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

Sat Jan 16 08:43:08 UTC 2021

It's a custom LF receiver I put together, a direct conversion downconverter
to 1kHz. Then bandpass sampling to an I/Q data steam, at 12 bit resolution
sent using RS422 o a PC. All frequency conversion and sampling is locked to
a master 10MHz clock.  The interface also includes time stamping from a GPS
module.

PC software takes the data steam, applies a small optional frequency
offset, needed to take out DDS frequency setting resolution, displays raw
input on a vector scope.  It then decimated filters the data down to
sampling rate in teh Hz to sub-Hz region for storage and further analysis
of display.

I originally designed the S/W for use with the  Ebnaut   LF data comms
work, hence the reference to .WAV files with time stamped file names but
its more use as a general tool for monitoroing LF signals.

You can find details here  http://g4jnt.com/Coherent_LF_Receiver.pdf  and
there's a better write up in Jan 2021 edition of RadCom (RSGB Members'
magazine)

Andy
www.g4jnt.com

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On Fri, 15 Jan 2021 at 23:55, paul swed <paulswedb at gmail.com> wrote:

> What was the program that you used for the plot please?
> Regards
> Paul
> WB8TSL
>
> On Fri, Jan 15, 2021 at 5:54 PM Andy Talbot <andy.g4jnt at gmail.com> wrote:
>
> > I did a plot of the phase of the UK 198kHz longwave transmission to me, a
> > path of about 150km, compared against an HP5061A Caesium standard
> > N
> >
> > early 24 hours duration, covering night time and day time propagation in
> > October.
> >
> > You can observe the wild wandering of both phase and amplitude  during
> > night time due to skywave/groundwave interaction as the ionosphere shifts
> > around.
> >
> > Plot also at
> > http://www.g4jnt.com/DropF/droitwichplot2a.bmp
> > if the attachment doesn't get through
> >
> >
> > [image: DroitwichPlot2a.bmp]
> > Andy
> > www.g4jnt.com
> >
> >
> >
> > <
> >
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> > <#DAB4FAD8-2DD7-40BB-A1B8-4E2AA1F9FDF2>
> >
> > On Fri, 15 Jan 2021 at 21:55, Gilles Clement <clemgill at gmail.com> wrote:
> >
> > > 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 <
> > > time-nuts at lists.febo.com> 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 <
> > > 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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