Hypothetical question - how do you calibrate highly accurate time base? - Page 1
Forum section: electronics / general

author: tkamiya
author_rank: Super Contributor
timestamp: November 21, 2018, 03:02:02 am
content: This is purely an hypothetical question -I have a GPDSO. Actually two of them running. I can't calibrate these except to compare both. Then I still don't know which one is correct should they differ.How do you calibrate even more accurate time bases? Rubidium, Cesium, and Hydrogen Maser. And just for heck of it, F1 time base at NIST. I understand Cesium is sensitive to earth's magnetic field. That means it will be affected even more for localized magnetic sources. What do you compare it with to calibrate this?Even worse, like NIST, if that is the most accurate time source, how do you know?? By definition, one cannot compare it with more accurate source. Average of lower precision unit will be uncertain.Likely, this is never going to be a problem I have to solve. But I'm very curious.

author: DaJMasta
author_rank: Super Contributor
timestamp: November 21, 2018, 04:27:41 am
content: NIST actually has a lot of information on their time sources, methods, and history available from their website. Stuff like this make a good primer on things:https://www.nist.gov/publications/nist-primary-frequency-standards-and-realization-si-secondThat said, the SI definition of a second is actually set as "The second is the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium 133 atom."https://physics.nist.gov/cuu/Units/second.htmlSo by definition, a cesium fountain of the right isotope is it, but there can still be factors that influence it from the outside or in methods of measuring those oscillations precisely. So you do what always is done, you gather a bunch of really good measurement devices and you measure each other over a long period. If you have enough methods/devices showing a characteristic drift or other instability of your source and you can rule out external factors, you're measuring the noise floor of that instrument, so you made a better standard.Even very good rubidium oscillators only get to low 10^-11 to high 10^-12 of overall stability, so a modern cesium source will be 10x more stable and can be used to calibrate them. Cesium sources are the definition, so then you're just dealing with the issues mentioned for maximum stability, and it seems 10^-13 is around the performance commercial hardware can offer before you go start going for laboratory setups.

author: MiDi
author_rank: Frequent Contributor
timestamp: November 21, 2018, 07:01:09 am
content: Even when comparing all primary standards, you never know which one is correct.But you can evaluate if they are in spec for their given uncertainty and that is what matters.In addition thePTB (german NIST) gives an uncertainty for their primary time standardsof below 1·10-15.The uncertainty of time is one of the best uncertainties humans can measure.

author: Mr. Scram
author_rank: Super Contributor
timestamp: November 21, 2018, 07:12:34 am
content: The problem with time is that it also changes when influenced by gravity or speed. Have two clocks match up and move one across the room or into a cabinet and you could be screwed already. Your brain is older than your feet are.

author: tkamiya
author_rank: Super Contributor
timestamp: November 21, 2018, 02:56:06 pm
content: Where does Hydrogen Maser come into this? Presumably they are 10x more precise than Cs based ones?I've seen somewhere, there is a system where average of 3 Cs steers Hm. THAT seem redundant.

author: DaJMasta
author_rank: Super Contributor
timestamp: November 21, 2018, 06:52:23 pm
content: Not 100% sure, but my guess is that the steering is to keep the drift down - could be that short term stability of the hydrogen maser system is better than a cesium standard, but the cesium is better for drift characteristics, so you effectively are using three to discipline it. Just a guess though, I don't know much about the tech, but having multiple references steering another may just mean that the performance is so close to the noise floor that you need multiple to average together to get a usable reference to steer with.The last fascinating time method I remember hearing about was the mercury ion clockhttps://www.nist.gov/news-events/news/2006/07/mercury-atomic-clock-keeps-time-record-accuracy, which in the last couple of years was miniaturized enough to be flown on a spacecraft.

author: rhb
author_rank: Super Contributor
timestamp: November 23, 2018, 11:38:45 pm
content: If you read the NIST documents on the topic, they use an ensemble of multiple clocks for which they have individual correction functions. It's a long exercise in iterative refinement.

author: cdev
author_rank: Super Contributor
timestamp: November 25, 2018, 01:49:24 pm
content: Do your two GPSDOs 'differ' to any measurable degree?Since you say 'hypothetical' no, right?Quote from: tkamiya on November 21, 2018, 03:02:02 am

author: edpalmer42
author_rank: Super Contributor
timestamp: November 25, 2018, 06:37:57 pm
content: Quote from: cdev on November 25, 2018, 01:49:24 pmThe frequencies of two properly operating GPSDOs will always average out to be the same. However, if you're measuring Allan Deviation, there will be differences between them. The differences are due to differences in OCXO performance and disciplining parameters. There will even be differences between supposedly identical units due to noise.Here's a web page that shows what happens when you measure two GPSDOs against each other.https://www.febo.com/pages/gpsdo_comparison/

author: awallin
author_rank: Frequent Contributor
timestamp: November 25, 2018, 08:10:30 pm
content: Quote from: tkamiya on November 21, 2018, 02:56:06 pm2 eurocents:If you buy a 5071A (Cs) it will be off in frequency by something like +/- 5ns/day - let's say you learn it's +3ns/day. Now keep it in a stable environment and be nice to it and it will be off in frequency by that +3ns/day for years and years.It's a bit noisy though, so from day to day it will wander around that ideal +3ns/day slope - up to maybe +/- 10ns or so...Now OTOH if you get an active hydrogen maser you first need to let it warm up a bit, 3-6 months is good. Then measure it's frequency which might be something like +20 ns/day or simiar (not really that important to have it close to the SI-second). However it will drift, the good ones are 1e-16/day (fractional frequency), and bad/ordinary ones are high 1e-15/day. That means that after a year (300days) when you come back to look at the freqyency it has changed (usually to a lower value) by 300days x (-1e-15/day) = +26 ns/day!The benefit wrt. to a Cs is that the H-maser is low-noise. Linear frequency drift means a parabolic phase, and a good maser stays within +/- 1ns from an a parabolic phase fit (or prediction) wrt. to an ideal SI-second clock.For timekeeping what matters is predictability of the free running clock - not so much it's absolute accuracy.primary frequency standards are another story...