Errors
If you are very conservative or if this is for some critical requirement then I'd get something other than another TB. That's a good point. There are 2 obvious single points of failure in using TBolts. One is TBolt firmware (or obscure design error). The other is the GPS system. If you want to protect against things like firmware bugs, I think you have to get 3 independent designs. If you have 2 TBolts and 1 other receiver, a bug in the TBolt firmware will out vote the sane 3rd receiver. I don't know how much to worry about problems in the GPS system. The military probably considers it to be very important so I expect they have thought hard about how to keep it going. Politics might be important. For example, somebody might decide to turn selective availability back on. At the receiver, you obviously have to worry about things like a shared antenna with a splitter. Even if you have separate antennas, the cables probably run near to each other and the antennas are probably close to each other where they might get knocked out by the same snowstorm, bird nest, or object falling from the sky. -- These are my opinions, not necessarily my employer's. I hate spam. ___ 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] Sneaky Errors
Yup, I agree.. If I could count on the Tbolt output going dead if there's a problem, that would be wonderful, but my worst fear is that it would keep on going but be wrong enough to matter. We are calibrating other equipment against the TB output to 1ppm so the TB would have to be pretty far off before it would matter, but I could see it happening. 1 ppm is pretty easy to detect. How about something like a counter and a PC. Use the crystal in the counter as a 3rd opinion. You don't care if the crystal is off a bit. You are just assuming it is reasonably stable. Of course, now you have added a PC to the system. -- These are my opinions, not necessarily my employer's. I hate spam. ___ 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] Sneaky Errors
1 ppm at 10 MHz is 10 Hz. It's been quite a while since I tested it, but if I remember correctly, if the Tbolt OCXO is off that far the GPS won't lock and the failure will be rather obvious. The typical Tbolt oscillator has a DAC gain of 3.5 Hz/V. 10 Hz would be 3V of DAC offset. Lady Heather can determine your actual oscillator gain constant using the oscillator autotune function ( A from the keyboard). From that you can calculate the DAC offset required to get you 10 Hz of shift. Put the unit into manual DAC control mode and set the DAC voltage to what is needed to shift the freq that much. See if it locks... ___ 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] Sneaky Errors
I have two thunderbolts, set up so that I can switch over to the backup unit if the primary fails. All is well with that, but what could I do to detect a less obvious failure, like 9.99 MHz output? If they disagree, I don't know how to resolve which is correct. ___ 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] Sneaky Errors
The thunderbolts produce status information and error estimates about how they are doing. If you are willing to trust that, you can remove the one that is falling out of tolerance. If you aren't willing to trust that, then I'm pretty sure you'll need a third frequency standard to compare each to. Thanks, -JP On Wed, Oct 19, 2011 at 3:46 PM, David VanHorn d.vanh...@elec-solutions.com wrote: I have two thunderbolts, set up so that I can switch over to the backup unit if the primary fails. All is well with that, but what could I do to detect a less obvious failure, like 9.99 MHz output? If they disagree, I don't know how to resolve which is correct. ___ 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] Sneaky Errors
I have two thunderbolts, set up so that I can switch over to the backup unit if the primary fails. All is well with that, but what could I do to detect a less obvious failure, like 9.99 MHz output? If they disagree, I don't know how to resolve which is correct. Get a 3rd. 2 good guys can outvote 1 bad guy. -- These are my opinions, not necessarily my employer's. I hate spam. ___ 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] Sneaky Errors
As Hal says and as in instrument flying, three independent sources of data before reaching a conclusion. If one disagrees, throw it out. A third source is in order. A third TBolt, an LPRO-101 adjusted to and compared regularly to each TBolt, or another GPSDO of another flavor. Also, a CS unit that can be turned on or off to compare. Joe -Original Message- From: time-nuts-boun...@febo.com [mailto:time-nuts-boun...@febo.com] On Behalf Of Hal Murray Sent: Wednesday, October 19, 2011 3:03 PM To: Discussion of precise time and frequency measurement Subject: Re: [time-nuts] Sneaky Errors I have two thunderbolts, set up so that I can switch over to the backup unit if the primary fails. All is well with that, but what could I do to detect a less obvious failure, like 9.99 MHz output? If they disagree, I don't know how to resolve which is correct. Get a 3rd. 2 good guys can outvote 1 bad guy. -- These are my opinions, not necessarily my employer's. I hate spam. ___ 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] Sneaky Errors
On Wed, Oct 19, 2011 at 12:46 PM, David VanHorn d.vanh...@elec-solutions.com wrote: I have two thunderbolts, set up so that I can switch over to the backup unit if the primary fails. All is well with that, but what could I do to detect a less obvious failure, like 9.99 MHz output? If they disagree, I don't know how to resolve which is correct. The only way is to have third or fourth 10Mhz oscillator. If you are very conservative or if this is for some critical requirement then I'd get something other than another TB. It is always remotely possable that the reason for out of spec performance is a design flaw. Having three identical units would never catch this kind of problem as it would hit all three at the same time.Using that same line of logic. Maybe your third redundant standard should not depend on GPS. I'd say you might want a Rubidium. Then you compare all three periodically check that the two TB track each other and that the Rb drifts slowly away. Later if ever the two TBs fail to track each other you will need all that drift data you collected to figure out which TB is correct because it's only 50/50 that the correct TB would be the one closest to the Rb. ___ 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. -- Chris Albertson Redondo Beach, California ___ 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] Sneaky Errors
Simple way is to use LadyHeather. From its output, you can tell if either or both are working correctly as well as how well they are working. That is assuming of course that there is at least one working GPS satellite in view at all times. If not it well tell you that also. ws David VanHorn D.VanHorn at elec-solutions.com I have two thunderbolts, set up so that I can switch over to the backup unit if the primary fails. All is well with that, but what could I do to detect a less obvious failure, like 9.99 MHz output? If they disagree, I don't know how to resolve which is correct. ___ 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] Sneaky Errors
This is the famous man who has two watches does not know what time it is problem. Lucent solved it for telecom with the RFTGm (Reference Frequency and Timing Generator) equipment, consisting of GPS disciplined OXO and Rubidium oscillator modules that continuously checked each other via 1 PPS and 10 MHz links. The frame housing the oscillator units selected one or the other for distribution to six connectors. Sadly, the output is 15 MHz. Otherwise, the only solution is more watches, preferably by different manufacturers. In some ways, this reminds me of the ancient parity check for memory locations. Parity is not used anymore for commercial computers, because a memory error is usually not alone, and errors soon make the computer lose its way and halt. In this case, plan on the TB either running correctly or failing due to some alarm. Alarms must be monitored, of course. You could set up a time interval counter to show the phase between the two outputs. The Racal-Dana 1992 does that at 10 MHz. Out of curiosity, what would be the consequences of a steadily increasing phase error? Would it offend your sense of perfection or would it have real consequences? Bill Hawkins -Original Message- From: David VanHorn Sent: Wednesday, October 19, 2011 2:47 PM I have two thunderbolts, set up so that I can switch over to the backup unit if the primary fails. All is well with that, but what could I do to detect a less obvious failure, like 9.99 MHz output? If they disagree, I don't know how to resolve which is correct. ___ 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] Sneaky Errors
Out of curiosity, what would be the consequences of a steadily increasing phase error? Would it offend your sense of perfection or would it have real consequences? Phase error wouldn't bug me. My worst fear is that the 10.00 MHz standard might be 10.02 MHz. I need to implement something that is at least relatively simple or best case, automatic. ___ 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] Sneaky Errors
On 10/20/2011 12:18 AM, David VanHorn wrote: Out of curiosity, what would be the consequences of a steadily increasing phase error? Would it offend your sense of perfection or would it have real consequences? Phase error wouldn't bug me. My worst fear is that the 10.00 MHz standard might be 10.02 MHz. I need to implement something that is at least relatively simple or best case, automatic. Steadily increasing phase error is to let there be a frequency error. Frequency is the derivate of phase, so it comes with the territory. So a 200 ns per second phase drift would provide a frequency error of 2 Hz on your 10 MHz. Can't have one without the other. 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] Sneaky Errors
Steadily increasing phase error is to let there be a frequency error. Frequency is the derivate of phase, so it comes with the territory. So a 200 ns per second phase drift would provide a frequency error of 2 Hz on your 10 MHz. Can't have one without the other. I understand, I'm just saying that if the absolute phase is wandering a bit over tens of seconds, it's NOT an issue. ___ 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] Sneaky Errors
On 10/20/2011 12:57 AM, David VanHorn wrote: Steadily increasing phase error is to let there be a frequency error. Frequency is the derivate of phase, so it comes with the territory. So a 200 ns per second phase drift would provide a frequency error of 2 Hz on your 10 MHz. Can't have one without the other. I understand, I'm just saying that if the absolute phase is wandering a bit over tens of seconds, it's NOT an issue. So, you are saying that the stability is not that important. Fair enough. I also assume that absolute phase biases is not of your concern... so that your second is shifted by say 4711 us doesn't hurt you, as long as you get your 10 MHz on average. What frequency stability do you need, over which time? (i.e. what ADEV value for which tau) 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] Sneaky Errors
Seems to me that 200 ns is 720 degrees of phase error, which is a lot. A person handy with logic circuits could build a simple phase detector with a flip-flop and an RC filter with a tenth second time constant. An analog circuit could detect 360 degree rollover and set off alarm bells. Note that you still have the two-watch problem. Two equal divider chips ahead of the flip-flop could allow larger errors before rollover. The error may reverse itself and run the phase error down, and then reverse again as the two ovens cycle at different rates. This would be normal behavior, unworthy of an alarm. An additional challenge would be to build logic to select the oscillator output to be distributed, then compare the output to the output of three oscillators in three phase detectors. The device that had phase rollover would put itself in standby, alarm, and leave you with the two watch problem. Perfection demands many oscillators with a voting system. Long winter nights could be spent solving these problems. I'm too old for that stuff. (I also post the most recent ideas first, so as not to reread old ideas.) Bill Hawkins -Original Message- From: David VanHorn Sent: Wednesday, October 19, 2011 5:58 PM Magnus said (note the attribution), Steadily increasing phase error is to let there be a frequency error. Frequency is the derivate of phase, so it comes with the territory. So a 200 ns per second phase drift would provide a frequency error of 2 Hz on your 10 MHz. Can't have one without the other. I understand, I'm just saying that if the absolute phase is wandering a bit over tens of seconds, it's NOT an issue. ___ 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] Sneaky Errors
I believe that the Thunderbolt firmware would catch such a thing.There is quite a bit of error checking and TRAIM (time receiver autonomous integrity monitoring) done. If the osc was off in freq, the firmware would try to use the EFC voltage to slew it back into agreement with the GPS signal. If the osc was off freq or not controllable, the DAC voltage would quickly reach it's control limit and the unit would alarm out. Of course, you have to be monitoring the alarms to know this has happened. Also, if the osc freq is not accurate, the unit can not decode the GPS signal since it uses the osc to generate the GPS receiver local oscillator. If the unit is tracking GPS satellites, your are pretty much assured that it is working. ___ 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] Sneaky Errors
Maybe this is too simple LadyHeather is always checking the Tbolt's Internal Osc value against the GPS. By watching it's plot outputs you can tell if the Tbolt is on freq. (compared to the GPS) If no plot outputs, then something is broken, at that point is does not matter what, can assume it is that Tbolt. A couple of other things to assume also for this to be the solution to the problem. 1)The GPS is Always right and is correct, IF one of the Tbolts fails IN any way. The GPS is used as the 3rd Osc to vote who is wrong when there is disagreement between the two Tbolts. 2) The Tbolt always has an output that is at the same freq as it's internal Osc. While not a critical fail safe for everything, this does cover the case requested. If the two Tbolt do not agree, what one is wrong. Answer is the one that is different than the GPS When they are both close to the same freq (with-in 1e-9), don't need the GPS's vote, if you assume they are both right. ws * As long as the Thunderbolt is working perfectly it's built in error checking will work fine. This should be obvious as anything that works, works. But I can think of plenty for failure modes where the built in error checking might not work. First off there could be a software or firmware bug or a stuck bit in a ROM. An open conecton on a DAC, lots of things. The assumption has to be the broken hardware is totally unpredictable We can not know in advance what it will do after it breaks. I'm not saying the built-in diagnostics are useless, just that they are never 100% reliable On Wed, Oct 19, 2011 at 2:18 PM, ws at Yahoo warrensjmail-one at yahoo.com wrote: Simple way is to use LadyHeather. From its output, you can tell if either or both are working correctly as well as how well they are working. That is assuming of course that there is at least one working GPS satellite in view at all times. If not it well tell you that also. Chris Albertson Redondo Beach, California ___ 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.
