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

[Pavel Kořenský]

Hello,

it will not be a commercial product, I am developing it just for fun and 
because I need a new frequency standard in my workshop.

When I will have at least version 1.0 (with PCB, power source, proper 
box etc.) I will put the design on Github for free use.
It will be a GPS/DAC solution only. One box with GPS/DAC and external 
connectors for 10 MHz input, DAC output, GPS antenna input, 1PPS out 
from GPS and two microUSB connectors (one for Pi Pico programming and 
second one for communication with F9T module) and also input connector 
for Lock Monitor from Rb standard. Most probably, there will be some 
switch which will allow to select two different types of DAC output 
voltage. One for OCXOs like HP-10811A with -5V to +5V output voltage and 
the second one for Rb standards with 0V to +5V output voltage.
The reason for this type of design is that I already have my Rd standard 
in a decent box with power supply etc. and I am too lazy to change it 
and I also like the possibility to disconnect the Rb and take it 
somewhere else, together with my frequency counter.
Take the whole unit with GPS somewhere else is problematic, because the 
GPS receiver is running in position fix mode and acquire the position 
fix at some other place takes a long time.

PavelK

Dne 14.05.2024 v 18:38 Neal Pollack napsal(a):
> Hello Pavel:
>
> Is your GPSDO going to become a product, or will it be a shared 
> project that other people can build?  The design sounds more accurate 
> than the recent one I built for use with my hobby radios.
> Although, for my use, I probably do not need better accuracy :-)
>
> Neal
> N6YFM
>
>
> On Tue, May 14, 2024, 08:46 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
>     >>
>     >>
>     >>
>     >>
>     >> <Snímek obrazovky 2024-05-13
>     202444.png>_______________________________________________
>     >> time-nuts mailing list -- time-nuts at lists.febo.com
>     >> To unsubscribe send an email to time-nuts-leave at lists.febo.com
>
>     _______________________________________________
>     time-nuts mailing list -- time-nuts at lists.febo.com
>     To unsubscribe send an email to time-nuts-leave at lists.febo.com
>