[time-nuts] HP 117/10509a...

[paul]

On 7/5/2012 11:02 PM, David I. Emery wrote:
> On Thu, Jul 05, 2012 at 04:19:25PM -0700, J. Forster wrote:
>> If propagation goes south, you loose track of the carrier phase, the basis
>> of the system. If your local standard is stable and close to right, that's
>> not a big deal. If not, you can easily go down the garden path.
> 	If I read this correctly, you mean you have a 180 degree
> ambiguity due to the BPSK - obviously losing track of the carrier phase
> in general with a significantly wrong local standard loses...
>
> 	I have not devoted enough time to this to be absolutely sure but
> it sure sounds like from what I read that if you know the accurate time
> to one second it should be possible to unambiguously predict the carrier
> phase sequences simply because you know the message format exactly, AND
> you know the exact time of day message that is being transmitted or most
> of it.
>
> 	There are of course two forms of encoding in PSK modulations -
> absolute, and differential (or transition) ... naively to me it would
> seem that if absolute encoding is used for this and you know most of the
> bits of the message most of the time you could predict which phase will
> be used a lot of the time, and also know when you don't know (message
> bits you might be uncertain about)...
>
> 	Differential encoding has the down side for this that UNLESS you
> know all previous message bits accurately starting from some phase
> reference datum you cannot predict what phase is in use at a particular
> moment.   Absolute encoding (eg 0 phase for a 0, 180 for a one) doesn't
> have that liability and if the time of day message is aligned to, well,
> the time of day if you know that with reasonable accuracy (and you do
> since you are being sent it in the first place) you should be able to
> predict a very large percentage of phases used accurately.
>
> 	Again, deferring to those who have done the experiments (which I
> have clearly not), it would seem that the ability to predict the phase
> most of the time would allow creation of a reliable local 60 KHz
> reference which could be used to disambiguate those bits you don't know
> apriori
>
> 	My naive scheme would be to drive a balanced modulator on the
> output of the 60 KHz loop antenna with either two or maybe three values
> (1 and -1 or 1,  0  and -1) using some cheapie micro (Arduino, PIC etc)
> with a software PLL to keep the bit timing in sync with the signal.
>
> 	For bits that one could not predict, one could either output 0
> to the balanced modulator for the entire bit interval  which would
> produce a drop in the 60 KHz carrier, or do a fast timed fraction of a
> bit look at the output of a synchronous detector and choose the most
> likely value for the bit and use that, maybe after a brief 0 no carrier
> interval to avoid a detectable phase glitch.
>
> 	Of course the other approach is to start with the assumption you
> have a pretty good stable source of clock or you would not be doing this
> to begin with, and simply A/D the 60 KHz with the stable clock (say at
> 10 MHz), delay it by storing samples in RAM for one bit time of the low
> speed code  and use that entire interval to decide which phase you were
> seeing and suitably adjust the output phase accordingly when you spit
> out the samples delayed by one bit time.
>
> 	This later approach would certainly be doable with modern
> processors mostly in software, certainly so if you could live with say 1-2
> MHz sampling of the 60 KHz or so... and quite possibly also pretty
> nicely with a modest FPGA complete with the sample storage in the chip.
>
> 	Both approaches would be helped a lot if the architecture of the
> system allows prediction of absolute phase (eg not differential encoding
> of unpredictable messages)... and AFAIK that is not yet set in stone and
> could be changed to allow this.
>
> 	The intent of both of these schemes would be to ultimately
> output a De-psk'd signal that older equipment could process using its
> antique analog circuitry without serious issues.   Thus the output
> would be an attempt at a phase stable corrected version of the original
> signal...
>
> 	Certainly using a lab reference stable 10 MHz derived 960 Khz
> or whatever sampling clock to delay the signal one time code bit time
> should not produce significant 60 KHz phase wanderings at all...
>
David I actually asked this ? to NIST and actually did not get an answer.
 From their documentation I believe that the the tick can actually be 
either direction. Its differentially encoded. That to me says it does 
not have to be in any particular direction. By not establishing a 
particular bias I believe there is an additional noise margin.
Regards
Paul