On Thu, 07 Mar 2013 11:39:45 -0800, Jeff Liebermann
wrote:

Fortunately, WWVB is on-off
keying with no amplitude component, so there's no incentive to add an
AGC controlled input amplifier in order to maximize the A/D converters
dynamic range.

Oops. WWVB does have an amplitude component and is not quite on-off
keying (OOK). There's a -17dB drop in RF signal level at the
beginning of each 1 second marker pulse. It was -10dB prior to 2005.
http://en.wikipedia.org/wiki/WWVB#Modulation_depth




--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

On 3/7/2013 2:39 PM, Jeff Liebermann wrote:
On Thu, 07 Mar 2013 13:11:03 -0500, wrote:

I understand. But this is intended to be *very* low power and I haven't
found an amp I can use that is in the low double digits uW power
consumption range. I plan to use no amp and go straight to digital.

I don't think that's possible. Unless your input A/D converter can
operate in the microvolt region, it's going to have a difficult time
dealing with the low signal levels. Fortunately, WWVB is on-off
keying with no amplitude component, so there's no incentive to add an
AGC controlled input amplifier in order to maximize the A/D converters
dynamic range. Still, you need to work with something more than a few
bits above the noise level. Incidentally, after midnight, you WWVB
delivers about 100 uV/meter or more to continental US.
http://tf.nist.gov/tf-cgi/wwvbmonitor_e.cgi (Java required)
I've seen it strong enough that I can see the waveform on an
oscilloscope after a 60Khz passive filter.

Yes, I have done my homework on the WWVB signal. 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. 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.

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.
But they also phase modulate the signal and I will be demodulating both
to see which one works best in my design.

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? conversion and got
code from the chip vendor for a simplistic implementation. I don't
think I have the power budget for that and am using a much simpler 1 bit
ADC at 4x the carrier rate. The bit stream is multiplied by quadrature
carriers at 60 kHz and each stream summed for 1/30 of a second to
implement what can be considered a DFT bin, a decimated FIR filter or a
decimated down conversion; take your pick, they are all mathematically
the same in this case because the sampling is synchronous to the carrier
(or very close to synchronous).

What comes out the other end of this processing gains nearly 40 dB in
SNR. My simulations show a recoverable signal when it is more than 20
dB below the noise.

Of course, I have not tested this yet on a real signal. I want to run
some tests on the antenna and coupling transformer to verify the
simulation. Then I will start working with the FPGA to see if I can
make the LVDS input do what I want. I have ideas on how to bend digital
circuits to do my bidding. This LVDS input is why I want as large a
signal as possible from the antenna. With the high impedance input on
the chip I should be able to boost the signal pretty well with just
passive devices and signal processing.

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.


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). Regardless, I am sampling at 30 Hz and if I
expect to see significant changes in phase or amplitude within one
sample time, I need an appropriate bandwidth.

Even so, that is not the limiting factor. The limiting factor is the
difficulty in holding tune with drift in passive component values. The
Q can be raised by increasing the turns ratio on the transformer, but it
becomes so sensitive to the parasitic capacitance that the sensitivity
drops 10 dB with a 1 pF change.


Incidentally, while Googling away merrily, I found this on SPICE
models for a loop antenna. It's not quite in your xformer format, but
it might be useful:
http://sidstation.loudet.org/antenna-theory-en.xhtml
I won't pretend to understand what the author is doing until I read it
more carefully.

Thanks. I will take a look at that.


Incidentally, I used a WWVB code simulator driving a signal generator
to test my receiver:
http://www.leapsecond.com/notes/wwvb2.htm

I will be needing a time code simulator. I designed a commercial
product that works with the IRIG-B time code which is similar. The
functionality is not hard, it is just a matter of generating the data,
encoding it into the modulation pattern, then impressing the carrier
with the modulation. Working in an FPGA this sort of stuff is easy.

The trouble is if you make the same mistake in both the generator and
receiver they work just fine in simulation, but not with other
equipment. lol

I'll take a look at this link.


If you're seriously into this, I suggest asking questions on the
time-nuts mailing list:
https://www.febo.com/mailman/listinfo/time-nuts

I might look into that. Certainly it can't hurt to get more input.

--

Rick

On 3/7/2013 10:17 PM, Jim Mueller wrote:
On Thu, 07 Mar 2013 09:50:11 -0500, rickman wrote:

On 3/6/2013 8:13 PM, Tim Williams wrote:
wrote in message

snip


I'm not familiar with the concept of voltage transformer vs. current
transformer. How do you mean that?

Current transformer measures current (its winding is in series),
potential transformer measures voltage (in parallel).

Series and parallel with what? I'm not following this.

snip

An electric circuit consists of a source of power, a load, and something
(like wires) connecting them. Transformers can be used if the source is
providing alternating current. A voltage transformer is connected in
parallel with the load so that the source, the transformer, and the load
all see the same voltage. It can also be used to match a load to a
source. A common example of a voltage transformer is the power
transformer in a piece of equipment that changes the AC line voltage to
whatever other voltages are required by the equipment.

A current transformer, on the other hand, is connected in series with the
load so that the source, load, and transformer all have the same current
flowing through them. The most common use of a current transformer is to
measure the current flowing into a load. A clamp-on ammeter is a common
example.

Historical examples of voltage and current transformers are the "picture
tube brighteners" that were commonly used in TV sets to prolong the
useful life of the CRT. There were two types, parallel and series. The
parallel types were used in transformer operated TVs and consisted of a
step-up transformer to raise the heater voltage of the CRT above normal
to increase emission. The series type was used in sets with the tube
heaters in series and consisted of a step-down transformer that raised
the heater current above normal. Of course, raising either the voltage
or the current also raised the other. These were, respectively, voltage
and current transformers.

A loop antenna is a distributed source with the voltage being generated
along the length of the wire and also having a magnetic field so that it
can be used as part of a transformer. This blurs the distinction between
a current and voltage transformer.

Is this a current transformer or a voltage transformer?
.--------. .--------.
| | | |
| C||C
VAC C||C Load
| C||C
| | | |
`--------' `--------'

--

Rick

"rickman" wrote in message
...
A Q of a million will get you gobs of "gain", but if it doesn't couple
into free space, it's only the thermal noise of the loss generating
that
signal.

I think you aren't reading what I am writing. I said I wanted a Q over
100, not 1 million. I don't get why you keep talking in hyperbole. What
you are describing is not even a tradeoff between signal strength and
SNR. If there is no coupling, there is no signal.

It may sound like hyperbole, but it's mathematically sound. The midpoint
theorem, for example, guarantees that, between two points, you must've hit
some point inbetween, somewhere, as long as the function is continuous.
More usefully, functions arising in electronics are often one-to-one, so
it's not only true that you are guaranteed midpoints, but you'll find them
in order, too.

If you aren't looking at the extreme cases, you aren't doing your job.
Whatever's left inbetween can simply be interpolated!

The point here being, an antenna which doesn't couple into free space
obviously has a crappy SNR. The signal level can be anything, it doesn't
matter. The signal need not be small, because internal losses generate
thermal noise. With sufficient Q, you can push that thermal noise up to
your receiver threshold (which you said is an ADC) and detect signal.
It'll be bandlimited, ~60kHz noise, a useless signal, but present
nonetheless.

In general, antennas which do couple strongly to free space have low Qs.
A 1/2 wave resonant dipole has a Q of only 1 or 2, so bothering to call it
resonant is actually kind of weak. This is similarly true for a large
loop, which of course would be highly impractical here. So there must be
some middle case where SNR is reasonably unaffected, which will be the
best choice antenna.

Since atmospheric noise dominates, the antenna can stand to be pretty
small.

Raw volts don't matter, you can always throw more amplifiers at it (as
long as they don't corrupt the SNR also!).

Maybe you didn't read my other posts. I am not using an amplifier. I
am running the antenna and coupler output directly into a digital input.

You hadn't mentioned that before...

The receiver input is high impedance, approximately 10 MOhms with a low
capacitance between the differential inputs of not more than 10 pF.

Any ESR? Example, the ATmega series 10 bit ADC specifies, I think, around
10pF + 10k ESR (somewhat depending on how many mux switches it's going
through to get there).

Your description of what is happening is very terse and full of
shortened terms that I don't understand.

I could write a book on the subject to explore it in detail, but there are
many available already, and there are too many holes in my knowledge to
really be worth it, plus this is Usenet, you get what you pay for. I was
hoping you'd Google in the blanks.

What do you mean "line up that
10p resonance with the operating frequency"? I assume you are referring
to the 10 pF input capacitance. How does this get "lined up" with
anything?

There's yet another theorem in networks that has to do with matching.

A resonant tank's impedance varies wildly with frequency. But it will
always be resistive at resonance. If you connect this to another network,
which has a resistive input impedance at the same frequency, you don't
care what the L and C are, it will simply work -- old fashioned resistor
divider action!

You *do* have to worry about L and C and reactance and bandwidth to solve
for the frequency response and stuff, but you can at least approximate
that with Q factor (i.e., how much loss is draining power out of the
system).

So if your ADC input is exactly 10p + 10M, you could resonate it with 0.7H
(well...), which has a resonant impedance of 264k, and thus a reasonable Q
of 38. (The real world typically bitchslaps the theorist at this point,
as 0.7H chokes with 10pF parasitic capacitance and Q 38 at 60kHz don't
exist.) If the capacitance's ESR is less than 6.9kohms (i.e., 264k / 38),
it won't have significant effect.

You can couple to this tank via parallel or series. If you did series,
the input impedance would be 264k / 38, or 6.9k, not horrible; going from
the 0.78 ohm loop to this in a single transformer requires a 1:100 CT,
which works fine at 60kHz. (This CT would require high inductance, so as
to avoid skewing results, but that's typical of a CT. An amorphous core
CT would probably suffice. So at least that part is physically
realizable.) Note the irony of coupling a current loop to a current loop,
where in both cases, the CT looks like a small impedance relative to the
loop it's within. That's simply how huge the impedance at the ADC is.

Since all these resistances are matched, the power transfer theorem holds,
and you're pushing as much voltage and power into the ADC as possible.
The bandwidth is about 1.6kHz, so the thermal noise floor is around 5uV at
the ADC. A received power of 1nW will generate 0.1V, which is probably a
reasonable figure. The SNR of the receiver is limited by quantization
noise for 14 bits, thermal for 14 bits. A 16 bit converter wouldn't be
too expensive at this sample rate (note it's the analog sample-and-hold
speed which limits direct conversion performance; a sigma-delta, running
at 100Hz, with no S&H, won't see jack).

When you talk about reactances canceling, that sounds a lot like a tuned
circuit at resonance. That is what I *am* doing and where this thread
started. One problem with that is the lack of precision or stability of
the parasitic capacitance. Any idea how to deal with that?

Considering theoretical 0.7H chokes aren't commercially available, you
might swamp it with more C, which stabilizes the value, and requires less
L to resonate. Rub: resonant impedance is lower, so the Q of the
components must be higher in order to achieve the same performance. Even
with a Q of 200, you still need over 0.25H, which is just as unlikely a
combination. Well, if you really wanted to try, maybe a gapped
ferrite-cored inductor could be made. Still, the only practical choice
seems to be lower signal level.

So ultimately, the question is, how little signal can you tolerate before
you need an amplifier? How many bits of conversion, how much sample rate
can you afford before a linear amplifier becomes cheaper on the power
budget?

Have you looked at the simulation data I had posted? I think you are
describing exactly the circuit we are simulating which I believe is an
accurate representation of the circuit I plan to build. Is that not
correct?

It's getting closer, but with adjustments (to the transformer inductance)
to make the resonances line up (same frequencies). Plus whatever
compromise you need to make on gain.

Tim

--
Deep Friar: a very philosophical monk.
Website: http://seventransistorlabs.com

"rickman" wrote in message
...
Is this a current transformer or a voltage transformer?
.--------. .--------.
| | | |
| C||C
VAC C||C Load
| C||C
| | | |
`--------' `--------'

Voltage. How about this?

.--------. .--------.
| | | |
| C||C
IAC C||C Load
| C||C
| | | |
`--------' `--------'

Tim

--
Deep Friar: a very philosophical monk.
Website: http://seventransistorlabs.com

On 3/8/2013 4:20 PM, Tim Williams wrote:

It may sound like hyperbole, but it's mathematically sound. The midpoint
theorem, for example, guarantees that, between two points, you must've hit
some point inbetween, somewhere, as long as the function is continuous.
More usefully, functions arising in electronics are often one-to-one, so
it's not only true that you are guaranteed midpoints, but you'll find them
in order, too.

If you aren't looking at the extreme cases, you aren't doing your job.
Whatever's left inbetween can simply be interpolated!

The point here being, an antenna which doesn't couple into free space
obviously has a crappy SNR. The signal level can be anything, it doesn't
matter. The signal need not be small, because internal losses generate
thermal noise. With sufficient Q, you can push that thermal noise up to
your receiver threshold (which you said is an ADC) and detect signal.
It'll be bandlimited, ~60kHz noise, a useless signal, but present
nonetheless.

In general, antennas which do couple strongly to free space have low Qs.
A 1/2 wave resonant dipole has a Q of only 1 or 2, so bothering to call it
resonant is actually kind of weak. This is similarly true for a large
loop, which of course would be highly impractical here. So there must be
some middle case where SNR is reasonably unaffected, which will be the
best choice antenna.

I was warned a long time ago to be wary of people speaking in
"glittering generalities". You seem to insist on using terms without
giving a mathematical basis. How about if we use some math?

V = (2 * pi * A * N * E * cos(theta)) / lambda

V is the voltage on the antenna,
A is the loop area,
N is the number of turns,
E is the field strength,
theta is the rotation angle of the antenna and the transmitter (just
consider this term to be 1,
lamba is the wavelength (c/f)

This is multiplied by the Q factor when resonated by a capacitor. So
higher Q, higher signal.

Where in here do you think I am having a problem?


Since atmospheric noise dominates, the antenna can stand to be pretty
small.

You are making assumptions that don't hold true in my design.


Raw volts don't matter, you can always throw more amplifiers at it (as
long as they don't corrupt the SNR also!).

Maybe you didn't read my other posts. I am not using an amplifier. I
am running the antenna and coupler output directly into a digital input.

You hadn't mentioned that before...

You didn't ask.


The receiver input is high impedance, approximately 10 MOhms with a low
capacitance between the differential inputs of not more than 10 pF.

Any ESR? Example, the ATmega series 10 bit ADC specifies, I think, around
10pF + 10k ESR (somewhat depending on how many mux switches it's going
through to get there).

FET input resistance. I will double check that though.


Your description of what is happening is very terse and full of
shortened terms that I don't understand.

I could write a book on the subject to explore it in detail, but there are
many available already, and there are too many holes in my knowledge to
really be worth it, plus this is Usenet, you get what you pay for. I was
hoping you'd Google in the blanks.

I thought I was doing well, but you seem to be telling me I am making
mistakes, but I can't figure out what they might be.


What do you mean "line up that
10p resonance with the operating frequency"? I assume you are referring
to the 10 pF input capacitance. How does this get "lined up" with
anything?

There's yet another theorem in networks that has to do with matching.

A resonant tank's impedance varies wildly with frequency. But it will
always be resistive at resonance. If you connect this to another network,
which has a resistive input impedance at the same frequency, you don't
care what the L and C are, it will simply work -- old fashioned resistor
divider action!

You *do* have to worry about L and C and reactance and bandwidth to solve
for the frequency response and stuff, but you can at least approximate
that with Q factor (i.e., how much loss is draining power out of the
system).

So if your ADC input is exactly 10p + 10M, you could resonate it with 0.7H
(well...), which has a resonant impedance of 264k, and thus a reasonable Q
of 38. (The real world typically bitchslaps the theorist at this point,
as 0.7H chokes with10pF parasitic capacitance and Q 38 at 60kHz don't
exist.) If the capacitance's ESR is less than 6.9kohms (i.e., 264k / 38),
it won't have significant effect.

You can couple to this tank via parallel or series. If you did series,
the input impedance would be 264k / 38, or 6.9k, not horrible; going from
the 0.78 ohm loop to this in a single transformer requires a 1:100 CT,
which works fine at 60kHz. (This CT would require high inductance, so as
to avoid skewing results, but that's typical of a CT. An amorphous core
CT would probably suffice. So at least that part is physically
realizable.) Note the irony of coupling a current loop to a current loop,
where in both cases, the CT looks like a small impedance relative to the
loop it's within. That's simply how huge the impedance at the ADC is.

Since all these resistances are matched, the power transfer theorem holds,
and you're pushing as much voltage and power into the ADC as possible.
The bandwidth is about 1.6kHz, so the thermal noise floor is around 5uV at
the ADC. A received power of 1nW will generate 0.1V, which is probably a
reasonable figure. The SNR of the receiver is limited by quantization
noise for14 bits, thermal for14 bits. A 16 bit converter wouldn't be
too expensive at this sample rate (note it's the analog sample-and-hold
speed which limits direct conversion performance; a sigma-delta, running
at 100Hz, with no S&H, won't see jack).

When you talk about reactances canceling, that sounds a lot like a tuned
circuit at resonance. That is what I *am* doing and where this thread
started. One problem with that is the lack of precision or stability of
the parasitic capacitance. Any idea how to deal with that?

Considering theoretical 0.7H chokes aren't commercially available, you
might swamp it with more C, which stabilizes the value, and requires less
L to resonate. Rub: resonant impedance is lower, so the Q of the
components must be higher in order to achieve the same performance. Even
with a Q of 200, you still need over 0.25H, which is just as unlikely a
combination. Well, if you really wanted to try, maybe a gapped
ferrite-cored inductor could be made. Still, the only practical choice
seems to be lower signal level.

So ultimately, the question is, how little signal can you tolerate before
you need an amplifier? How many bits of conversion, how much sample rate
can you afford before a linear amplifier becomes cheaper on the power
budget?

Have you looked at the simulation data I had posted? I think you are
describing exactly the circuit we are simulating which I believe is an
accurate representation of the circuit I plan to build. Is that not
correct?

It's getting closer, but with adjustments (to the transformer inductance)
to make the resonances line up (same frequencies). Plus whatever
compromise you need to make on gain.

We are still having communications difficulties. You keep talking in
terms I can't relate to. I don't need you to write a book, but I do
need you to communicate clearly.

I am using a 1 bit ADC. Don't assume that I am doing what you have done
in the past.

--

Rick

On 3/8/2013 4:30 PM, Tim Williams wrote:
wrote in message
...
Is this a current transformer or a voltage transformer?
.--------. .--------.
| | | |
| C||C
VAC C||C Load
| C||C
| | | |
`--------' `--------'

Voltage. How about this?

.--------. .--------.
| | | |
| C||C
IAC C||C Load
| C||C
| | | |
`--------' `--------'

Tim


I have to say I don't follow the distinction. It is a transformer, no?

--

Rick

On Fri, 08 Mar 2013 13:44:36 -0500, rickman 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.

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

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.

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.

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

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.


--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

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

On Fri, 08 Mar 2013 16:51:57 -0500, rickman wrote:

On 3/8/2013 4:30 PM, Tim Williams wrote:
wrote in message
...
Is this a current transformer or a voltage transformer?
.--------. .--------.
| | | |
| C||C
VAC C||C Load
| C||C
| | | |
`--------' `--------'

Voltage. How about this?

.--------. .--------.
| | | |
| C||C
IAC C||C Load
| C||C
| | | |
`--------' `--------'

Tim


I have to say I don't follow the distinction. It is a transformer, no?

The second one is a current transformer. They both consist of coils
around a magnetic core driving some kind of load. The difference is the
source of power and that causes them to behave very differently as well
as being constructed differently.

Let's assume ideal components (a good place to start when learning a new
concept). The voltage transformer is driven by a source that provides a
constant voltage, no matter what the load. The transformer takes this
voltage and converts it to some other voltage depending on the turns
ratio; Vout = Vin * Ts / Tp. For example, if the primary has 100 turns
and the secondary has 20 turns and the primary is supplied with 50 volts,
the secondary will provide 10 volts. As the secondary load changes, this
voltage remains the same but the current changes. If the secondary is
open-circuited, the voltage still stays the same. If the secondary is
short-circuited, the current becomes infinite; that's why real voltage
transformers are protected by fuses or similar devices.

Now for the current transformer, it is driven by a source that provides a
constant current no matter what the load. The transformer takes this
current and converts it to some other current depending on the turns
ratio; Iout = Iin * Tp / Ts (note the inversion of the turns ratio).
For example, if the primary has 1 turn (a common number for real
transformers) and the secondary has 5 turns and the primary is supplied
with 5 amps, the secondary will provide 1 amp. As the secondary load
changes, this current remains the same but the voltage changes. If the
secondary is short-circuited, the current still stays the same. If the
secondary is open-circuited, the voltage becomes infinite; that's why
real portable current transformers have a shorting switch on the
secondary that the operator must close before disconnecting the load.

Also, note the difference in the number of turns, voltage transformers
have a lot of turns and current transformers have few turns.

For a loop antenna with an external resonating capacitor, a voltage
transformer would be connected in parallel with the loop and capacitor;
all three in a parallel circuit. A current transformer would be
connected in series with the antenna and capacitor so that the three form
a series circuit. If the loop itself is used as the primary of the
transformer and another winding is used as the secondary, the distinction
between the two types is blurred. Also, a real antenna is neither a
voltage source nor a current source but something in between.

--
Jim Mueller

To get my real email address, replace wrongname with dadoheadman.
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