[Lf] [Fwd: LF: Re: Phase meter for propagation experiment]

[Andre Kesteloot]

Stewart Nelson wrote:

> Hi Wolf and all,
>
> With some additional programming, it should be possible to
> eliminate most or all of the special hardware.  That might
> let you get a number of folks to participate in reception.
>
> One possibility is to use the LORAN sidebands already present
> in the received signal.  They can calibrate both receiver LO
> and soundcard errors.  Of course, if you are not close enough
> to the selected LORAN station for its groundwave to always
> be dominant, that might not provide an adequate reference
> for your needs.  But that's also true if you use hardware
> based on MSF or DCF77, and you're not really close to them.
>
> I don't know how the TV broadcasters lock their sync, but
> if the signal is suitable, and the test signal is near the
> high end of the band, then just turn on the TV and the flyback's
> ninth harmonic of 15625 will be at 140.625 kHz.  It may
> be somewhat out of the receiver's passband, but should be
> strong enough to be trackable.  For calibrating the sample
> rate, use the field rate modulation inevitably present.
>
> If one has a GPS with a 1 PPS output, you can get really
> accurate results.  Again, no special hardware is needed.
> Couple the 1 PPS directly into the Rx antenna input (along
> with the desired signal) and do the rest in software
> (G3PLX algorithm).
>
> 73,
>
> Stewart KK7KA
>
> ----- Original Message -----
> From: <DL4YHF at aol.com>
> To: <rsgb_lf_group at blacksheep.org>
> Sent: Wednesday, April 10, 2002 7:36 PM
> Subject: LF: Phase meter for propagation experiment
>
> > Dear LF group,
> >
> > I am planning an experiment based on an idea from Alan G3NYK. The idea is to
> > monitor the phase variation of strong signal on LF over day and nighttime. It
> > may be interesting to find out how much phase variation is there; how much
> > the propagation path length varies etc.
> >
> > At least two receiving sites are required: One close to the transmitter, and
> > another far away from it. On both ends very accurate phase meters must be
> > used. Of course, the phase of the transmitted signal must connected to an
> > atomic clock like MSF or DCF77.
> >
> > I think I found a possibility now how the required accuracy can be achieved,
> > here is the basic principle. Most is done by software, only a decent hardware
> > is required:
> >
> > - The RX stations have clock sources locked to GPS, MSF, DCF77 or TV sync
> > (15625 Hz) of certain broadcasters.
> >
> > - The reference clock (from the locked source) is divided down to a frequency
> > which can be handled with a standard soundcard. For reasons explained below,
> > an audio tone of 1...3kHz is required. Assume 60kHz/24=2.5 kHz,  or
> > 77.5kHz/31=2.5kHz, or 15.625kHz/6=2.604166666kHz. This audio frequency must
> > be entered in the software's "sample rate calibrator".
> >
> > - The divided reference frequency (or the 15625 Hz signal) is used in the
> > software to PERMANENTLY monitor the soundcard's sample rate. This is
> > important because the sample rate may drift by a few millihertz which is
> > unacceptable here.
> > The software can already detect the sample rate from a very weak reference
> > signal, so it is enough to add a small fraction of the reference frequency to
> > the receiver's audio output because it is in another audio frequency band
> > (longwave RX: 100...2000 Hz, reference: 2.5kHz or 15625 Hz). So there is no
> > need for a stereo soundcard !
> >
> > - The 2.5kHz reference is formed into a square wave like a 'frequency marker
> > generator'. Odd harmonics are the result. One harmonic must be in the
> > longwave receiver's passband, for example 55*2.5kHz = 137.5 kHz, or
> > 53*2.604166666kHz = 138.020833333kHz. A small fraction of this harmonic is
> > added to the antenna signal which goes into the receiver. We need this to
> > compensate the VFO drift of a "normal" shortwave- or longwave receiver via
> > software as explained below.
> >
> > - Assume your SW receiver runs in USB, the VFO tuned to 136 kHz. For the VFO
> > drift compensation (which is completely done in software), the received audio
> > should contain a weak 'audio peak' at 137.5-136= 1.50kHz, or 138.02083333-136
> > = 2.02083333kHz. This audio frequency must be entered in the software's
> > "frequency offset calibrator".
> >
> > With this system of two "calibrators" (one for the PC's audio sample rate,
> > the other for the longwave receiver's slightly drifting VFO) it is possible
> > to make very accurate long-term phase measurements.
> > I have such a system running now, but not perfect yet, because my
> > DCF77-locked source sometimes unlocks for a few seconds which spoils
> > everything. I tried to convince my pocked GPS receiver to produce a
> > 1-pps-signal which could drive G4JNT's GPS locked source but no success. At
> > the present time I use the german ZDF TV broadcaster which has a precise
> > 15625 kHz signal.
> >
> > If someone likes to participate in this experiment, he may try to get the
> > last version of SpecLab running. The two 'calibrator' routines are
> > implemented but not explained in the manual yet, if there is interest in this
> > experiment I will continue development and tell you how to use it. Or offer
> > the calibration routines (written in C) to anyone who can program nice and
> > clean user interfaces...  ;-)
> >
> > Regards,
> >   Wolf DL4YHF.
> >
> >
> >