[time-nuts] Long Wave Radio-Frequency standard testing

[JF PICARD]

 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 <clemgill at gmail.com> 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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