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.

-John

===============

Hi

And possibly if the bpsk does something useful, you can identify a carrier phase slip and correct for it….

Bob

On Jul 5, 2012, at 7:19 PM, J. Forster wrote:

On Thu, Jul 05, 2012 at 04:19:25PM -0700, J. Forster wrote:

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...

--
Dave Emery N1PRE/AE, die@dieconsulting.com  DIE Consulting, Weston, Mass 02493
"An empty zombie mind with a forlorn barely readable weatherbeaten
'For Rent' sign still vainly flapping outside on the weed encrusted pole - in
celebration of what could have been, but wasn't and is not to be now either."

Glad to know that it is not finalised as yet. When I read about this
wrinkle, I was about to put my units up for sale.

Merchison

On 2012-07-05 1:54 PM, Bob Camp wrote:

David,

Most of what has been tried is an analog squareing, then a divide by two.
No additional PLLs in the system, beyond what is already in the Rx.

The BPSK rate is 1 bit per second, There are 120,000 half cycles in that
time. Fades can last seconds, minutes, or hours. It comes down to how long
does it take your local standard take to drift roughly 4 uS.

At the moment we are not looking at the message at all.

Certainly a correlating receiver that uses the message as well as the
carrier could be built. But, IMO, that'd be a whole lot easier done from
scratch with a micro. The object here is a small, fairly simple, retrofit
for the existing receivers. The message format may not be fully defined as
yet. The squareing approach is message independant.

If you used the signal to set your local clock, and knew the format, it
should be easy to predict at least a good part, if not all, of the
message.

This is what Equatorial did, in TTL, 30+ years ago.

This is what NIST is planning, I think.  Make a new receiver, then
synthesizing 60 kHz from the internal locked clock. It's kinda like a TV
'Converter Box'. It will continue to provide the functionallity, but at
what price? At $50 it would be a good deal; at $5000 not so much, IMO.

-John

=================

On Thu, Jul 05, 2012 at 11:13:32PM -0400, Merchison Burke wrote:

Put them up for sale.  If you can find a buyer.

I asked Mr. Lowe this week and was told there'd be no residual 

carrier or workaround for existing phase locking receivers, even time
of day receivers.

Debating what to do with mine, really, keep them as nice pieces

of hardware (albeit in my own personal museum) -- they can go next to
the LORAN and GOES receivers.  The pile is getting pretty big these
days.  Anyone have an OMEGA receiver they want to part with?

Sorry, but if you want something besides GPS, you're on your

own.  The US government has made its priorities clear -- if it's not
GPS, it's an 'obsolete waste.'

Somehow, we lose out vs "12 million" WWVB clocks, despite the

fact that not 3 years ago they were willing to obsolete all those
clocks with an added 40 or 75 kHz station.  At least, as long as
'stimulus' funds were being waved around.

Along the lines of developing a receiver:

Since they haven't settled on the format, there's no additional

documentation available [that was the second part of my question..]

I'm torn on the subject anyway...part of me wants the challenge,

and part of me thinks that if the format can be changed without a public
comment period or a phase out timeframe, that it may not be worth the
risk of developing one.

As it stands I guess I'm back to WWV/WWVH as a backup.

But I can have all the self setting wall-clocks I want -- provided I
don't mind flipping them between PST and MST, since few allow you to
disable DST.  :)

--msa

No residual carrier is required.

-John

================

Lets see if this comes through. Not sure gmail is sending.
As John has mentioned we have been working on this and I have concluded
that something needs to keep the local oscillator in 1/2 of the cycle.
Hate going back to some vco approach. But that seems to be the case.
Tried forcing the miller divider into a given 1/2 cycle and because of
its nature really could not. Sometimes it just can't be simple it seems.
But I have not at all given up.
Several comments. It is a 1hz modulation. The modulation formats quite
complex. So setting a clock with it though useful and the future will
take some work. The amplitude modulation is still there and as proven
still locks the cheapy $12.95 clocks just fine. Itd oes not at all work
with spectracoms

XXX I am looking for a couple of spectracoms of the 8163 class for
experimenting (Self inflicted torture). But I am looking for the ole
flea market price. XXXX
If we can get a fix going I want to confirm the fix works with them also.

I have spent a lot of time doing much. But an approach that intrigues me
and I have done nothing with is to remember the old phase compare the
new phase per cycle and flip the phase back to the old if it changes.
This can be a feed forward behavior. Not a PLL style solution.
Granted it might take 2or 3 cycles of 60 Khz to get it figured out, but
the old receivers most likely would smooth that out.

Most likely will do this with a micro or some logic which would be fast.
Kind of a missing edge or what direction is the carrier moving in
detection approach.
Regards
Paul
WB8TSL

On 7/5/2012 11:56 PM, J. Forster wrote:

Hi

My guess is that $50 is in the ball park for parts cost of a pretty good receiver for the new format. That does not include things like the external standard, antenna, frequency comparison stuff, power or case. I'd bound the range of the guess as $25 to $100.

Bob

On Jul 5, 2012, at 11:56 PM, J. Forster wrote: