On 3/10/2013 1:32 AM, Jeff Liebermann wrote:
On Fri, 08 Mar 2013 13:44:36 -0500, wrote:
I am at the fringe of
the 100 uV/m contour. I would very much like to see the signal on an
oscilloscope when I test this.
I built a passive 60KHz bandpass filters out of a collection of
ferrite cores from an old modem front end. I left it at a previous
consulting job, but can resurrect the design if necessary.
Incidentally, during my limited testing at home, I found that the
biggest determent to decent reception was all the switching power
supply noise found around the house. I finally ended up using a
battery power oscilloscope
http://802.11junk.com/jeffl/pics/drivel/slides/tek213.html
a gel cell for powering the RF amp, and turning off the main power to
the house. Then, I could sorta see a signal.
Holy crap! That's a lot of trouble to see a signal. By "see" I assume
you mean on the scope. How large was the signal?
The place where I am working currently is not very close to much and
there isn't much in the house. I'm told the fridge is the biggest
source of noise. We'll see how the CFL lamps do.
Funny, last night my two RCC's both updated like they should. One is an
analog clock and runs at 8x speed to get the hour ahead. In the fall it
does this to go 11 hours ahead. Quite a sight! They both did the job,
but my PC didn't update until it had been on for awhile, without being
connected to the I'net.
They have a receiver not far from here
in Gaithersburg, MD and the signal is often strong during the day. So
much so that I don't follow why they say there is this day/night signal
strength fluctuation. It looks much more random to me.
http://tf.nist.gov/tf-cgi/wwvbgraph_e.cgi?5636103007
Very random. Compare the above graph with Santa Clara which looks
less random:
http://tf.nist.gov/tf-cgi/wwvbgraph_e.cgi?5636105007
On the east coast, besides a weak signal, you also have the potential
for 60KHz interference from the UK:
http://en.wikipedia.org/wiki/MSF_time_signal
Loop antennas have a null that can be steered toward the source of
interference. I expect that will solve that problem...
I had a 100KHz LORAN antenna on the roof of a former employer. The
signal was just fine, until someone turned on the mercury vapor arc
parking lot lamps at night. They were changed to low pressure sodium,
which made testing possible at night.
Incidentally, got any clue as to the vertical scale? My guess(tm) is
20 uv/meter signal strength per division, but I'm not sure.
The WWVB signal is not truly on-off keying. I believe they use a 10 dB
modulation factor for the AM signal. This is close to on-off I agree.
It's now 17dB drop at the beginning of each UTC second. The change
came in about 2008.
But they also phase modulate the signal and I will be demodulating both
to see which one works best in my design.
The BPSK signal is much better at rejecting interference and digging
the signal out of the noise. I don't know exactly how much, but I'm
sure it's in a NIST publication somewhere.
That's for an ideal receiver. I have my limitations and I have no idea
how that will impact the reception.
The ADC in my design is truly one bit. It is an LVDS input on an FPGA.
I looked at delta-sigma (or is it sigma-delta? 
It's delta-sigma.
Actually I always say that with a smiley as it can be either.
The loop antenna is rather large. I would like to end up with something
smaller. Once I get this working with a shielded loop antenna I will
check out the ferrite core antennas. My understanding is that they
don't produce as much signal.
Not exactly. Small loopsticks receive a proportional amount of noise.
The ratio of signal to atmospheric noise remains roughly the same
within a fixed bandwidth for any antenna. That's why tiny little
loopsticks, inside "atomic time" wris****ches work. The small
loopsticks also use the magnetic field instead of the electric field,
which is why they can be made so small.
http://en.wikipedia.org/wiki/Loop_antenna#Small_loops
In my case I am not worried that the SNR isn't better, I just need a
strong enough signal to drive the LVDS input. I will be providing
feedback to eliminate any DC bias, but even that will only be so good.
The input is claimed to have no hysteresis, but even a tiny amount can
ruin this design. I will only know if this will work when I try it.
As for bandwidth, the code is sent at 1 baud (1 bit/sec) which
produces about a 2Hz occupied bandwidth. Therefore, the maximum Q of
the antenna would need to be:
60Khz/ 2Hz = 30,000
before the antenna bandwidth becomes a problem.
I'm not sure how you came up with 2 Hz for the bandwidth. In this case
the bandwidth is not just twice the bit rate. I believe the stated
"system" bandwidth is around 5 Hz (from a 1995 paper prior to addition
of the phase modulation).
Ok, I made a bad guess(tm). Even at 5Hz BW, the maximum Q of
60KHz / 5Hz = 12,000
is not going to happen in a loop or loopstick antenna.
Even so, that is not the limiting factor. The limiting factor is the
difficulty in holding tune with drift in passive component values.
Agreed.
--
Rick