WWVB BPSK Receiver Project?
Bill wrote:
[BPSK] leaves all the real Timenut type people, actually
using the system for its intended purpose, out in the cold
To be fair to NIST, there really aren't many people using WWVB as a
source of laboratory-grade timing signals. As others have pointed
out, it isn't accurate enough for true time nut performance, and to
get all of what it is capable of requires heroic efforts. So in
truth, the real market for WWVB is not time nuts -- it is people who
want to know the time of day to within a second (the "atomic" clock
crowd). And there are LOTS of them. So the change is likely to
provide a modest upgrade path for the vast majority of actual users,
at the expense of a few die-hards (hobbyists, mostly) who are trying
to get more out of an LF timing source than it is really capable of
delivering.
From a public policy standpoint it seems to make good sense, however
much it may offend time nuts' sensibilities.
Best regards,
Charles
In message 20120315043646.1BC3F11BB72@karen.lavabit.com, "Charles P. Steinmet
z" writes:
As others have pointed
out, it isn't accurate enough for true time nut performance, and to
get all of what it is capable of requires heroic efforts.
And isn't that what being a time-nut is all about ?
VLF signals, once they have phase-code, are pretty good for frequency
stabilization, you just need to use an averaging time of 24 hours.
--
Poul-Henning Kamp | UNIX since Zilog Zeus 3.20
phk@FreeBSD.ORG | TCP/IP since RFC 956
FreeBSD committer | BSD since 4.3-tahoe
Never attribute to malice what can adequately be explained by incompetence.
Hi Charles:
There's another thing the WWVB (& WWV) do that GPS does not and that's Daylight Saving Time.
Pop quiz. . . . what are the dates DST is turned on and off?
http://en.wikipedia.org/wiki/Daylight_saving_time_around_the_world#United_States_of_America
Have Fun,
Brooke Clarke
http://www.PRC68.com
http://www.end2partygovernment.com/Brooke4Congress.html
Charles P. Steinmetz wrote:
Bill wrote:
[BPSK] leaves all the real Timenut type people, actually
using the system for its intended purpose, out in the cold
To be fair to NIST, there really aren't many people using WWVB as a source of laboratory-grade timing signals. As
others have pointed out, it isn't accurate enough for true time nut performance, and to get all of what it is
capable of requires heroic efforts. So in truth, the real market for WWVB is not time nuts -- it is people who want
to know the time of day to within a second (the "atomic" clock crowd). And there are LOTS of them. So the change is
likely to provide a modest upgrade path for the vast majority of actual users, at the expense of a few die-hards
(hobbyists, mostly) who are trying to get more out of an LF timing source than it is really capable of delivering.
From a public policy standpoint it seems to make good sense, however much it may offend time nuts' sensibilities.
Best regards,
Charles
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I'm not clear how accurately one can resolve the phase transition
in the new scheme, but I suspect probably unambiguously to 1 cycle of
the 60 KHz... and from there is merely a function of how accurately one
can resolve the phase of the 60 KHz. This potentially can supply a
much higher resolution time hack than the AM envelope.
I think the low transmitting-antenna bandwidth will prevent an
unambiguous identification of the exact cycle of phase inversion, just
as it smears out the AM transition from high power to low power.
Fitting a model to the signal's AM exponential decay (or PM
transition) would be better than a simple threshold, but at best it
might get down to 50 us territory (excluding the propagation delay and
iono uncertainties).
Cheers,
Peter
In thinking about it a bit further, one might be able to take the 60 kHz
received sine at some point in the receiver, full wave rectify and HP
filter it (which doubles the frequency) then divide by two in a Flip-Flop
and heavily filter the resultant. This is a hybrid solution... analog and
digital... with not a uP in sight!!
That would preserve the frequency, but ditch the phase reversals of the
BPSK. Depending on the guts of the particular receiver, it might be
possible to simply retrofit a PCB.
There would be an SNR penalty for this, though, called "squaring
loss". A PIC that knew when the transitions would happen and inverted
the original signal would be free of squaring loss, since its
reversing-signal would be noiseless.
My worry, though, is that even this preprocessing doesn't look like it
would give as good a signal as the original WWVB. Eyeballing the
phase data derived from John Seamons' capture seems to show some phase
variation from bit to bit, even those bits with the same nominal
carrier phase. Some nonlinearity in the transmitter when hit with
these phase transients perhaps. How quickly does it average out in a
carrier-phase receiver? Unknown.
Cheers,
Peter
Brooke wrote:
There's another thing the WWVB (& WWV) do that GPS does not and
that's Daylight Saving Time.
Doesn't that reinforce my point? Automatic adjustment of time-of-day
clocks for DST is not really a time nut priority, is it? Very
convenient in daily life, yes -- but to the general public, time nuts
included, not to time nuts qua time nuts.
Best regards,
Charles
Part of the processing gain comes directly from the BPSK modulation and that
amounts to a little over 10 dB improvement, but there's a further 18 dB gain
to be had by accumulating an hours worth of data and processing that.
That part of the paper bothered me. There's nothing preventing a
receiver from averaging the current AM-only signal for a long time.
They shouldn't be taking credit for that.
A receiver capable of integrating over a few hours using the existing
signal would arguably achieve many of the stated goals of the paper,
including the jammer resistance.
Maybe the new signal is an improvement, and I would have nothing
against it if it doesn't hurt the overall phase stability, but apples
should be compared with apples.
Cheers,
Peter
How about this: Generate a precise 60 KHz signal from a GPSDO's 10 MHz.
Modulate it with 1 bit audio generated by a Linux program which would know
about DST. Feed this to a loop around the house to give a good 60 Khz
signal
inside but little outside.
I have thought of this to keep my Atomic Clocks working :-)
--
Chuck Forsberg WA7KGX N2469R caf@omen.com www.omen.com
Developer of Industrial ZMODEM(Tm) for Embedded Applications
Omen Technology Inc "The High Reliability Software"
10255 NW Old Cornelius Pass Portland OR 97231 503-614-0430
On Wed, 14 Mar 2012 18:14:56 -0700
WB6BNQ wb6bnq@cox.net wrote:
His enthusiasm was aimed totally at new products. Although he admitted
it leaves all the real Timenut type people, actually
using the system for its intended purpose, out in the cold, he really
did not seem to care. Pointing out that a failure
with the GPS system left WWVB as the only alternate did not seem to
matter either.
Could someone be so kind and could explain me what the problem with
the BPSK modulation is? I mean the phase of WWVB shifts around several 10us
during sunrise/sunset already... Not to talk about the changing propagation
conditions. Just see [1] for an example of what's happening.
Yes, for those devices that lock on the phase, you'd have to change
their correction/detection loop, but overall, they should still work.
Attila Kinali
[1] http://www.febo.com/time-freq/wwvb/spectracom/index.html
--
The trouble with you, Shev, is you don't say anything until you've saved
up a whole truckload of damned heavy brick arguments and then you dump
them all out and never look at the bleeding body mangled beneath the heap
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On 3/14/12 9:14 PM, J. Forster wrote:
On 3/14/12 8:07 PM, J. Forster wrote:
John
Like your thought. I seem to remember costas loops work like that to
recover the carrier.
Paul,
It recovers a bipolar signal to steer the local VCO as well as the
data..
It also needs a quadratue hybrid at the VCO frequency (although it might
be fairly easy to make a quadrature oscillator vat 60 kHz.)
One easy scheme is to make your VCO run at a multiple and divide down to
generate the two quadrature square waves.
Doesn't look like that works with the HP 117A. I don't know about other
receivers.
Had seen it in amsat many years ago. So perhaps an approach is to limit
if possible the incoming signal.
I'm not sure if it works properly with clipped (digital) dignals, off
hand.
Yes it will.
Not w/o a quadrature drive to the mixer/multiplier. A square wave,
multiplied by itself, has the same output as input.
Oh... I was assuming you had the two quadrature square waves (which are
just like the saturated LO for the mixer in RF land)
Though further simple dumb thought. A NE602 or SA602 or also teh 612
series. All the same mixer circuit (Or multiplier)will double the
incoming
frequency if you delay the incoming by 90 degrees I think.
Sine and Cosine are orthogonal. You need to do (Sine)*(Sine)
sin^2 (wt) = 1/2(1 - cos (2wt)
This is like the classic squaring technique to receive PN coded signals
without knowing the code. (it's used in some "codeless" GPS receivers..
you can retrieve frequency and phase)
A Costas Loop recovers the bit stream and the carrier frequency (from the
local VCO) from a BPSK. It is self syncronizing.
Yes.. but if you don't care about the bitstream, and you want simpler
hardware, squaring works. (especially if the modulator doesn't have good
carrier suppression)