On Wed, Sep 26, 2012 at 11:59 AM, Majdi S. Abbas msa@latt.net wrote:

If I do this the PC would likely be a good large distance from the RF
front end.  With an SDR type receiver you'd place the RF stuff very
close to the antenna and the long cable from RF section to computer is
a balanced audio cable, like an XLR microphone cable.    By current
60MHZ WWVB receiver lives outdoors inside a 1 foot length of PVC pipe
far from the house.  Only serial data leaves the pipe and DC power
goes in.

Also the RFI from a PC depends on who built it.  Typical
self-assembled computers are very poor.  Notebooks can be good. bUt
thebest plan is to use the inverse square law and some distance and
RFI goes away.

Chris Albertson
Redondo Beach, California

On 26 Sep, 2012, at 11:19 , Majdi S. Abbas wrote:

This is fine, though to make it maximally useful for time and
frequency purposes I believe the hardware might need to provide a
way to synchronize the ADC clock to an external reference, and likely
some way to time-mark the incoming data (e.g. a quick-and-dirty version
might feed a PPS signal to the second channel of a stereo ADC, if no
more elegant solution is available).  A control loop to discipline an
oscillator's output might use that oscillator to clock the ADC and adjust
the oscillator to zero the ADC's phase alignment with the input signal,
if that can be made to work.  A system to measure WWVB propagation delays and
signal levels might instead clock the ADC and the time marker with a
known-accurate frequency and PPS (e.g. a GPSDO).

RFSpace makes commercial LF/MF/HF SDR equipment with almost the right inputs
for this (an external frequency input and a timing trigger).  What I'd like
is a tiny-budget version of this just for LF stations.

I guess the trouble with this is only that the availability of brute force
can sometimes make it unnecessary to deal with a lot of complexity.  If your
job is to do a convolution of a model of what you know was transmitted
against the incoming signal to measure the time alignment then using a
platform where you can store big blocks of data and do Fourier transforms
with wild abandon can provide really good results without having to spend
a lot of time thinking about it.  Even quite modest modern PC hardware comes
with a boatload of memory and is exceedingly speedy, and for some purposes
it can save a lot of time and effort just to make use of that compared to
trying to do without.

I have a quick-hack DCF77 PM detector which runs on PC hardware and makes use
of one of the above-mentioned RFSpace receivers for the data acquisition.  While
it is now in boxes being moved, when I get it back up I would love to lose the
RFSpace receiver in favor of something much less costly, but would hate trying
to make this work with something less capable than the PC.  Using a microcontroller
like that to do the A/D conversions and send the data collected out (say) an
ethernet port to a PC which does the heavy computational lifting (that's what the
RFSpace receiver does) would appeal to me, but trying to do without the PC would
not.

Dennis Ferguson

Might be a bit of a cost. The SDR runs $1495.
Regards
Paul

On Wed, Sep 26, 2012 at 5:36 PM, Dennis Ferguson <
dennis.c.ferguson@gmail.com> wrote:

On 26 Sep, 2012, at 14:43 , paul swed wrote:

The ones with the clock input options (the SDR-IP
and the NetSDR, I think) are significantly more than
that. But they are also huge overkill if all you want is
a digital LF receiver.

That's why I'd like to replace it with something cheap,
but that something wouldn't be nearly as useful without
the clock and timing edge inputs.

Dennis Ferguson

Hmmm SDR. Does that mean spensive darn radio?
Joking aside its clearly massive overkill.
The rf front end is very reasonable and 3-4 stages of opamps like the TL08X
class will do a very fine job and you can even use stages as active
bandpass filters etc. Have built a few over the last 6 months. Its the
various other things that get messy. Compression to deal with the amplitude
modulation, agc etc. Forget hard limiting what a mess that made. Then after
you have that settled you can jump into the technology of your interest.
I will say that there are lots of high level concepts but the devil is in
the detail.
What is clear is that the system has to know whats going on in the channel.
Impulse noise and fades here on the east coast make the design difficult.
As compared to say a Kansas station in the 500 uv signal region. The weak
signal and channel behaviors tend to make lots of great ideas useless.
I did also tinker with spectrum lab and a pc. Looked interesting and maybe
could have value.
But running a PC 24X7 seems like a Kluge. I prefer low power solutions
measured in sub 100 ma. Hey its got to be green because wwvb really isn't.
Regards
Paul
WB8TSL

On Wed, Sep 26, 2012 at 5:53 PM, Dennis Ferguson <
dennis.c.ferguson@gmail.com> wrote:

On 09/26/2012 07:13 PM, Tom Van Baak wrote:

Looking at it, the BPSK receiver strategy often becomes the Costas loop
should maybe not be that much of a modification to some of these. The AM
and PM modulation should be fairly trivial to crank out of them.

In a Costas loop, you mix-down your input signal in both I and Q signal,
by using both the cos and sin variant of an oscillator, which can be
realized by several means. The I and Q mixer outputs is then low-pass
filtered. The I and Q is mixed to provide the phase detector signal,
which is then used for the normal PLL loop.

Modifing a standard loop into a Costas loop involves adding a 0-90 phase
splitter (see polyphase filters), a pair of mixers and a pair of
low-pass filters. If the oscillator is available at 4 times the target
rate, a cheap digital trick can be used to create an oscillator
phase-split by xoring the bits or using other alternating methods. The
Tayloe detector might be a thing to look at in that case.

The Costas loop is a good vehicle towards a MAP receiver if you as so
inclined.

There was a paper on the new signal, and some receiver strategies.

Another trick to remember is that both the AM and PM signal has a very
high degree of predictability as many bits re-occur precisely or can be
predicted out of earlier state. The redundancy bits provide redundancy
within each frame rather than between frames. Thus, the actual
information flow or "news" in the channel is essentially zero once you
have locked into the signal. What changes is your oscillators phase, and
the amplitude and phase of the transmission-path.

I'm not near the WWVB transmitter to care enough to rebuild a receiver,
but I would guess that the handy time-nuts would not have too hard to
hack their existing receivers into a Costas setup.

Cheers,
Magnus