Cheap GPSDO - Page 1
Forum section: electronics / projects

author: Theboel
author_rank: Frequent Contributor
timestamp: May 26, 2015, 04:53:45 pm
content: Dear AllI like to ask You who has more experience about my plan for "USD75 GPSDO" before I explain my plan let me tell You some "real world" limitation1. The parts I choose must be available in local store (custom bussiness cost a lot of time)2. I do not aim for something special a magnitude better than my MTI milliren OCXO is welcome.Now What in my mindI will use Ublox 7m GPS board that can program from 0,25Mhz to 10Mhz(https://sites.google.com/site/g4zfqradio/u-blox_neo-6-7)But the problem are I found someone (http://www.ra3apw.ru/proekty/ublox-neo-7m/) already do the spectrum analysis for this board and the result is like this :and the last picture is far better, correct ? so what I need to do I have 2 plan :1. use the board 10Mhz output than use very narrow (100Hz bandwith) Crystal filter2. Use the 4Mhz output than again filter it use Crystal filter to clean it than use it as reference for PLL for controling 10Mhz OCXOWhich one you think a better way.last I also like to know because MTI only spec the OCXO model 240 harmonics to down to -30dB (http://www.mti-milliren.com/pdfs/240.pdf) is there any benefit to use a crystal filter in the output to clearer it more.any opinion is very welcomeThank YouAnton

author: dom0
author_rank: Super Contributor
timestamp: May 26, 2015, 06:33:07 pm
content: You want a digital frequency-locked loop with a good quality OCXO as tuned oscillator.

author: paulie
author_rank: Frequent Contributor
timestamp: May 26, 2015, 06:53:46 pm
content: In case you missed my post yesterday here's the dannyf thread for building extremely precise GPSDO that costs less than $5:https://www.eevblog.com/forum/projects/toyo-tco-919-%27high-stability%27-crystal-oscillators/15/It turns out 1pps is more useful than 10mhz for locking because this is default for all low cost GPS. No need for expensive GPS modules or nasty configuration software. I built something similar using NEO-6 and AVR and found it much more convenient than zero beating WWV for this project:https://www.eevblog.com/forum/projects/3-dollar-precision-frequency-standard/ps. Now I have 10mhz reference that stays 0.2ppm no jitter without spending many early morning hours straining to hear NIST radio signal.

author: Theboel
author_rank: Frequent Contributor
timestamp: May 26, 2015, 07:28:38 pm
content: Quote from: paulie on May 26, 2015, 06:53:46 pmYes I already see Your thread the Neo-6 cost USD20 and the 7M cost USD27 both can adjusted. only the neo-6 until 1Khz and 7M go to up 10Mhz. and both has 1PPS so no problem we can try to lock the OCXO with 1 PPS later.in my first plan I will use 10Mhz generated by the TCXO in the 7M board. only I use crystal filter to clean it up, I assume very narrow crystal filter with very high Q can filter the harmonics well and help to achieve better phase noise.

author: PaulAm
author_rank: Frequent Contributor
timestamp: May 26, 2015, 08:02:32 pm
content: It costs a little more, but if you're in the US you can get a NOS Lucent/Symetricon Z3810as GPSDO timing system still in the box for $125.Look on ebay for item 221777430088Apparently a new stock of these have shown up. No connection with the seller, other than I have one on the way. There's a lot written up on these units in the time-nuts mailing list.It has a 15MHz output, but there's a note onwww.ko4bb.comon a modification to get a 10MHz output. Look under the GPS Timing section. It just involves adding a connector, emitter follower and a couple of passives.I was going to build a GPSDO, but this is much easier. I plan on hooking it into my ntp server to get a stratum 0 time server.

author: Theboel
author_rank: Frequent Contributor
timestamp: May 27, 2015, 12:51:56 am
content: Quote from: PaulAm on May 26, 2015, 08:02:32 pmIts really tempting but consider ($125 + $91(shipping)) X 1,33 (for tax) I really hate to push the buy button also think about what will happen after 16000 miles traveling from USA its No No option to me

author: edpalmer42
author_rank: Super Contributor
timestamp: May 27, 2015, 04:47:17 am
content: Quote from: Theboel on May 27, 2015, 12:51:56 amI bought one of these a few months ago. Take a look at the packaging shown in the auction pictures. This isn't your typical random box and whatever packing is handy. This is custom designed to protect the units from the evil shippers. It's the original Symmetricom packaging. Each of the two units is inside a plastic bag. It's then put in a cardboard box and supported with spongy pink foam so that it's about 30mm from the cardboard. The two boxes are then put insideanothercardboard box that's just big enough to hold them. Basically, the units are protected by double-corrugated cardboard and spongy foam that will take as many hits as you want to give it andstillprotect them from impact.Ed

author: Dragon88
author_rank: Regular Contributor
timestamp: May 27, 2015, 10:56:39 am
content: The Z3810AS GPSDOs on eBay are quite good for the price. At $125, almost a steal. They are packaged extremely well and NOS. Also there is useful free software available (SatStat) to monitor the units.Why should you buy an off the shelf unit? Well when you build your GPSDO, which is a great idea, you need something to compare it against. And what you compare it against has to have BETTER performance than what you are building in order to collect sensible data. You can't just build a "reference", power it on, cross your fingers and hope it's accurate. It's not a reference until it's validated against something better.

author: paulie
author_rank: Frequent Contributor
timestamp: May 27, 2015, 11:57:19 am
content: The 1pps signal from cheap GPS modules is an excellent reference with very little wishful thinking involved. Over long term in the parts per billion. NIST not sci-fi. Those with money to burn may enjoy spoon fed off-the-shelf but anyone on a budget or who really want to learn the technology might be advised to go DIY.

author: Theboel
author_rank: Frequent Contributor
timestamp: May 27, 2015, 11:49:38 pm
content: Quote from: paulie on May 27, 2015, 11:57:19 amI understand the PPS is stable but the problem are every manufacture has different way about duty cyle, jitterfor some useless to have a stable signal but with a lot of jitter.

author: paulie
author_rank: Frequent Contributor
timestamp: May 28, 2015, 02:08:14 am
content: Duty cycle does not matter because it's only period (edge to edge) that is important. 1% or 50% works fine. As far as jitter I have one running right in front of me at this time and there is no detectable jitter. At least not on parts per billion scale. Even if there were it's long term stability that matters for GPSDO so again, not an issue.I understand the simplest and lowest cost solutions are not always chosen in situations like this so it will be interesting to see what you finally settle on.

author: Dragon88
author_rank: Regular Contributor
timestamp: May 28, 2015, 02:50:36 am
content: Quote from: paulie on May 27, 2015, 11:57:19 amThe problem with a home-built gadget is that you don't know poor or how good it is. If you just want to play around then go for it, but a homebrew "reference" is simply a toy without verification of its performance.

author: paulie
author_rank: Frequent Contributor
timestamp: May 28, 2015, 03:02:23 am
content: In addition to regularly doing a zero beat check of the 10mhz signal against WWV I also verified less than one millisecond off after two weeks using the NIST internet facility. Probably a lot less but that is the limit of my measurments. So shall I take word of one who considers $200 store bought unit "cheap GPSDO" or worlds defacto standard for time/length/mass? Hmmmmm... tough one.