Scullcom Hobby Electronics # 35 - Design & Build a GPS locked Frequency Reference Standard 

Loved this video. Firstly, I used to work in the Racal-Dana design department in Maidenhead in the 1980's and worked briefly on the 1990 series of frequency counters, so it was great to see one in action, still. The top models had pretty awesome 10MHz OCXO's in them.

The projects you come up with are great. You explain what you are doing really clearly and the outcome is a properly useful device. Thanks for doing these, I am learning a lot.

Awesome content. Thank you. I am subscribing right away! . A word of caution for bread-boarders, avoid taking the timing pulse from the GPS module without proper shielding as it can cause some severe RFI ( Radio Frequency Interference) limiting the ability of GPS receiver to lock on the signal. Remember GPS signal is the weakest signal among all wireless technologies.

May you live long and healthy. Thanks for your videos.

As has been your usual, another well thought-out, well designed and engineered project. I certainly hope you gain more viewers and quickly. You have the ability to explain things very clearly and enjoyably. No BS, no click-bate only hardcore projects. I cannot thank you enough for your work and the contributions you make to (not only) my continuous learning.

Very cool. There's really not enough information out there that helps to compare the GPS modules out there and what one should pick, this video helps.

I have become your subscriber... I must admit most of your project are almost complete... and you can also learn from them. Kudos!

I have one of these modules, they're great but at 10MHz output you get a large amount of jitter, the on board reference crystal is 48MHz so it's not an integer divide to get 10MHz. Selecting 24, 12, 8, 6, 4, 2 ,1 MHz gives a vastly improved signal. I believe it's also possible to add a DAC to one of these units and have it steer an external OCXO which will yield even better results. Setting the unit going with a clear sky view and asking it to run a 'survey' will improve results as well but the supplied patch antenna may not be good enough. As far as I can see it's very accurate, I've compared mine to two Rb standards and it's within millihertz of 10MHz. It might be a good idea to provide the receiver with some thermal insulation so it's less prone to drift as room temperature changes

I've got a couple of these modules so will definitely be making one of these. Thanks for making the video Cheers John

Very interesting, and so nicely explained even I can follow it! Many thanks.

A few corrections and comments if you don't mind. First, the EEPROM is an Electrically Erasable Programmable Read Only Memory meaning that it is nonvolatile and won't loose its contents when power is removed. The only reasons you could be loosing your settings is either it is damaged (too many writes for example but that requires more than 1,000,000 writes), not being written to correctly (not likely as that routine is in the firmware by the manufacture), or you are not correctly telling it to save the settings. If you take a look in the Configuration View window and then the CFG settings, on the right side under Devices you likely only had "0 - BBR" selected. That only saves the settings to the Battery Backed RAM. You can select more than one "device" by holding down the button. Send (execute) that and you should be fine. Now to save the hassle of having to connect to the computer and using the U Center program every time you get a new GPS unit or to save the end user the trouble you can actually program your Arduino to set the configuration you want every power up. As long as you don't save the settings you can still easily use that same GPS with other projects without having to go back into U Center to change them. Using either the Arduino or the correct "device" when saving settings also saves not having to connect up a battery to your circuit of course. You get jitter like that when you are trying to produce a signal that doesn't divide the source frequency evenly. An example that you can draw on a piece of paper would be if you wanted to generate a 3 Hz signal from a 10 Hz source you can see that while you could get 3 complete cycles per second each cycle isn't going to be the same shape or length. Now say you have a 2 kHz source and want 1 kHz out. That works perfectly of course. But what if you want 990 Hz? You can get 990 cycles out per second but there will be some longer cycles to make up the difference. Using a phase locked loop can correct that but it still has its own problems and limitations. I was getting about a 21ns jitter on a NEO-6 and both the NEO-6 and NEO-8 data sheets claim an accuracy of the time pulse of 30ns RMS. Hit the run/stop button on your scope and you will see the jitter easily when you zoom in. Dividing the signal down of course will make the 21ns jitter a much smaller percentage. Side note... are you sure you want that capacitor between the GPS and 74AC14? The Schmitt trigger is expecting an input between 0 volts and Vcc. That's likely why you are not getting a nicer square wave at 10 MHz. My NEO-6 has a rise and fall time of less than 12ns. I would try it without the cap. Enjoyed the video though.

This was excellent! I actually have a GPSDO which I have been struggling to get a usable signal from and this has not only given me some ideas for that, but also a fallback if I can't get them to work.

Fantsatic project and one ive been wanting to try for a while and yours is way more eligant than what i was thinking. Will be giving this a try. Thanks Brian G0WZB

For the CR2032 I would use Schottky diode, one with the smallest possible forward voltage. That way the battery can be used for a longer period of time.

Thank you for sharing Louis, nice implimentation of these devices, 73

Excellent as usual. Thank you.

Thank you for another great video. I am looking forward to the build and video No.2. Great job....................Berni

Awesome series of projects, I need to build one myself.

Wow it's working on my U-blox NE0-7N you saved me hours and hours of work ! and alot of money Thanks for sharing this info with us. :-)

Great project Louis...

Great project, great video i have been trying to make something like this for quite some time, buying a number of timing GPS units and creating CLPD board to set a OCXO, all have failed and it's good to see a proper project i can do to finally get this done i have already purchased the module and can't wait to build something like this up

Great project. One to build for certain. Thanks.

Good morning...another great super project...loving it.

building one! love the detail in your videos.

Great video, very informative.

30:00 For Simplicity I would use a red-green LED to indicate if it's locked or unlocked. Green is locked and red is unlocked. And the Arduino should have no problem power that led. Or you can even just have a green LED when it's green it's locked when it's off it's unlocked. For simplicity's sake.

wow right man in right place:) good job mate

Nice project, thanks. I think I'll build that one myself.

Excellent video and project.

Hi Louis, let me tell, that i'm a big, big fan of your videos, from them i learn a lot. Also i'm a fan from Daves EEVBlog, etc., but yours are at least so constructive of your own positive interesting way, that i don't wont to miss yours. ;) Thank you for all your work. I'm looking every day, to gather all your stuff and benefit from your experiences. Greetings from germany, and a big thumbsup.

Nice project!

So well done

Also, I've seen people combine multiple of the schmidt triggers together (through the appropriate resistor) so that they have enough current to truly drive a 50 ohm output into a 50 ohm load.I suspect you already checked the output capacity of the single gate, but if it's not high enough, this might be an option for you. Thanks much for the great video.

Thank you for this great video. You explained this very clearly and it was pleasant to listen to you. I have a question. I would like to know if I made several of these, would all square waves be in-phase among all of them? I am interested to achieve phase synchronization among these units.

31:00 "A bit of jitter"... That jitter is 25% of the period! 🙈 However, thanks, the rest of the tutorial is very useful. I plan to take a cleaner 10 kHz signal from the GPS and use a PLL with a tight loop bandwidth to lock a 10 MHz OCXO to that.

Nice. I'm currently building my own GPS disciplined 10MHz reference based on an old OCXO from ebay. Also controlled by an arduino nano. So I find this a very interesting project. I also did not know there were GPS receivers with 10MHz output. I'm using another approach, counting the 10MHz signal edges between a 200s timespan of the 1pps pulses from my uBlox GPS which have a 10ns accuracy.

Another great video from Scullcom. Many thanks! Now, a dumb question...how far from the GPS Module can the antenna be located??? Maybe an outside antenna with a long lead-in...obviously I know nothing about GPS modules.

very interesting

Love the video. If you don’t mind me asking, why not use a simple ATMEGA328P instead of the Arduino Nano?

awesome hack, thanks

why not use a RTC?! i mean... i GET IT.. but man... GREAT VIDEO! not trying to discourage you at all!!! keep em coming LOVE YOUR WORK!! 73

amazing video, but the question is when module is unlocked 10mhz comes from local oscillator, but when the module is locked the 10mhz comes from satellite?! or some calculation will depend to gps module and finally it come to local oscillator, or local oscillator will take some thing from gps module! in fact what is difference for local process when it is locked and unlocked? Thanks for your Fast Answer.

Great video as always, I really want to build one of these.... and by the way, what nimrod would give this video a thumbs down???

The NEO-7 modules are 3.3V devices, and they don't have 5V-capable inputs. But the Arduino Nano is a 5V board, so you might destroy your GPS module. (With the USB-to-UART converter it depends - my CP2102 module is a 3.3V device so its safe to use)

Why is the smith trigger not doing its job? Its supposed to clean the waveform and provide us a good squarewave right?

hi I've been playing around with one of these ublox modules having seen your video, I found like you that when set to 10 MHZ the signal is fairly poor, luckily my racal counter (same as yours) has options 10 installed so works fine with 1 MHz. The other thing I've tried which you may be interested in is putting the signal through a 74hc4046 phase locked loop chip this cleans up the 10 MHz considerably. another way of doing it is to set the module to 1 MHz and then multiplying it by 10 using the 74hc4046 and a hc390 divide by 10 in the feedback loop, this seems to work well also, I will probably use this in my completed project and have both 1 & 10 MHz outputs, hope this is of some interest

Hello! This is a great project. Is there a way to get a sine wave output rather than a square wave? Thanks, Chris

hi Louis! I have a question: what is the expected life of the back-up battery? btw...Happy New Year! i2NDT Claudio

@18m00s: "..add some extra Schmitt triggers..." Or just move U2A pin 4 to the output U1B pin 4. Shouldn't need endless Schmitt triggers; just one at the start should feed the rest of the dividers.

Can you recommend a replacement for the ublox Neo-7M GPS module. I cannot find one anywhere

I kinda hopes you'd do external OCXO and then dicipline that to the GPS with PLL... maybe the next project? :D

Great video and project! As a follow-up project, have you considered using the GPS 1 Hz signal, a microprocessor, and a D/A to discipline an OCXO? I know many others have done this project, but it seems like it would be a great video.

At 29:00 into the video where you are talking to the GPS, mine wasn't working at first and apparently had some bad setting stored in it so I had to click "Revert To Default Receiver Configuration" which is the icon along the top at the far right, to get it working.

just curious why run the signal through a Schmitt trigger (btw I would use this approach as well), rather than a opamp with a large hysteresis? would that not do a better job to rid the noise?

I allready have a homebrew gps locked standard that uses the ublox, I have issues with the ublox forgetting the setup, I assumed the supercap was a battery!, I'll do the mod you did. My system uses a pll and a 10mc voltage controlled xtal oven, the ublox o/p's 10kc not 10mc, the pll removes the phase noise from the ublox, I'm thinking of phase synchronizing the 1hz o/p to the 10mc xtal oven as I use it for a real time clock, though thats a little overkill. One thing I like and I'm going to make use of is the gps lib, I'd like to be able to use mine standalone with an lcd and know the ublox is locked and I have satellites.

Good evening I come back again after I changed everything that we said, and I just put the material on you, but unfortunately it does not catch with any satellites, so I do not have a frequency lock. The screen light normally says it is Ver. 6 the frequencies change it, with C, I see the voltage with D, I see the frequency I put, with # I see satellites 0 and hangs there and does not go back and again I turn the trend and put it again to work. The only difference we have is that to pass the program, I have to convert it to .hex from Arduino and I go through the XLoader because my Arduino does not pass the program directly and I think something is wrong with the conversion! Do you have any idea for top what's going on ??? Yours sincerely Vangelis

Do you make your PCB's available for us to buy? It might make it easer on everyone and also make you a few bucks.

If the 10MHz output is jittery because it is derived from a 48MHz clock (30:57) what would happen if you set the clock out put to a multiple of the master clock? For example if you set the output frequency to 9.6MHz (48 * 2 * 100,000). Would the source of jitter due to different multiples be eliminated?

good work! vy 73 de DL1LEP

Looking at the output LED at 30:00 it does seem to run at 10Hz as it should BUT when the system is locked at 35:00 the LED then seems to be flashing at 1Hz, why is this as you did set the output frequency in locked and unlocked mode to 10MHz ?, I did wonder if for a cheaper version without the Arduino and LED display you could set the unlocked output frequency to 1Mhz so you would get a slow flashing LED while the unit was unlocked and set the locked frequency to 10MHz so when you get the fast flashing LED you know the unit is GPS locked, I think a good idea would be to have a buffered 10MHz output on one BNC and then a selector switch to enable the 1MHz to 100Hz output to be fed into a second output socket via another buffer/50 Ohm output BNC, that would allow the you to do lower frequency tests and keep the 10MHz feeding other equipment as a workshop master reference, great project, look forward to the next update, Thanks, Dave

so, a low pass filter set at around 20Mhz should work to rid the harmonics right? roughly 1.4uF (if my math isn't lying to me again).

Anyone know where the tp5 option is in ucenter v18.08

I suspect this will be very jittery at 10MHz

TCXO = "Temperature COMPENSATED crystal oscillator" not controlled.

33:00 scope needs a calibration.

sir please make projects on woman safety and arduino security devices

Why does the EEPROM not remember the contents with the power off ?, I thought the idea was they held the memory until they are electrically erased !, nice project, parts on order already, keep up the good work, I love these projects and I buy in most of the major parts to supply via our www.basicmicro.co.uk web site, thanks, Dave

you didnt tell the arduino nano code

Looks like Neo 7 modules are unavailable today....will this also work with a Neo 8 , I mean has anyone actually built it with a Neo-8 ?

Where can I get the code for programming the board with keyboard?

Hi Louis; You said that the board uses a 80mF capacitor, but more later in the video, it looks like to me that the board uses a MS621 type battery, can you check that? Also, from the datasheet; " 1.12.2 Real-Time Clock (RTC) The RTC is driven by a 32kHz oscillator, which makes use of an RTC crystal. If the main supply voltage fails, and a battery is connected to V_BCKP, parts of the receiver switchoff, but the RTC still runs providing a timing reference for the receiver. This operating mode is called Hardware Backup Mode, which enables all relevant data to be saved in the backup RAM to later allow a hot or warm start." www.u-blox.com/sites/default/files/products/documents/NEO-7_DataSheet_%28UBX-13003830%29.pdf Love your work and your excellent artwork with the schematics!

EEPROM don't need battery backup

Since you are pushing the GPS module a little too hard at 10MHz and since you are trying to design an accurate freq standard, why not set the GPS to output 1MHz instead and use a PLL to gen the 10MHz? This would then give you more stability in the output noise and jitter of and would also lock to a satellite quicker.

Can you give me a reliable address where I can order the neo7m?.... I buy one at ebay, but that's a fake. I can not change the configuration. Cheers, Hans

Это любой сможет, настроить NEO на10 МГц. Я думал у вас будет алгоритм синхронизации по сигналу 1PPS

You lost me at "for windows...", since I don't run that here.