On Wed, 11 Jan 2006, Robert Rolf wrote:

> 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.

The point is that the frequency proper is changing all the time but the 
integrated frequency (period count) over 24 hours is probably traceable 
to a good standard.

>> Maybe that is locked to a rubidium standard somewhere. 
>
> Nope. Except perhaps for SCPC (single channel per carrier).

That has nothing to do with it. Think about it. A studio somewhere sends 
TV (say analog). This goes up, then down, is converted, demodulated etc. 
The frequency stability may not be so famous. But the *count* of say 
frames or lines sent, per day, depends on the initial standard, not on 
the link.

F.ex. for PAL a studio will send 1.35x10^9 line pulses per day. Nobody 
cares as long as they all get to the other side on the same day. If one 
pulse falls 'tomorrow' instead of today, who cares. By counting the 
pulses over 24 hours one should achieve 1msec/month.

Same for V rate. 4320000 V pulses are sent in 24 hours (50Hz) with the 
*studio*'s timebase accuracy. A normal mains operated clock can be 
operated almost directly off this signal (just tap to the V yoke's stray 
radiation with a coil). I do not care how they get to the V yoke as long 
as they come from the studio.

Imho this is too good for not to be tried with an old TV and a LED clock 
mated to it temporarily and observed over a week or so.

>> 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.

I do not agree. The *total* pulses sent per day must come from the 
studio and it should have the accuracy of their output TBC, which is 
locked to the studio timebase. Maybe this last TBC is not so accurate 
but it should be within 2ppm or less even if it is not locked to 
anything afaik.

>> 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. $$$

But you are integrating pulses over 24 hours (or more)!. Even if the 
phase noise is lousy (say 50dB) the integration effect will improve this 
by 66dB or more for V pulses and even more for H.

More to the point: assuming you have a PIC using an ordinary Xtal and 
also receiving 15625Hz picked off a TV that somehow receives a studio's 
signal. It will see 1.35x10^9 H pulses per day. Assuming it can detect 
that it has received the pulses without interruption (that many pulses), 
at the end of the day (or whenever convenient), the count from the 
internal oscillator is compared. Say it runs at 4MHz, then there will be 
8.64x10^10 pulses in the clock counter when the H counter reaches 
1.35x10^9 (=24 hours). If so, then the quartz clock is exact. Suppose 
the quartz clock counter shows 8.64x10^10 - 10 pulses when the H counter 
reaches 1.35x10^9. Then the quartz is slow by ~1.1x10^-10. This 
information can be used to operate a pulse 'stealing' constant that 
effectively disciplines the quartz. The constant will be written to 
EEPROM and used all the time until overwritten (usually this is done 
with two constants, to achieve fractional approach but for such a huge 
accumulator a signed integer should work as well).

Further, assuming the user switches channels about 20 times per day and 
every time he does that, the TV jumps about 100 H pulses until resyncing 
then the uncertainty will be +0/-2000 pulses per day. So it still works 
out to better than 2ppm (as long as he does not watch DVD or video or 
Nintendo or Playstation).

Wiring a LED clock into an old TV for timing should not be so hard. 
Maybe someone can try this and see what happens after a week, by 
comparing to NTP (Internet) or similar ? The LED clock should show 
seconds for best results.

Peter

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