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'[PIC] Fine-tuning oscillator. Only as good as the '
2006\01\11@175955 by Robert Rolf

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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

2006\01\12@164440 by Peter

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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.

{Quote hidden}

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

2006\01\12@172446 by Jan-Erik Soderholm

face picon face
Peter wrote :

> Further, assuming the user switches channels about 20 times
> per day...

What if the user switches the TV set off ?

:-)

Jan-Erik.



2006\01\12@173953 by Neil Baylis

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--> > 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. $$$ <--

You can buy a GPS timing receiver that will give you a local 1Hz reference
that's within 20ns of UTC for less that $100. Check out the Trimble
Resolution-T for example. It would be hard to do better than this for the
money.

Neil

2006\01\12@175448 by Robert Rolf

picon face
Neil Baylis wrote:

>> 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. $$$ <--
>
>
> You can buy a GPS timing receiver that will give you a local 1Hz reference
> that's within 20ns of UTC for less that $100. Check out the Trimble
> Resolution-T for example. It would be hard to do better than this for the
> money.

True. And for most time of day applications, that's good enough.
To do VLB interferometry @ 1420 MHz, one needs better than 100ps accuracy.
To do gamma ray burster direction finding 1 ns is OK for amateur
level work.
Ideally, the various sites would have a 10 MHz reference which was
absolutely in sync (<100ps) to use as reference for time stamping
the samples, and GPS time to get gross phasing of the data.

Robert
If you have only two clocks, with different values, what time is it
really?

2006\01\12@182335 by Robert Rolf

picon face
Peter wrote:
> On Wed, 11 Jan 2006, Robert Rolf wrote:

>>>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.

And how many frames get dropped as sources get changed, machines
burp, etc.
I used to work in a TV station. We tried very hard to use station
sync, for everything but there were always hiccups over a day.
Today the is much less likely since the frame synchronize would
fill in the holes.

> 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.

Maybe.

I have the advantage of being able to look at several TV stations
at once (both off air, satellite and cable). Frequency accuracy
is all over the map. If one beats one station against another
(two inputs to an old analog switcher set to 10% mix between them),
one can easily see the 'spin' of one stations frame rate
compared to another. Some stations were offset by as much as
10 lines /second (the fastest I could count).
10x64uSx86400s/day =5.4 seconds a day DIFFERENCE!
I quickly realized that you can't use a TV station as a time reference.

> 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.

Yes, there is enough stray field that it is easily picked up.

> 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.

It's probably easier and less power hungry to use an old VCR.
Easy enough to make an R/C sync separator to pull out the vertical
or horizontal sync. (LM1881 if you want a chip to do it)

> 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.

2ppm is quite could compared to what I've seen for REAL.

PAL has much more stringent timing specs that NTSC.

www.microsoft.com/whdc/archive/VidSynch.mspx
"Most decoder crystals in mass production have a rated accuracy of more
or less 100 parts per million (ppm), which on a 27-MHz crystal equals
more or less 2700 parts per second."

{Quote hidden}

This ASSUMES that the television signal is broadcast with NO dropped
frames and it that has been referenced to a good source.
That is not always the case.
So you will then argue that one could
look for dropped frames and adjust the count accordingly. Sure.

In the 'old' days, networks did use Rubidium standards.
Today, with cheap frame synchronizers, most stations are closer to 50ppm
than 2ppm accuracy. At least thats what my comparison of off air
sources in Canada shows.


> 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).

Right.
Which is why a VCR or Cable convertor with baseband out is a better
starting point.


> 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.

It's certainly doable.

And if you used the power line AND the TV signal as interrupts to a PC,
(RI and RX on a serial port) you could see the jumps in phase very readily.

And just for interest's sake NTSC vertical rate is 59.940052333
14318180/455/525

Robert

2006\01\13@132006 by Peter

picon face


On Thu, 12 Jan 2006, Jan-Erik Soderholm wrote:

> Peter wrote :
>
>> Further, assuming the user switches channels about 20 times
>> per day...
>
> What if the user switches the TV set off ?

What if the unit is a portable lcd tv that is used only for this ?

Peter

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