[time-nuts] Re: New design for a GPS disciplined OCXO or Rubidium

[Magnus Danielson]

Hi Pavel,

Do you happen to have a link to a paper or something that present the 
results of the measurements of F9T you refer to?

Cheers,
Magnus

On 2024-05-13 23:35, Pavel Kořenský via time-nuts wrote:
> Hello,
>
> I know about this problem. This is the reason why I am using the 1.25 
> MHz from F9T.
> Last year, my friend did some serious measurements od various F9T 
> frequencies. Because he is working in CERN, he has a possibility to 
> use hydrogen maser disciplined by cesium fountain as a primary 
> frequency standard and compare various sources to this primary 
> frequency standard.
> And the result was, that 1.25 MHz is one of the "best" frequencies for 
> F9T.
>
> I am further trying to minimise such errors with averaging inside PI 
> loop. Also, because the duty cycle measurement in Pi Pico is using the 
> free running Pico 125 MHz internal clock as a source for incrementing 
> the PIO counters, the results are always a bit averaged (or smudged) 
> and this also helps.
>
> BTW, it seems that the picture which I attached to my former post was 
> too big to be sent to the mail list, so here is the link: 
> https://www.rajce.idnes.cz/pkorensky/album/agilent-53132a/1635108363
>
> PavelK
>
> Dne 13.05.2024 v 23:14 Bob kb8tq napsal(a):
>> Hi
>>
>> What you will run into on the F9T output are often called “hanging 
>> bridges”. They do not lock the LO on the F9T to the GPS signal. They 
>> simply correct it to the “nearest edge” when they can. How well this 
>> works and what sort of issues it creates is a “that depends” sort of 
>> thing. You may find that some output frequencies are “nicer” than 
>> others.
>>
>> The result (at any output frequency) is that you can get a fairly 
>> long “wrong phase” out of the device. It’s not so much a phase step 
>> as a phase lock at the wrong location. The classic answer to this is 
>> to look at the “sawtooth correction” information out of the F9T. 
>> Getting that to match up with the HF synthesized output can be tricky.
>>
>> Does any of this matter? It very much depends on your application and 
>> objectives.
>>
>> Fun !!!
>>
>> Bob
>>
>>> On May 13, 2024, at 4:04 PM, Pavel Kořenský via time-nuts 
>>> <time-nuts at lists.febo.com> wrote:
>>>
>>> Hello,
>>>
>>> I used a modified Brooks Shera GPSDO (with some Gollidge OCXO an 
>>> Motorola OnCore UT+ GPS) since 1999 as a timebase for my workshop 
>>> (signal generator, counter etc.).
>>>
>>> At the beginning of 2024, the unit died an I found that the whole 
>>> thing is beyond repair, because capacitors are old, the whole 
>>> plastic box is already decomposing etc. etc. So, I decided to design 
>>> a brand new GPSDO with a different approach.
>>>
>>> I bought the uBlox F9T timing GPS module: 
>>> https://www.sparkfun.com/products/18774 and I designed a relatively 
>>> simple circuit with 74HCT4046A and Pi Pico.
>>>
>>> The whole design works as follows:
>>>
>>> The 4046 is using its phase comparator OC1 (exclusive-or) for phase 
>>> comparison of 10 MHz signal from oscillator (divided by 16 with 
>>> 74HCT393) and 1.25 MHz signal from F9T GPS (divided by 2). Ideally, 
>>> if those two signals are locked, there should be exactly 50% duty 
>>> cycle signal on the OC1 output. The sinus signals from local 
>>> oscillator and GPS are connected to the 4046 via AC-coupled Schmitt 
>>> buffers (74LVC1G97).
>>>
>>> The OC1 output duty cycle is measured constantly by two PIO modules 
>>> of Pi Pico. Pi Pico software implements a PI regulator (loop run 
>>> each 100ms) and is steering a local oscillator with 20bit SPI-DAC ( 
>>> MAX5719) with a precision 5V reference (LT1021).
>>>
>>> The GPS output signal 1.25 MHz is in F9T internally locked to 1PPS 
>>> signal from GPS satellites and is pretty precise by itself. Far 
>>> better than was the 1PPS from my old Motorola Oncore UT+ back in 
>>> 1999. At those times, there was a SA in GPS signal.
>>>
>>> But the GPS signal is not ideal, there are phase differences and 
>>> spikes couple of ns here and there. So, with the help of my 
>>> colleague, we implemented the Kalman filter which is used for 
>>> measured duty cycle filtration in PI loop.
>>>
>>> The whole solution is able to recover from smaller sudden phase 
>>> differences during couple of seconds. From large sudden phase 
>>> differences, the recovery time is around 30-35 seconds, because 
>>> firstly the Kalman filter is disconnected and cleared, the normal PI 
>>> loop lock fast again and after 30 seconds of lock, the Kalman filter 
>>> is switched on again.
>>>
>>> Currently the whole design is running on breadboard, using one old 
>>> HP-10811A double-oven OCXO as a local oscillator,  the power supply 
>>> is a chaotic mess of cheap DC-DC converters etc. but the measured 
>>> results are quite good. See the picture, where my design is compared 
>>> to TM4313 GPSDO. I measured both devices with my Agilent 53132A 
>>> which has a non-disciplined Rubidium standard (Efratom FRS-C) as a 
>>> timebase.
>>>
>>>
>>> In the next phase, I will build a decent linear power supply for the 
>>> whole thing, I will put the design (without local oscillator) on the 
>>> PCB and in the box. I want to use my existing Efratom FRS-C as a 
>>> local oscillator with much better short term stability than the 
>>> HP-10811A. The only difference (from electronic point of view) is 
>>> that FRS-C has a control voltage 0-5V and HP-10811A has a control 
>>> voltage -5V to +5V.
>>>
>>> The only thing which I do not know is how to test the final design, 
>>> because I do not have access to third, more precise "reference" 
>>> frequency in the form of either cesium frequency standard or 
>>> hydrogen maser.
>>>
>>> What do you think about my design  ? Did I missed something ?
>>>
>>> PavelK
>>>
>>>
>>>
>>>
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>>> 202444.png>_______________________________________________
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