Hi JF,
DCF77 is more distant, less powerful and probably more polluted
(77kHz).
Anyhow I would probably not be able to measure better than 10e-11 with
current setup (need a better reference)
Indeed a good and stable phase lock was not easy to reach.
I experienced the day and night huge differences (as documented in
post)
but nothing specific to phase shifts during sunrise or sunset.
No big difficulties with the ferrite antenna and the receiver design
either (thanks to good stuff from the old radio days probably).
Found that magnetic field antenna (ie: ferrite) appeared much less
sensitive to pollution than electric field antennas.
Naturally bad experience with Led bulbs and vapor gas lamps. You have
to
chase them all and change to old filament lamps in and around the lab.
No
issues with computers though.
What I found most challenging (and hence interesting) was the
following :
- Temperature control, high resolution and high stability (Crystal
oscillator but also for the controller parts, ADC, DAC… )
- PI loop stability (very tricky)
- Matching theory with practice (still work in progress…!)
- Understanding the logic and physics behind behaviors, the real root
cause of problems,
and especially why a « really clever » enhancement - more than often -
actually leads to… performance degradation...
Gilles.
> Le 15 janv. 2021 à 16:57, JF PICARD via time-nuts <
[email protected]> a écrit :
>
> Hi,
> 800Kw according to the press release of ANFR. I doubt it is the best
choice : DCF77 is more precise (active hydrogen maser) but a little
bit
more distant...
> But the phase lock of a quartz on a VLF signal is not as easy. There is
a considerable phase shift in the evening and in the morning with the
sun
position, big instabilities at these moments and you have a hudge
difference between day and night (10 e-9/8)... Have a look at the
Adret
receiver 4101 with its step motor phase lock...The engineering of the
ferrite road antenna is very tricky : temperature coefficient of the
ferrite, subtle tiny out of resonnace tuning, problem of the
interferences
from domestic electrnic pollution (computers with sync of monitors,
led
drivers...). The archiyecture of the receiver is also tricky : no AGC
(introduces phaseshift), heavy filtering (where : antenna,
receiver...)
> On Friday, January 15, 2021, 03:54:40 PM GMT+1, Gilles Clement <
[email protected]> wrote:
>
> Hi,
>
> This is to share current results on a "Long Wave RadioFrequency
Standard" project I have been pursuing for a while.
> Attached are typical ADEV plots and a block diagram of the system.
>
> I live in a crowded city (Paris, France) with no - or very limited -
access to open sky. Not good for GPS.
> However a long wave broadcasting public service is (still) available,
broadcasting time signal for clocks.
> The transmitter is located in Allouis, central France (200km for Paris).
> The signal is quite powerful (1MW) and the carrier (162kHz ) is
stabilized with a Cesium-standard.
>
> I decided to test how far I could go in disciplining a local VCO with
this signal.
>
> As well known, long wave RF has interesting features:
> - Signal is available (almost) everywhere, anytime, in the country
especially inside buildings (even underground !)
> - Quite stable and strong ground wave in day time.
> - Relatively easy antenna and RF signal processing (ferrite rod)
> And there are naturally a number of drawbacks (especially with the
Allouis signal) such as:
> - Much more unstable signal at night (interferences with ionospheric
path)
> - Large phase modulation of the carrier (time signals bits +/- 1 rad
phase modulated).
> - RF perturbations on the signal path.
> -Stop broadcasting for maintenance every Tuesday morning….
>
> Design of the « LWRFDO » was derived and inspired from many references
(including this list naturally).
> Principles are summarized in the attached pdf, with the following
specific feature to get rid of the phase modulation:
> The incoming signal has large sections of « un-modulated » segments
between the time signal bits.
> (Including a whole quiet section during the 59th second)
> Such « quiet zones » are detected - where the 162kHz base carrier is
untouched - and measurement of phase difference
> with a local OCXO is then performed inside these quiet zones. Then PI
controller to a 20bits DAC (see picture).
>
> Latest results show ADEV approaching 10E-11 at 1000 seconds on the « D2
» graph (day time only).
> « DN123 » is a three days uninterrupted run, combining day and night
signals, showing the impact of night instabilities.
> The frequency standard stability at the transmitter site is given for
10e-12.
> LWRFDO PPS is measured against an HP10811A PPS (about 10e-11 stability a
100s) with a TICC,
> So I believe 10-11 is not far from the best one could get.
> Which is actually not too bad, isn’t it ?
>
> Still working on improving the OCXCO (currently home brewed)
>
> Comment and suggestions welcomed,
> Gilles.
>
>
>
>
>
>
>
>
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