[Fwd: LF: Frequency stability]

[Andre' Kesteloot]

vernall wrote:

> dave wrote:
> > No problem, I will look for any other "signatures" that may aid
> > identification. The Algerian or Russian TXs may well get over there better
> > than Droitwich so the sidebands may be more than 20dB down as some of the
> > carrier energy could be from another TX. They all seem to be netted within
> > a fraction of a Hertz (cycle!).
> >
> > 73, Dave G3YXM
>
> There is an ITU recommendation for highly controlled frequency offsets
> to deploy in sychronous networks, for LF and MF AM broadcasting.  I
> think the offset is a good fraction of 1 Hz but I would need to check
> that out (elsewhere, not in my local info) before trying to give any
> detail.  The idea is to reduce the audible effects to their listeners
> who happen to be in the "mush area" where the signals strengths are
> about equal.  Synchrounous networks are likely to be entirely within a
> given country.  Frequency re-use around the region would be "random" in
> what specific carrier is set, but would likely be within a few Hz
> (again, there is an ITU recommendation on frequency stability).
>
> Out of interest, I have checked the carrier frequency of some local
> aeronautical beacons (they use the band limits of some 200 - 400 kHz in
> New Zealand) and it is rare to find one that is more than one Hertz
> out.  However, less than 1 part per million (1 Hz in 1 MHz) is not much
> trouble to a decent oscillator.
>
> Which perhaps raises another matter .....   How good is the "timebase"
> in each of the PC based spectrum displays?  Each must rely on some
> internal oscillator or bus data for reference, and it is probably an
> oscillator with other than a tight specification.  I have used GRAM
> (uses a SB card) for some audio checks, and it seems to be agreeably
> "correct" from a general point of view, but I do not know the actual
> tolerance.  No doubt there are readers who can fill in the gaps on this
> ....
>
> Bob ZL2CA