Lars DIY GPSDO with Arduino and 1ns resolution TIC

[nealix]

Yes, thanks, that helps a lot.
I also ran across this:   https://elvistkf.wordpress.com/2016/04/19/arduino-implementation-of-filters/
which, with your answer, now make perfect sense :-)

Thanks!

[Mike99]

I've just seen this, which appears to be a Trimble GPS antenna/receiver for almost no money:

https://www.ebay.co.uk/itm/Acutime-2000-GPS-Smart-Antenna-for-Precise-Timing/233702099796

Would this be a good source for the 1pps signal?

Mike

[bingo600]

I've just seen this, which appears to be a Trimble GPS antenna/receiver for almost no money:

https://www.ebay.co.uk/itm/Acutime-2000-GPS-Smart-Antenna-for-Precise-Timing/233702099796

Would this be a good source for the 1pps signal?

Mike

I have one of these (in a box somewhere).
I seem to remember the 1pps is accurate to 40ns , not stellar.
The multiplug is a funny beast ... Can you source it ?

/Bingo

[thinkfat]

I've just seen this, which appears to be a Trimble GPS antenna/receiver for almost no money:

https://www.ebay.co.uk/itm/Acutime-2000-GPS-Smart-Antenna-for-Precise-Timing/233702099796

Would this be a good source for the 1pps signal?

Mike

The press release says something like "synchronized to UTC within +/- 100 nanoseconds (one sigma)". I guess that may be before "sawtooth correction"? The user manual says "50ns against UTC". Not stellar. But as far as bang-per-buck is concerned, a timing receiver ready for montage for 10 bucks is hard to beat.

[texaspyro]

The multiplug is a funny beast ... Can you source it ?

You can use regular "Dupont" jumper wires to populate the connector.   How you secure them is up to you.   I've used cable ties to bundler them.  Maybe candle wax or casting resin to fill the housing?

[Mike99]

The multiplug is a funny beast ... Can you source it ?

You can use regular "Dupont" jumper wires to populate the connector.   How you secure them is up to you.   I've used cable ties to bundler them.  Maybe candle wax or casting resin to fill the housing?

It's a standard military connector, you can find them on eBay.

[Mike99]

The press release says something like "synchronized to UTC within +/- 100 nanoseconds (one sigma)". I guess that may be before "sawtooth correction"? The user manual says "50ns against UTC". Not stellar. But as far as bang-per-buck is concerned, a timing receiver ready for montage for 10 bucks is hard to beat.

The User Guide says "Over-determined Clock Mode with a timing accuracy of <15 nanoseconds (one sigma) to UTC, GPS, or GNSS time."

I don't know how that compares to my NEO-7 which quotes "30ns RMS"?

[thinkfat]

If you have a general purpose NEO-7 receiver, the Trimble will likely get you better stability. Maybe you can even improve on the 15ns with a longer position self-survey (seems you can configure that).
My NEO-M8T can do better than +-10ns accuracy (peak, not RMS, after sawtooth correction), but it needs a really good position uncertainty. I ran a survey-in setting a boundary of < 200mm position uncertainty, and after that it claimed 2ns time accuracy.

[Mike99]

It's an attractive price but I think I'll give it a miss. It would need a cable making up and an RS-422 to USB converter plus yet more software to learn.

I do need a better antenna installation though as my NEO-7 can only see 3 satellites despite being wall mounted outside the house.

Oops software glitch. It's 13 now ...

Funny how my smartphone can see 16 from inside the house 

Mike

[thinkfat]

Funny how my smartphone can see 16 from inside the house 

Your smartphone is cheating. It has a lot of assistance from the network (time, coarse position, almanac and ephemeris for all constellations) so it only needs exact time (easy to receive). Then all it has to do is track the satellites.

[Mike99]

Your smartphone is cheating. It has a lot of assistance from the network (time, coarse position, almanac and ephemeris for all constellations) so it only needs exact time (easy to receive). Then all it has to do is track the satellites.

Thanks for that. I'm getting there slowly ...

My external antenna is seeing 10-12 satellites which should be enough for the GPSDO. I'm getting a splitter so I can feed my GPS clock as well.

Mike

[nealix]

@Dbldutch

Hi:  I notice on your Oscillator Circuit V2 schematic from GIThub, that the voltage control line going into the OCXO goes through a DC choke filter, a 33uH L102 inductor.   Can you share more detail on this?   I understand the purpose, in filtering out higher frequency noise on the voltage control line to the OCXO, but is it a 1/4 watt, 1/2 watt, or 1 or 2 amp inductor?   I assume since the average OCXO can take a little over an amp while heating, then around 600ma running, that the inductor might be bigger than 1/4 watt types?   But how do you calculate the size/wattage/part needed?   What specifically is L102 if I want to use one (or wind it myself)?

Second question:   On that Oscillator Circuit V2 schematic, the main schematic is using a Bliley NV47M1008 with an R109 of 27K ohms.  But the little table at the bottom
right of the schematic shows R109 as 330K ohms for a Bliley.  Quite a difference.    What do you suggest would be the proper starting point for R109?

Third newbie question :-)   :   I have both Arduino Nano 5 volt modules and also some Arduino Pro Mini 3.3 volt modules.   If I decide to start with the Arduino Pro Mini
modules so that the 3.3volt logic will all match, should I still use the series 100 ohm resistors that you show on your Arduino Nano digital I/O lines?

Last question for now;   On your Controller Schematic V2_1, you have 68 ohm resistors in series with the LM35 output.  What made you choose this value?
Is that a form of the low pass filtering based on R12/C11 for the LM35?

Sincere Thanks.

@everyone

I finally decided to compare my china fake U-blox Neo-6 GPS modules (all 5 are fakes), to a real U-Blox purchased via reputable USA supply chain.
I ended up buying a Mikroe GNSS 7 Click module from Mouser (also carried at DigiKey), which has the U-Blox Neo-M9N that just started shipping last month.
Ref:  https://www.mouser.com/ProductDetail/Mikroe/MIKROE-3922?qs=GBLSl2AkirvsES1Toi%252BOeA%3D%3D

WOW!.   The Neo-6 modules are good, even as fakes.  But, the Neo-6 only gets USA GPS sats.   The new Neo-9 series will simultaneously receive all four
systems (BeiDou, Galileo, GLONASS, GPS / QZSS).  With an indoor GPS puck antenna near a window, Neo-6 with just USA GPS could give me maybe 5 to 6
SATs.   But the new Neo-M9N give me closer to 20 satellites indoors.   Impressive little module for $40 USD.

Cheers,

Neal

[Dbldutch]

Hi Neal,

Q#1: I use two choke filters.

One for the VCF input, and I just used what I had available. I once ordered a selection from a China source. Unfortunately, no documentation. They look like .25W resistors and cannot carry much of a current. They are there to keep the 10MHz from the OCXO from getting in the DAC adjustment circuit. The value is not very critical but they do make a large difference.

The second one is used in series with the powerline to the OCXO. I used a Ferrite I already had available. It has a few holes (we call it a pig-snout in Dutch) where the lead is winded through a few times. It can carry several Amps. It is there to keep hf noise from the outside (switching power supply) from getting in the OCXO. For a picture for this one, have a look at one of my pictures of the PCB  in an earlier post, or look for a picture on my blog.

Q#2: I used the values in the table. The resistors in the diagram where just placeholders for the OCXO that I used first. Don't forget that these values are dictated by the gain setting procedure, and are highly depending on the quality of your OCXO and the sensitivity of the adjustment.

Q#3: The small resistors in series are there to remove some of the sharp edge transitions that may cause cross-talk or ringing. Leave them in.

Q#4: I used what somebody on this blog recommended. 68 Ohm in series with the data line and a 10uF capacitor to ground. Yes it is a low-pass filter.

You are right, only NEO's at model 7 and up can be configured to receive all the constellations. If you don't have a clear view of the sky, get a timing version like the 8MT.

Good luck!

[Mike99]

Hi Neal

Just to add to what Dbldutch said (I have built his circuit and it works very well).

The voltage control input to the OCXO is high impedance so almost any size choke will be OK. I used these 0.25W 33uH chokes:

https://www.ebay.co.uk/itm/Inductor-Choke-Coil-1-4W-0-25W-Values-0-1uH-to-1mH-Various-Pack-Sizes/152687409289?ssPageName=STRK%3AMEBIDX%3AIT&var=452270326536&_trksid=p2057872.m2749.l2649

And for the supply ferrites I used these:

https://www.ebay.co.uk/itm/PACK-OF-4-FERROXCUBE-MHC6-6-10-4B1-MULTI-HOLE-FERRITE-CORES-10MM-BY-6MM/122831367380?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2057872.m2749.l2649

The Mikroe module looks like a bargain (£29 in the UK). Does it have a 1pps output line or do you have to tack a wire on?

Mike

[Dbldutch]

The two that Mike took the effort to look up are indeed the ones I use.
Thanks Mike!

[Fennec]

But the new Neo-M9N give me closer to 20 satellites indoors.   Impressive little module for $40 USD.

Cheers,

Neal

M9N  Accuracy of time pulse signal RMS  30ns
99%                                                     60ns

M8T   20ns

You don't need 100 sats, you need a stable Module.

[Mike99]

You are right, only NEO's at model 7 and up can be configured to receive all the constellations. If you don't have a clear view of the sky, get a timing version like the 8MT.

Is the M8T pin compatible with the 7N? If so I could swap the module on one of my boards.

Mike

[Dbldutch]

As far as I know, the 6, 7 and 8 are all pin compatible.
I swapped my Chinese copy of the 6 with a genuine M8T on the same carrier.

[Fennec]

Is the M8T pin compatible with the 7N? If so I could swap the module on one of my boards.

Mike

Yes we did it in the BG7TBL thread.

[nealix]

M9N  Accuracy of time pulse signal RMS  30ns
99%                                                     60ns

M8T   20ns

You don't need 100 sats, you need a stable Module.

@Fennec:

Thanks, I do understand that, and did at the time.  But I could not locate any supplier of a breakout module board with the M8T on it
for anywhere less than $100 USD.    Where do you guys find a timing module on a breakout board that can actually be used without
doing your own wave soldering and PCB design?     I did find a person selling on Tindie selling the SkyTraq timing module and ordered that:
   SkyTraq Venus838LPx-T Timing GPS module breakout
   Add a u.FL to SMA pigtail?: No
   Add an amplified u.FL patch antenna?: No
   Connection style: SIP
   Unit Price: $50.00 + free shipping.
   Quantity: 1

But I also ordered the Ublox M9N breakout module I mention above at the same time, just to compare, since I knew
that module would arrive in a few days, and the Skytraq takes about a month to get here.

Cheers,

Neal

[nealix]

Hi Neal

The Mikroe module looks like a bargain (£29 in the UK). Does it have a 1pps output line or do you have to tack a wire on?

Mike

Yes, it does have 1-PPS output.   Here is the schematic of the board;
https://www.mouser.com/datasheet/2/272/gnss-7-click-schematic-v100-1863770.pdf

It also comes with the backup battery installed.

Neal

[Fennec]

But I could not locate any supplier of a breakout module board with the M8T on it
for anywhere less than $100 USD.    Where do you guys find a timing module on a breakout board that can actually be used without
doing your own wave soldering and PCB design?   

You like it accurate, so you have to pay. If you need just a "frequency" you can set a cheap $US15 UBlox module output with UCenter.
I do not understand why you want to use an "High End Lars-Board" with the lowest quality pulse generator. 
Contradiction in terms
See att

https://www.ebay.com/itm/1x-Ublox-LEA-6T-2-000-50-channel-15ns-1pps-Timing-GPS-module-WD22UGRC/313149500246?hash=item48e92a2b56:g:IR8AAOSwULhfEG5T

[Mike99]

OK it's time to ask the experts a question please.

I've been running my GPSDO for a while now with tc=200s and tempCoeff=0 just to put some aging on the OCXO, which was sold as unused. The first graph shows what happened over a three week period.

Regarding the time constant, I thought 200s would be a reasonable number to start with, but I've now started a run at tc=4s to determine the optimum value. The second graph shows the current result but I will leave it running to improve the accuracy.

Here's the question. With tc=200s it loses lock, then recovers, several times every 24 hours. How do I determine the cause?

Mike

[Dbldutch]

Mike,

You can't tell that from the MDEV chart.
You'll need to store the report from Lars' program over a 24hr period and use Excel to chart the ns, DAC and OCXO temperature values to try to make sense out of what's going on. Can you publish those graphs, or post the excel file with the report?

[nealix]

But I could not locate any supplier of a breakout module board with the M8T on it
for anywhere less than $100 USD.    Where do you guys find a timing module on a breakout board that can actually be used without
doing your own wave soldering and PCB design?   

You like it accurate, so you have to pay. If you need just a "frequency" you can set a cheap $US15 UBlox module output with UCenter.
I do not understand why you want to use an "High End Lars-Board" with the lowest quality pulse generator. 
Contradiction in terms
See att

https://www.ebay.com/itm/1x-Ublox-LEA-6T-2-000-50-channel-15ns-1pps-Timing-GPS-module-WD22UGRC/313149500246?hash=item48e92a2b56:g:IR8AAOSwULhfEG5T

RE:  I do not understand why you want to use an "High End Lars-Board" with the lowest quality pulse generator.

:-) :-)  If you read a little higher, you may notice that I purchased both a Skytraq Venus838LPx-T Timing GPS module breakout,
AND the U-Blox M9N.   The M9N is a very good enough receiver for hobby level work on my projects, and got here in days,
so I could play and learn while the Skytraq takes one month to arrive.

Some of us are "time-nuts" {tm}, but I am not.    My personal goal for this project is to learn more about how a GPSDO
really works, learn time lab and lady heather, and compare the result  to my $175 commercial GPSDO.   If I wanted or
needed "Aerospace" level accuracy, I could indeed choose to rip the wallet wide open and spend spend spend.  But I like
the idea of seeing if my parts spend can be less than or equal to the commercial unit, yet match or exceed it's performance.
Each person has a different reason.  :-)

As far as accuracy for the $50 Skytraq Venus838LPx-T Timing GPS module:

The Venus838LPx-T module will provide PPS quantization error correction messages, allowing you to remove the quantization
error present on the PPS output. Without correcting this error, the PPS will have approx. ±6 ns of jitter.

Back to the non-timing and typically fake eBay U-Blox Neo-6 modules;  They are more than good enough for
my GPS disciplined Wall Clock projects, where a human can't tell if you are half a second off;

https://nealix.zenfolio.com/img/s/v-10/p4012064259-4.jpg
https://nealix.zenfolio.com/img/s/v-10/p4012064258-4.jpg

https://nealix.zenfolio.com/img/s/v-10/p4012064260-4.jpg
https://nealix.zenfolio.com/img/s/v-10/p4012064255-4.jpg

and finally, when I receive the Skytraq Venus838LPx-T Timing GPS module and can build Lars/PaulV's schematic,
this is what I wish to compare results to (The simple China BG7TBL GPSDO that uses FLL instead of PLL):

https://nealix.zenfolio.com/img/s/v-10/p4012055982-4.jpg
and yes, accurate to half a Hertz is more than good enough for my amateur hobby needs.

So anyway, thanks again for putting up with another newbie here.  But I am learning from you guys and having fun
at the same time.  For that, huge thanks!

Cheers,

Neal