GPS Locked Frequency Standard - Page 1
Forum section: electronics / projects

author: t1d
author_rank: Super Contributor
timestamp: February 20, 2017, 07:27:25 am
content: Louis, of Skullcom Hobby Electronics, a YouTube channel, has developed a cheap, easy to build, DIY, frequency standard. It is a complete, free, open-source project, including firmware. He starts with a particular Chinese GPS receiver and powered antenna, runs that through a few bits and pieces into an Arduino Nano and displays the information on an LCD. You get atomic clock accuracy... I think it is a worthy endeavor... Kudos to Louis!

author: netdudeuk
author_rank: Frequent Contributor
timestamp: February 22, 2017, 09:54:29 am
content: He has a great channel. Check out his DC load project too.

author: CJay
author_rank: Super Contributor
timestamp: February 22, 2017, 02:51:00 pm
content: It's a nice project but it's terribly dirty on the output, lots of jitter because of the way the Neo 7 generates frequencies.There are far better ways of doing this.

author: t1d
author_rank: Super Contributor
timestamp: February 22, 2017, 05:29:20 pm
content: Quote from: CJay on February 22, 2017, 02:51:00 pmIIRC, Louis is aware of the dirt and is working on that; this is an on going project.Quote from: CJay on February 22, 2017, 02:51:00 pmIf you have other design ideas, I would really like to see them. Maybe you could use his design criteria... Cheap, use of existing modules, single-sided board, DIY doable, etc. Those methods would help me.Thanks to everyone for contributing!

author: CJay
author_rank: Super Contributor
timestamp: February 22, 2017, 06:49:01 pm
content: He won't get far I'm afraid, I'm not fully conversant on the reasons why it's jittery but my understanding of it is that the Neo 7 (and many other GPS devices) 'drop' pulses on the output so errors average out over a specified number of seconds to keep it 'on frequency'.The way to improve it without extra hardware is to only output a frequency that's an integer division of the onboard reference clock (I think 48MHz) so he will notice an improvement on the 'scope if he sets it to exactly 12MHz or some other integer quotient of 48 (I have a Neo 7 here and have proven this for myself, on a frequency counter it's 'precise' but the gate time averages out the errors, on a 'scope it's all over the place as seen in the video unless you choose that integer divisor, then it improves rather a lot but is still far from perfect)That kind of limits the usefulness.If it was possible to add hardware for one fixed frequency then I think* an external Voltage controlled OCXO can be steered directly by the GPS receiver with the addition of a simple, cheap Microchip DAC, there are details in the Neo documentation and there are hints in the U-Blox software.If you need to be able to generate a random, spot frequency then it's a case of adding external hardware like a PLL/VCO or DDS** and using the GPS frequency (integer divisor remember) output as a reference for that, to make the PLL/DDS more immune to the effects of the pulse dropping you could drive the OCXO from the GPS and use that as a reference for the PLL.(* may be limited to only some models and specific firmwares, may not even be possible, I gave up researching it after I got my Rb standard and a decent sig gen)(** PLL and DDS solutions add their own complications and are 'a bugger' which need skill and experience to design and implement properly or you can end up with a system that's orders of magnitude worse than the original)

author: DimitriP
author_rank: Super Contributor
timestamp: February 22, 2017, 07:00:28 pm
content: QuoteA) Sculcom Electronics is one of my few favorite channels.B) As far as the GPS project goes it's not a GPSDO and you don't seem to know the difference, so maybe turn down the volume and stop waving your arms around about it.

author: CJay
author_rank: Super Contributor
timestamp: February 22, 2017, 07:50:33 pm
content: Umm, if that was aimed at me...I know it's not a GPSDO, he's using it as a variable frequency sig gen, I know the difference.My point was that to get a decent variable output that's 'clean' he will need to doa bit more hardware work, one step towards that would be to add a PLL to help remove the jitter, I don't think he will be able to remove it any other way, the GPSDO idea would just make it that bit more clean.I'm not criticising the channel, I like the channel and am a subscriber.If you take offence at my comments then mute me.

author: DimitriP
author_rank: Super Contributor
timestamp: February 22, 2017, 08:52:12 pm
content: QuoteMost certainly not aimed at you. It was aimed at the OP.