In message 4F6116CE.7080809@pacific.net, Brooke Clarke writes:

I've been playing with SDR and VLF signals for ages.  What you
want is an antenna, a 1MSPS ADC and a fast-ish CPU.

One very interesting thing you can do with that, is to make a
buffer 1000 samples long, and continously average the received
signal into it, round-robin format.

That amounts to a comb-filter for every n1kHz signal, and a
trivial sin/cos multiplicator will give you the phase and
amplitude of every single radiotransmitter on n1kHz up to
your antialias filter at the same time.

If you have CPU power, you can also receive Loran-C by making the
buffer GRI*10 (or *20, if you want the code) samples long.

I've long thought about building a board with one of the faster
ARM CPUs and a 1MSPS 16bit ADC for this, but nobody else seemed
interested, so I've just used my hacked up rig.

--
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.

The first move will be to familiarize with this new modulation format. Of
course I can't receive the WWVB but the DCF77 maybe a good test for me.

On Wed, Mar 14, 2012 at 11:08 PM, Brooke Clarke brooke@pacific.net wrote:

Dear Time-Nuts,

(new at this list, but reading for long time excellent timekeeping &
oscillator articles)

I have no experience with WWVB, since I live in central Europe, but some
time ago I received quite well German DCF77 (77.5kHz) using absolutely
simplistic circuit with no tuned parts except very tolerant ferrite rod
antenna.

The point was direct sampling into an ADC and doing all the business
in a SDR fashion. I wanted to do PRBS PSK tracking and also PLL-less clock
disciplining this way, but there were another priorities, though.

However, if anybody would be interested in, I would be happy to return to
these nice LF circuits.

Greetings from Marek

P.s> A very little bit from DCF77, but only the pre-SDR stage:
http://www.youtube.com/watch?v=Fx9bas49Uow

All very nice, but if this change renders all existing receivers useless.
How does that improve things?

All it does is wipe out all the existing phase tracking infrastructure.

The only benefit is to the government who can reuse the WWVB transmitter
and frequency allocation. Everybody else will have to buy new stuff.

Sounds a lot like HDTV fiasco. Making jobs (in China or Korea) by making
scrap.

YMMV,

-John

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

[SNIP}

On Wed, Mar 14, 2012 at 3:08 PM, Brooke Clarke brooke@pacific.net wrote:

I'd say to go "100% SDR".  In other words a simple front and that
pushes as much of the functionality into software as possible.  The
carrier is only 60K.  That is low enough that one can directly
digitize the RF using an ADC that samples at only 192K/sec.
192K/Sec is a common sample rte for high-end audio and you can buy a
24-bit dual channel interface for under $200.

So I'd use an antenna (the best would be a shielded loop with many
turns of wire but ferrite "loop stick" could work) Follow that be an
RF amp and very narrow filter and then the above 24-bit 192K ADC.
With a 24-bit ADC you may not need any automatic gain control.  So yo
are almost sampling the voltage off the antenna, so that's why I
called it "100% SDR"

Once the data are inside the computer the very next step might be an FFT.

Some good easy to use software is this:
http://gnuradio.org/redmine/projects/gnuradio/wiki/GNURadioCompanion
Using this you simply drop function blocks on a screen and connect
then with lines.  It's a visual drag and drop way to build a signal
processor

As  an example to build a spectrum analyzer you drop a block the
represents your audio interface, another for the FFT operator and a
third for a graph.  Connect them together.    Then plug in a
microphone and point it as something you want to plot.

If you do use the simplest possible RF front and that can still work,
followed by a common off the shelf audio interface and then a simple
graphical programming environment you then will have a wider community
of people working on this.    You could use more complex technology
like an FPGA or a DSP chip but then the number of people who would
know how to help will be a number close to zero.

The RF front end does not need to be sophisticated because much of the
selectivity and gain control is done in software.  You just need a
hard low pass filter to remove everything above 60KHz

--

Chris Albertson
Redondo Beach, California

Asus has a $30 Xonar PCI soundcard that should do the job.
I have two of the the more expensive  pci-e versions.  Some motherboards
can do a/d at 192 but not as well as the Xonar.

I made a 60 KHz antenna by winding a zillion turns on a ferrite
rod and a padder going into the gate of a FET.  This was in the
1970s.

On 03/14/2012 03:35 PM, Chris Albertson wrote:

--
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

I will share my few bits of worked experience. But it may seem obvious.

Not necesarilly. I received 77.5kHz very well in first sampling mirror,
sampling using ADS7813 16bit ADC @44ksps, yielding carrier at 10.5kHz in
discrete-time domain.

Beware, there are lots of sigma-delta ADCs for this purpose and I am in
doubt whether they could perform better than less-bits SAR ADC.

I must object a little bit against "RF" and "very narrow" -- I have used
very sloooooooow amplifiers (they were in a shack, original purpose DC
measurement up to some 100s of kHz) and nothing narrow (or even tuned) --
except the ferrite rod itself. The rest were 2 ICs (amp & ADC) and simple
RC network.

Worked very well, including few centimeters from laptop's CCFL inverter.

Best regards,
Marek

In message Pine.LNX.4.64.1203142345310.2459@tesla, Marek Peca writes:

Here's a really interesting platform for VLF SDR work:

http://www.seeedstudio.com/depot/dso-nano-v2-p-681.html?cPath=174

1MSPS 12 bit ADC, input amplifier/attenuator, display, USB interface,
and rechargeable lithium battery.

For $89...

Too bad it doesn't have a 10MHz reference clock input for time-nuttery.

--
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 John:

They are going to maintain the existing AM modulation format so all the WWVB "Atomic Clocks" will still work.  The phase
modulation is added on top of that.

Yes, I expect my HP 117 may no longer work, but I'd much rather have the improved s/n and timing accuracy.

Have Fun,

Brooke Clarke
http://www.PRC68.com
http://www.end2partygovernment.com/Brooke4Congress.html

J. Forster wrote:

Brooke,

As I've said, I don't care about the Time. The time determined by the
start of TV or radio programs is plenty good enough to keep any
appointments.

My only interest is as a standard of Time Interval as a reference for
synthesizers, counters, etc.

If you think about it, unless you are doing something like occultation or
eclipse timing or eBay, the ToD rarely matters.

They killed LORAN, which worked beautifully.

Now it looks like they are going to kill WWVB, which is a bit more
involved, but works.

GPS is not an option without a tall tower.

This is NOT progress, IMO.

-John

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

I am afraid that like John my concern is the frequency reference. Time?
Heck it comes by the internet, WWV or GPS and lastly good old watches that
do pretty well these days. No comments on celphones. So the term is
screwed. All of the sampling and computer processing may indeed loose the
primary reference quality for frequency measurement.
So is all lost?
Well maybe not completely.
Those old receivers are actually pretty nice for filtering the incoming
signal and such. A Singer I have has a good collins 60 Kc filter. So
perhaps as a gain stage they still have value. It gets interesting at the
next step and thats what to do about the reversals of the carrier.
A question I have is this. Since the samples are actually slow on the
comparison. Would a 117 even see it. Is it perhaps just adding additional
filtering. All speculation on my part.
I need to read the dock we have just received more carefully to get a
better understanding.
Happy to run up the fluke 207 and a 117 perhaps on the next set of tests
and see what happens. (207 is actually Johns old unit) Also have a
spectracom 8170. But thats really a clock and as stated should work fine.
Regards
Paul.
WB8TSL

On Wed, Mar 14, 2012 at 8:13 PM, J. Forster jfor@quikus.com wrote:

Brooke,

In speaking with John Lowe of NIST (Group Leader for Time & Frequency service), he stated that the absolute time recovery of
their intended new modulation scheme is 10 milliseconds.  Nothing stellar there !

BUT you are right, all of us that have hp-117 type receivers are just out of luck.  John Lowe did say they are going to
produce a PIC (Microchip) project that will grab the data stream and reconstruct the carrier signal so that can then be fed
back into a hp-117 type receiver so it can still be used.  However, he did say that is a dream at the moment as they have
not really started to work on it.  He then said I could do it and they would consider my efforts.  While I had a number of
thoughts running though mind when he said that; I did hold my comments back.

I have to tell you, John Lowe sounded like he was drinking the Kool-Aid because I told him I was quite negative to the whole
idea and he went into a nonstop mode of telling me all the good things that were going to come about with this modulation
scheme.  The same kind of hype that occurred with the HDTV.  By the way, all those good things have nothing to do with
anything Time-nutty except for pissing us off, as it were.

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.

OH Well,

Bill....WB6BNQ

Brooke Clarke wrote:

OK thats great a maybe pic chip answer. They do cure all ill's after all.
Really scratching my head here. But I do think there is an answer as long
as the phase reversal is accurately controlled and still referenced to the
reference standard.
A I say I need to read.
Regards
Paul

On Wed, Mar 14, 2012 at 9:14 PM, WB6BNQ wb6bnq@cox.net wrote:

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.

The 180 degree phase reversal of the BPSK is just about the worst possible
thing for a PLL of typical receicers. If the ratio of 1s to 0s is 50% the
loop just thrashes.

-John

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

On Wed, Mar 14, 2012 at 05:13:47PM -0700, J. Forster wrote:

Everything you say up to this makes perfect sense, but what makes

you think GPS timing fails to work with less than a tall tower ?

I believe it is readily possible to get to the 10-30 ns of

UTC/TAI TOD area with just reasonable sky view, not 100% as implied by a
tower. And certainly 1E-11 or 1E-12 frequency accuracy is also readily
available with less than perfect sky view depending on your taus...

Perhaps ultimate performance requires really unobstructed sky view

in order to absolutely minimize multipath but then you are probably
talking 1E-13 or better...

Virtually ANY GPS timing solution ought to easily get you inside of

a couple of microseconds of UTC/TAI, I am pretty sure it is quite difficult
to get within 10-100 us with the current AM modulation of WWVB, possibly
even 1-10  ms is difficult.  And anything close to this requires accurate
knowledge of geographic position and 60 KHz propagation corrections.

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.

The real question being how important is preserving backward

compatibility with antique equipment versus better performance...

I agree that ALWAYS is a trade off...

--
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."

John
Like your thought. I seem to remember costas loops work like that to
recover the carrier.
Had seen it in amsat many years ago. So perhaps an approach is to limit if
possible the incoming signal.
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. Its a sensitive
chip and has a 17 db conversion gain and is $2.40 at digikey. 8 pin dip.
though what ever the delay at 60KC thats a long delay. ;-)
Regards
Paul

On Wed, Mar 14, 2012 at 10:12 PM, David I. Emery die@dieconsulting.comwrote:

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.)

I'm not sure if it works properly with clipped (digital) dignals, off hand.

Sine and Cosine are orthogonal. You need to do (Sine)*(Sine)

sin^2 (wt) = 1/2(1 - cos (2wt)

delay. ;-)

The delay (phase shift) is not needed.

Best,

-John

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

On 3/14/12 8:07 PM, J. Forster wrote:

One easy scheme is to make your VCO run at a multiple and divide down to
generate the two quadrature square waves.

Yes it will.

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)

Doesn't look like that works with the HP 117A. I don't know about other
receivers.

Not w/o a quadrature drive to the mixer/multiplier. A square wave,
multiplied by itself, has the same output as input.

A Costas Loop recovers the bit stream and the carrier frequency (from the
local VCO) from a BPSK. It is self syncronizing.

I'm beginning to think that, for the HP 117A at least, a fix could be
built on a small daughter board.

Also, I think that NIST should do the engineering and maybe run the boards
too.

-John

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