Yet another DIY GPSDO - yes, another one

[Scrachi]

Hi,

I've exported as SVG the top and bottom layers of both PCB versions. Please see attached.
Let me know if it is ok.

Mick

[geggi1]

The copper traces and component placement would be nice.
I will try to make the PCB according to the method in the link.
https://www.instructables.com/Heatless-cold-Toner-Transfer-for-PCB-Making/any experiance?
Does anybody have any experience with this metode?

[jpwolfe31]

Another happy builder here.  Thanks Andrew et al for putting this all together.  I added a 2004 LCD display with a  button that lets you move through various data screens.  Your code was a breeze to work with!

[AndrewBCN]

Another happy builder here.  Thanks Andrew et al for putting this all together.  I added a 2004 LCD display with a  button that lets you move through various data screens.  Your code was a breeze to work with!

Nice build, and it seems to be working fine, plus that large display makes it easy to read. Well done !

[jpwolfe31]

Completed unit with the display mounted in a repurposed Extron ADA 4 video distribution amplifier purchased on ebay for $22 plus $15 shipping. 

20 bncs in the back and 4 added in the front for sine, square, pps and counter input. 

GPSDO power was taken from the existing Extron power supply.

[AndrewBCN]

Well done, very nice build !     

[mendip_discovery]

Hi All,

The PCB is here, but..... not perfect... The footprint I've made for the OCXO was reverted, the connector for the INA219 too and finally I've also reverted the pins GND and VCC on the 74HC14. I need to check if my eyes are not reverted too.... After a surgical operation with some wires all is working as expected.

Attached I've uploaded the schematic and the PCB's Gerber files (with all above issue fixed, but I've not manufactured theses PCB for testing purpose).
I've made 2 versions of the PCB :
- 1st one for 3D printed case, very similar with the version I've already manufactured, I will design a 3D case for printing soon.
- 2nd one to fit on a 100mm large extruded case, all components need to be directly soldered on the pcb except the lcd and the outputs will be connected using u.fl connectors.


EDIT 11-16-2022 : Schematic updated, to correct 74HC14 GND and VCC pins

Hope it will be useful


Mick

Thanks for uploading a PCB layout, I have some on the way. Anyone in the UK wants a board I might have 3 spare ones.

Any chance of a list of parts so I can be sure I am getting the right parts. Going to order enough to make at least two. I have the optional boards just need the PIC and v-reg etc.

[Scrachi]

Hi,
Find attached the BOM extracted from Kikad for both version, the 3D case and the extruded case.

Mick

[mendip_discovery]

Hi,
Find attached the BOM extracted from Kikad for both version, the 3D case and the extruded case.

Mick

So I have started my shopping list. Just have a few questions.

F1 = PollyFuse = What fuse rating?
L1,L2 = L_Ferrite = Inductor with a Ferrite core. What inductance?

[Scrachi]

So I have started my shopping list. Just have a few questions.

F1 = PollyFuse = What fuse rating?
L1,L2 = L_Ferrite = Inductor with a Ferrite core. What inductance?

Hi,

The PollyFuse is rated 1A and the inductors 10uH

Mick

[mendip_discovery]

Thanks to @Scrachi I have now just received my boards.

Got a few bits on the post so that I can make them up.

Anyone in the UK wanting a board pls ask. I have 3 going spare at the moment.

[mendip_discovery]

So I have got the boards mostly assembled. But I have run into an issue with the Black Pill.

The Arduino software just won't find it. I do the Boot press reset thing and my W11 PC shows it as STM32 BOOTLOADER and the STM software finds it happily.

IDE 2.0.4
stm32duino 2.5.0

[AndrewBCN]

So I have got the boards mostly assembled. But I have run into an issue with the Black Pill.

The Arduino software just won't find it. I do the Boot press reset thing and my W11 PC shows it as STM32 BOOTLOADER and the STM software finds it happily.

IDE 2.0.4
stm32duino 2.5.0

Hi,
Make sure you have your Arduino IDE properly configured for the Black Pill, and that ambient temperature is between 20 and 25C (yes, I know that sounds odd, but it's like that). Also post a few screenshots, that will help find the problem.

[LADmachining]

So I have got the boards mostly assembled. But I have run into an issue with the Black Pill.

The Arduino software just won't find it. I do the Boot press reset thing and my W11 PC shows it as STM32 BOOTLOADER and the STM software finds it happily.

IDE 2.0.4
stm32duino 2.5.0

I had similar sounding issues, and found that I had to have the STM32Cube programmer application open in the background but not connected before I could program the board.  Had other problems using the USB interface to program after initially flashing the board, so use an ST-Link USB interface all the time now.

Anthony

[mendip_discovery]

It's in a room that is currently 19.4C but I have moved it to onto the PC where it is warmer.

Attached are the screenshots. I think I got it right.

[AndrewBCN]

Hi mendip_discovery,
Sorry for the very late reply.
I have just re-installed a new (actually refurbished) laptop, a Thinkpad T470, with both Ubuntu Linux 23.04 and Windows 11 in a dual boot configuration.
This will be my main MCU software development machine from now on.
In Ubuntu Linux, I also manually installed both Arduino IDE 2.1.0, the STM32 board support, and STM32 Cube Programmer and tested these on a Black Pill STM32F411CEU6, and this combination works fine, I have verified that I can program the Black Pill using the Cube Programmer DFU method. The Black Pill must be put in DFU mode as usual by pressing the boot button and then connecting the USB-C cable, then releasing the boot button. One can check that the Black Pill has entered DFU mode by checking the Linux kernel messages with the dmesg command. Note that sometimes the Black Pill fails to enter DFU mode on the first try (this time it only worked for me on the second try, but I remember it taking quite a few tries sometimes). Room temperature was 21.5C.
I am next going to test the same software combo but using Windows 11 to come as close as possible to your setup.
I'll add some screenshots ASAP.
Thanks to all of you for your patience ! 

Notes :
1. The Arduino IDE 2.1.0 is a free download from the Arduino website.
2. The STM32 board support is automatically installed following the instructions from the STM32duino GitHub Wiki.
3. The STM32 Cube Programmer is a free download from the STM32 electronics website, but requires one to register. The registration is free.
4. To get the Arduino Serial Monitor to have permissions to communicate with the STM32F411CEU6, you have to include yourself in the dialout group, using the command:

Code: [Select]

sudo usermod -aG dialout <your user name>

[mendip_discovery]

I used the W10 laptop I keep near the sofa for when the TV is on and I need to look stuff up.

That allows me to use another STM32 driver that does work but sadly that driver won't install on W11 as it's not made for that system.

Anyway I managed to get to talk to the laptop and finally found all the files for it. It didn't like the ++.h filename so had to change the file name. But after all that it gets funny about syntax. At this point I was just pleased i had a working connection.

Tempted to find a Pi to use as that would mean Linix and then a whole new set of fun to get it to talk but at least it would allow me to do it rather than windows yeah I can see the device but I wont let you talk to it.

[AndrewBCN]

So, I just installed the same combination of software on the T470 laptop but this time in the Windows 11 partition. And it worked out of the box for flashing the STM32F411CEU6 Black Pill. However, I could not get it to talk with the serial monitor at first. After a good 40 minutes of head scratching, I decided to plug the USB cable in another USB port on the laptop, and then the USB serial interface showed up immediately as COM4. Problem solved ! 
Tomorrow I shall post the screenshots for both the Linux and Windows setups.

[ErnestB]

Hi, I am new to designing accurate CLK. Maybe a stupid question but I am wandering why would one need (in conjunction) already very very very  accurate OCXO if you have the GPS looked signal that provides even higher accuracy? Is there a specific reason for that?

[mendip_discovery]

Hi, I am new to designing accurate CLK. Maybe a stupid question but I am wandering why would one need (in conjunction) already very very very  accurate OCXO if you have the GPS looked signal that provides even higher accuracy? Is there a specific reason for that?

The way it is sitting in my head it is like using a capacitor on a voltage output. It helps filter out some of the noise/spikes/jitter. Due to a multitude of things that happen with the signal path from there to here the GPS 1pps and even the location flickers about a bit.

Plus also the GPS generated a 1pps signal aka 1 Hz and using the OCXO you can get a 10MHz signal which is what a lot of test gear can accept as a reference signal.

[MIS42N]

Hi, I am new to designing accurate CLK. Maybe a stupid question but I am wandering why would one need (in conjunction) already very very very  accurate OCXO if you have the GPS looked signal that provides even higher accuracy? Is there a specific reason for that?

There's a difference between accuracy and stability. An OCXO is stable, provides a constant frequency. But that frequency may not be accurate. A GPS is accurate measured over hours. But not stable second to second.

[ErnestB]

Clear 👍 As what I have seen, in short, that a OCXO is voltage controlled. It would be then a good thing to have some register / memory to remember the right setting, once you have a good GPS reception. In that way the output of the OCXO would stay reasonably accurate until the next "good reception". I have seen one design example where the GPS 10MHz output is the whole time compared with the 10MHz output of the OCXO, and the difference fed back to the OCXO voltage input to adjust the output. But if the GPS output is not stable the whole time you just shift the instability directly from one place to another.

[MIS42N]

Clear 👍 As what I have seen, in short, that a OCXO is voltage controlled. It would be then a good thing to have some register / memory to remember the right setting, once you have a good GPS reception. In that way the output of the OCXO would stay reasonably accurate until the next "good reception". I have seen one design example where the GPS 10MHz output is the whole time compared with the 10MHz output of the OCXO, and the difference fed back to the OCXO voltage input to adjust the output. But if the GPS output is not stable the whole time you just shift the instability directly from one place to another.

I think you have some research to do. There are many designs that have 'hold over' capability - remember the last good control voltage and apply it until it can be verified by GPS.

The short term instability of GPS receivers is overcome by averaging. This can be done by analog means, a phase locked loop, or by digital means (like the method in this thread). It can also be mitigated by using timing GPS modules that are 'surveyed in' to a particular location then they compensate for any apparent deviation caused by atmospheric changes, swapping satellites, etc. This can be further refined by receiving on two bands (L1 and L5? don't remember the detail) which can be used to further reduce uncertainty.

The simplest GPS + OCXO combinations will usually generate a frequency accurate to 1 part in 10^9. Very expensive OCXO may achieve this accuracy for weeks but will eventually drift. With a GPSDO, 1 part in 10^10 is common. Absolute time is not easy as it requires an accurate clock to set the time in another clock. However, GPS will give time to within a microsecond or better. For interval timing GPS + OCXO is much simpler, within 30ns without much trouble.

Did you have an application, or just thinking about it. There are many, many designs so it is unlikely you can raise a question which someone has not already answered.

[ErnestB]

Thanks for elaborate answer. Today I have received the NEO-8 and will start the research with that, as I have no experience with GPS. As this is very popular subject I believe that there are a lot of different designs. It is also interesting to develop your own design.

[PCB.Wiz]

Thanks for elaborate answer. Today I have received the NEO-8 and will start the research with that, as I have no experience with GPS. As this is very popular subject I believe that there are a lot of different designs. It is also interesting to develop your own design.

Yes, see also
https://www.eevblog.com/forum/projects/lars-diy-gpsdo-with-arduino-and-1ns-resolution-tic/1150/
And
https://github.com/erikkaashoek/Arduino_SI5351_GPSDO
The added Time Interval Capture using a FF and diode helps improve lock by ‘seeing’ the GPS correction steps, and also lets you see the correction in action. 20 second corrections seemed to be a good trade off, in #2

I ran up a spice variant of that TIC using a 74LVC1G80 !FF and 74LVC1G38 open drain NAND gate that eliminates the diode (but flips the phase polarity, easily corrected in the MCU)

And for a hardware only approach  ( no MCU ) PLL this thread has some options
https://www.eevblog.com/forum/projects/gpsdo-question-378182/msg4877936/#msg4877936