Idea cheap & precise frequency verification of an GPS trimmed Rubidium source - Page 1
Forum section: electronics / general

author: pelule
author_rank: Frequent Contributor
timestamp: April 29, 2018, 11:40:35 am
content: I have an idea how to continoulsy verify the precision of my DIY GPS disciplined rubidium oscillator.See Rb GPSDO:https://www.eevblog.com/forum/projects/rb-frequence-standard-or-a-gps-disciplined-for-the-home-lab-both-is-better/.I kindly as the forum to discuss the idea for "proof of conecpt".My idea:use the GPS time/date and a by software extended counter of the MCU used in the Rb-GPSDO to continously verify the Rb frequency:- the HW counter counts the Rb-GPSDO frequency (10 MHz)- the MCU comunicate with the GPS kit (to get UTC time/date as reference for the measures)The GPS give a quite correct date & time information (absolute precision down to below a second isn't required just relative one).The HW counter counts the 10 MHz output - counter overrun is counted by SW to extend the resolution to the required length.At a 1st GPS day&time, say UTC 01-May-2018 @ 01h 00min 00sec the counter is cleared and started.At a 2nd GPS day&time, say at UTC 02-May-2018 @ 01h 00min 00sec (24hrs) the counter values is read as end value.The measured frequence could be calculated and due to the long counting time any error of the measuremnt could be eliminated with acceptable precision.It is quite clear, this measured value does not give any information regarding short term frequency stability.Just the frquency accuracy with a precision of the GPS accuracy.BR/PeLuLe

author: Kleinstein
author_rank: Super Contributor
timestamp: April 29, 2018, 11:58:13 am
content: Over a long time the involved satellites will change and so will the atmospheric conditions. So the precision is kind of limited in comparison to a fast test with the 1 pps signal over a few minutes.Even worse, reading the time via UART or similar may include rather large errors, because the test message has a limited resolution and the transmission time may vary. 1 day with something like 1/10 second accuracy is not yet useful. Also using the same GPS receiver as the GPSDO would not include errors of the receiver. So the date test would be more like checking for gross programming / divider errors.To a certain degree a GPSDO should be kind of self checking: if the 1 PPS signal shows too much jitter there is something wrong.

author: rhb
author_rank: Super Contributor
timestamp: April 29, 2018, 04:25:34 pm
content: Atmospheric effects may be inferred from the shift in position computed by the GPS receiver. I don't know whether you can get data for each satellite. There should be a *lot* of literature on this. I'd start with the NIST website. I have one of their papers somewhere that went into great detail on the mathematics and how accurately you could discipline various reference oscillators.The position shifts of a thousand or so stationary GPS receivers are used to measure atmospheric moisture over the US.Take a look at RTK. I was sent this by cdev, but have not yet gotten around to more than a cursory read.Quote

author: zhtoor
author_rank: Frequent Contributor
timestamp: April 29, 2018, 04:27:50 pm
content: Quote from: rhb on April 29, 2018, 04:25:34 pmhello Reg,could you point to any text/paper on this?best regards.-zia

author: rhb
author_rank: Super Contributor
timestamp: April 29, 2018, 06:34:57 pm
content: Quote from: zhtoor on April 29, 2018, 04:27:50 pmUnfortunately, no. There was a news article quite a few years ago that about 1500 high end (i.e. $10K) Trimble receivers had been purchased for this. I'm pretty sure NOAA bought them and I'm sure there are professional papers on the subject. I'd like to read them myself, but my reading list is absurd. At the time I retired from the oil industry I was getting 37 publications a month at a cost in excess of $1500/yr. Without any prospect of employment, I pretty much lost interest in geophysics. It's fun, but so are a lot of other things.Now I just get QST, QEX, American Lutherie, The Home Shop Machinist, Machinist's Workshop and Digital Machinist plus a few trade rags.

author: texaspyro
author_rank: Super Contributor
timestamp: April 30, 2018, 02:03:55 am
content: Lady Heather has the ability to set the system clock from the GPS receiver time message. To make this more accurate I added some code to measure when the last byte of the receiver time message arrives and the standard deviation of the times.Attached is a list of the results from several GPS receivers.

author: David Hess
author_rank: Super Contributor
timestamp: April 30, 2018, 06:49:43 pm
content: Quote from: Kleinstein on April 29, 2018, 11:58:13 amThe GPS satellite positions repeat every 12 hours. Atmospheric conditions weakly repeat every 24 hours.QuoteThat is what I would do especially when a Rubidium source is available. Monitoring the variation of the GPS timing signal (faster than the PLL time constant) and the correction to the Rubidium source over 24 hours (slower than the PLL time constant) should tell all.

author: ArthurDent
author_rank: Super Contributor
timestamp: April 30, 2018, 07:54:51 pm
content: I have a modified Trimble Thunderbolt GPS receiver with the oscillator removed and connectors to connect an external oscillator that I want to test. Displaying the 10Mhz gives me the instantaneous error while the pps graph gives me the total error. Here is a graph over time for a rubidium I was adjusting.