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

[Magnus Danielson]

Hi Zdenek,

How is edges of the 1.25 MHz corrected? Often?

Just a straight divided down of 125 MHz gives that clock, but not the 
GNSS result, so some form of hand-over I expect to occurr.

Cheers,
Magnus

On 2024-05-14 19:44, Zdenek Chaloupka via time-nuts wrote:
> Hi,
>
> the reason why 1.25 MHz is so clean, is that internal clock for 1PPS/freq. output is based on internal 125MHz clock (that is why there is 8ns quantization error for 1PPS). Sawtooth correction information is of no use at 1.25 MHz, as it is output only for some tens of hertz, if I remember correctly.
>
> Best regards
> Zdenek
>
>
>> On 13 May 2024, at 22:35, Pavel Kořenský via time-nuts <time-nuts at lists.febo.com> 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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