It's Feb 2016, Which ebay GPSDO? - Page 1
Forum section: electronics / projects

author: NivagSwerdna
author_rank: Super Contributor
timestamp: February 19, 2016, 10:07:08 am
content: I'm torn between...http://www.ebay.co.uk/itm/Symmetricom-Inside-GPS-Receiver-GPSDO-10MHz-1PPS-GPS-Disciplined-Clock-GPS-Ant-/181948031179?hash=item2a5cf2a0cb:g:rlcAAOSwyQtVkpxIi.e. Recovered crusty old Symmetricomandhttp://www.ebay.co.uk/itm/10MHZ-OUTPUT-SINE-WAVE-GPS-DISCiPLINED-CLOCK-GPSDO-GPS-Antenna-Power-supply-/181793953652?hash=item2a53c39774:g:R-YAAOSwT6pVmeari.e. BG7TBL. Modern, clean. Slight systematic phase difference.. i.e runs slightly slowPlease can someone advise me! It's just for use as a source for calibrating my equipment and I'm only ever likely to buy one!Thanks in advance

author: voltsandjolts
author_rank: Unknown
timestamp: February 20, 2016, 09:27:16 am
content: What are your calibration requirements exactly? How many digits do you need?For calibrating a frequency counter, this does the job just fine:https://www.eevblog.com/forum/testgear/economical-option-for-precision-frequency-reference/msg754025/#msg754025

author: Gyro
author_rank: Super Contributor
timestamp: February 20, 2016, 12:43:38 pm
content: Interesting. Are you able to verify that they are actually UBLOX 6Ts, ie. the timing variant with TCXO?Edit: Spelling corrected

author: DimitriP
author_rank: Super Contributor
timestamp: February 20, 2016, 01:21:20 pm
content: LEA-6T Also discussed here :https://www.eevblog.com/forum/reviews/ubox-max-7c-based-module-ryn25ai-use-for-gpsdo/msg541854/#msg541854

author: DimitriP
author_rank: Super Contributor
timestamp: February 20, 2016, 01:25:44 pm
content: ...and it's not a box, is a blox.ublox. Makes it sooo much easier to find the threads too

author: Gyro
author_rank: Super Contributor
timestamp: February 20, 2016, 01:34:03 pm
content: Oops, typo. Thanks for pointing it out, I've corrected it.Thanks for the link, that's why my search didn't pull it up too.

author: voltsandjolts
author_rank: Unknown
timestamp: February 20, 2016, 05:04:08 pm
content: Quote from: Gyro on February 20, 2016, 12:43:38 pmUBlox PC software sees it as 6T but that doesn't mean its genuine I guess. Regardless, it compared very well with a high spec freq counter so for me its good enough.

author: Gyro
author_rank: Super Contributor
timestamp: February 20, 2016, 06:33:07 pm
content: Thanks voltsandjolts,I've just ordered one from Hong Kong that has a photo showing LEA-6T-0-000. Worth a punt at that price.http://www.ebay.co.uk/itm/Ublox-LEA-6T-GPS-Module-w-Compass-for-APM2-6-APM2-8-Flight-Controller-Multirotor-/251682364176?hash=item3a99701b10:g:-wMAAOSw1x1UQg9qThe 8MHz will do for me for counter checking until I finish my Resolution-T based GPSDO (assuming I don't switch it over to the 6T that is).Oh, I found an article comparing the 6T with the old Motorola M12+...http://www.gpstime.com/files/PTTI/Low_cost_GPS-based_time_and_frequency_products.pdf

author: NivagSwerdna
author_rank: Super Contributor
timestamp: February 20, 2016, 06:54:05 pm
content: Quote from: voltsandjolts on February 20, 2016, 09:27:16 amIt's just for fun. I would use it for my antique Racal Dana 1998 counter (into which I planned to put a OCXO)Interestingly I do actually have a couple of ancient Motorola VP Oncores which produce 1PPS to within 50nS... So I guess I could just use that... I just need to calibrate it for 1Hz?

author: Gyro
author_rank: Super Contributor
timestamp: February 20, 2016, 10:30:25 pm
content: Yes you could, so long as your counter has sufficient resolution in reciprocal (time interval) mode.

author: NivagSwerdna
author_rank: Super Contributor
timestamp: February 20, 2016, 11:33:27 pm
content: Quote from: Gyro on February 20, 2016, 10:30:25 pmI tried it... not convinced by result. The counter claims to be 10Hz+ so this was a bit out of it's band.I did get some numbers though that at least suggested it was in the right ball park... I'll do some more investigation

author: DimitriP
author_rank: Super Contributor
timestamp: February 21, 2016, 11:20:57 pm
content: BTW, if you have a GPS that will output 10KHz (and I believe the 6T will do), very few beat this for simplicity.Especially of solder is your favorite programming language.http://www.george-smart.co.uk/wiki/GPS_Frequency_Standardhttp://www.george-smart.co.uk/w/images/c/cc/GPSDO_Schematic.pdfand if you want to go with an even more minimalistic approcach, you can skip the "fancy display"/"arduino" part and get the 10MHz directly out of the GPS/ (although I have no info on stability etc with 10MHz straight out of the GPS)If you are going to buy a "new" "ebay unit" built out of recycled parts, you might as well build your own.

author: Gyro
author_rank: Super Contributor
timestamp: February 22, 2016, 09:41:35 am
content: The 10MHz out of the GPS (the ublox 6T that is) is jittery, that's why voltsandjolts picked 8MHz - it's an integer divide of the 24MHz TCXO.As 2MHz is also an integer divide (obviously), hooking it up to a 10MHz OCXO with a 5:1 divide PLL has to be the simplest solution I've see so far, even easier than the 10kHz versions! Ublox have an app note:https://www.u-blox.com/sites/default/files/products/documents/Timing_AppNote_%28GPS.G6-X-11007%29.pdfMy Trimble Resolution-T (1pps) based option is looking unnecessarily complex in comparison.Edit: just has another look at the app note again - the 6T's internal TCXO is 48MHz, not 24.

author: jpb
author_rank: Super Contributor
timestamp: February 22, 2016, 09:55:48 am
content: Quote from: Gyro on February 22, 2016, 09:41:35 amIt would be very straight forward to adapt Jim Miller's 10kHz version:http://www.jrmiller.demon.co.uk/projects/ministd/frqstd0.htm

author: Gyro
author_rank: Super Contributor
timestamp: February 22, 2016, 10:08:00 am
content: I don't know why I was suggesting 2MHz... 1MHz and a 10:1 divide would be more sensible wouldn't it!

author: dom0
author_rank: Super Contributor
timestamp: February 22, 2016, 12:39:46 pm
content: Well, either one is a common divisor and neither is a power of two. Does 1 vs 2 MHz make a (relevant) difference in noise rejection?

author: Gyro
author_rank: Super Contributor
timestamp: February 22, 2016, 01:28:13 pm
content: Probably not. I was thinking that 1MHz would be a more useful output to have... and more importantly would allow a 74HC90 divider to provide a 1:1 mark/space into the phase comparator.

author: tonyarkles
author_rank: Regular Contributor
timestamp: February 22, 2016, 08:33:54 pm
content: I recently bought this one:http://cgi.ebay.ca/ws/eBayISAPI.dll?ViewItemVersion&item=181822333620&view=all&tid=1440186469008to use with a Ettus B200 software-defined radio. It has been working awesome so far; unfortunately, I don't have any test equipment with enough precision to measure the accuracy directly, but using it as a 10MHz clock for the B200 and using kalibrate-uhd, it measures around +-5Hz offset when compared to my local GSM850 towers at 870MHz. If my brain isn't toast today, that works out to around 6ppb which is definitely good enough for what I'm doing.