Peter wrote:
> Probably true. The odd part is that cable and sattelite digital framed 
> signals are very accurately timed, probably traceable to a rubidium 
> standard somewhere, and the cable or sattelite box loses that and uses a 
> lousy 50ppm crystal to generate the output subcarrier. The reason the 
> cable and sattelite signals are very accurate is, afaik, the fact that 
> they have to keep a large switched network operating. For sattelite they 
> also have sideband and intermodulation concerns. E.g. I do not know what 
> stability the *carrier* of a sattelite transmission has but it should be 
> very very good imho.

It depends on what is being sent through the 'bent pipe'.
There will be a LO (local oscillator) on board the satellite to 
frequency shift the uplink frequency to the downlink. Very stable LO of 
course.

If the signal is FM (old analog C or Ku band) there is NO 'stability'.
In fact it is deliberately 'dithered' at 60 Hz to prevent interference 
with terrestrial links that use the same frequency band.

If digital, there will be the instability introduced by the modulation
process, and the up/down conversions.

> Maybe that is locked to a rubidium standard 
> somewhere. 

Nope. Except perhaps for SCPC (single channel per carrier).

> The same should be true for the bit rate in the digital cable 
> signal.

Nope. Sloppy as heck. That's why the DCT (digital cable terminals)
can be so cheap. They have a reasonable PLL to recover the data even if 
it is 30Mbps.

> Perhaps the future of precision timing for the masses will require a 
> PLLd LNB on a sattelite dish and little else ? (An ordinary LNB can be 
> PLLd for sure, some already do this and the 2nd IF can be divided down 
> directly for frequency measurement). Now that would be nice. 10GHz with 
> +/-0.1Hz over 24 hours would be really nice even if weather doppler 
> would cause some problems.

The phase noise of the PLL and general instability of the crystal used 
to create the lock will be the limiting factor.

As with ANY chain, it is only as good as it's weakest link, and there 
are at least a DOZEN frequency shifts by the time a satellite signal 
gets from it's source to it's destination.

I looked at many ways to try and get a wide area, very accurate timing
signal for an amateur radio astronomy project, and a GPS disciplined 
ultra stable frequency standard was about the only way to go. $$$

Brooke Clarke would be the best person to chime in here on time and
frequency references.

http://www.pacificsites.com/~brooke/timefreq.shtml

Robert
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