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
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