On Sun, 10 Mar 2013 17:09:45 -0400, rickman wrote:

On 3/10/2013 1:32 AM, Jeff Liebermann wrote:

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?

Turning off the house was easier than finding the multiple sources of
noise at 60KHz. What drove me nuts for about an hour was that much of
the noise was coming from my bench oscilloscope. Argh.

This is typical. WWVH through an active preamp showing the effect of
power line noise (probably from attached switching power supplies).
http://www.prc68.com/I/Images/AMRAD110.GIF
and after adding some better line filtering:
http://www.prc68.com/I/Images/AMRAD_BT.GIF
Main page:
http://www.prc68.com/I/LF-Ant.shtml

I didn't log the setup or take pictures. So, let's do the math and
guesswork.
http://vk1od.net/calc/FS2RPCalc.htm
I plugged in some guesses and recollections as to what the antenna
(Q=30) and amp (+20dB gain) were doing and got:
http://802.11junk.com/jeffl/crud/WWVH-rx-signal-estimate.jpg
-15.8dBm or about 36mv into 50 ohms. I amplified this about 20dB with
two or three U310 JFET's (I forgot what I did) to about 3V rms on the
scope. I didn't bother with the 50 ohm to scope input Z conversion.
Most of what I saw was noise, noise, and more noise. However, if I
was patient, I could see the data fade in an out. As I vaguely
recall, it was less than 1 division or about 0.1v change.

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.

Sigh. Most of what I found at 60KHz was coming from lightning storms
over Florida. The local sources were all switching power supplies,
including those in my test equipment. I didn't have an CFL or LED
room lights at the time. I've recently found them to be a rather
nasty noise source. Also, the switching power supply wall warts were
rather awful. My standard test is to fire up my antique IC-735 HF
xceiver, attach a long length of RG-58c/u to the antenna with a
resonant loop at the end, tune it to 100KHz (as low as it will go),
and sniff around the house.

What you'll see on a spectrum analyzer.
http://www.prc68.com/I/Spec_0002.shtml
If you're thinking of removing all that junk with a 5Hz wide digital
filter in software, please note that you'll need to have the input A/D
handle the total power of almost all that junk. Also, the amplifier
that you're trying to avoid between the antenna and A/D will also need
to be rather linear, and therefore rather high power, in order to
avoid producing more spurious junk via intermodulation products.

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.

So that's how they change daylight savings time. If I had known, I
would have stayed and watched. Thanks for the tip.

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...

The depth of the notch seems to be less as the antenna shrinks in
size. I'm not sure about this as I haven't attempted to recently
model a 60KHz magnetic loop with 4NEC2, but that's what my tinkering
shows. If there were a deep notch, most of the home "atomic clock"
receivers would be orientation sensitive and I would expect warnings
in the docs.

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.

Well, you have to start somewhere, and an ideal receiver is a good
place to start. The advantage is that reality only makes everything
worse, never better. You should be able to build the BPSK
demodulator, and then use a PC to decode the data. I've seen several
such programs that do not require I/Q outputs. Here's one based on
FreeBSD intended to sync the system clock to WWV/WWVH:
http://docs.freebsd.org/doc/4.0-RELEASE/usr/share/doc/ntp/driver36.htm
I'm sure there are others.

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.

Gain at 60KHz is very cheap. Watch out for overload issues. If you
design it to work at full scale with whatever you get at 50uV/m, and
the signal climbs to 100uV/m, your input A/D isn't going to be very
happy. AGC will help, but I don't think it will be needed if you
calculate your signal levels so that the A/D input amp isn't clipping.

Out of service for a day. It seems that about 30 years of chemistry
experiments has finally destroyed much of the kitchen sink plumbing. I
hate plumbing.

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

rickman wrote:
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.

In Europe we have DCF-77 which is at 77.5 kHz.
The trouble receiving it is similar to WWVB.
I have several clocks in the house but some of them have only very
weak sync. Also, to save battery they only sync once every 12 hours
or so. At DST change, they may display the wrong time for a couple
of days, especially the one in the kitchen. I need to relocate it
to a place where I know there is better signal.

The problem is (harmonics of) switching power supplies here as well.
Once I had a big open-frame SMPS that I used to power my radio equipment
and that switched around 25 kHz. Under the right circumstances, the
3rd harmonic wiped away all DCF-77 receiving within 5 meters or so.
Old CRT computer monitors also were problematic.

I presume you have some specific needs, low power being among them,
to stay focussed on WWVB for your clock sync. Most computer users
would use GPS now, or simply sync via the internet. GPS has a different
receiving conditions problem, but at least it isn't so much affected
by prominently present local interference.

Of course a GPS receiver requires a lot more power than a WWVB receiver,
although this has come down over the years. Using some sort of on/off
switching (syncing with the received time and then having it run free
for some time) may help a bit, the battery powered radio clocks do
that as well.

On 3/10/2013 9:25 PM, Jim Mueller wrote:
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.

I can't say I understand the distinction.


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.

This is ok so far.


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.

I don't follow how any of this has to do with a difference in the
transformers. Bth transformers obey both equations you have presented.
Both transformers change the voltage as well as the current, no?


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

I don't see how this follows from what you have written. What is there
about these two transformers that define the number of turns? The
current transformers I am interested in using use 100 or 300 turns. Is
that a lot or just a few?


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.

What I have gotten from this is that Tim's original usage of the terms
implies how the transformer is connected to the antenna. As you say, a
voltage transformer will be connected across the coil in parallel with
the capacitor and a current transformer will be connected in series with
the antenna and capacitor.

I was planning to use the antenna wire itself in the middle of the
antenna loop as the primary of the transformer. So I guess that will be
a current transformer. I may try a simulation to see just what happens
with a parallel connection.

--

Rick

On 3/10/2013 11:42 PM, Jeff Liebermann wrote:
On Sun, 10 Mar 2013 17:09:45 -0400, wrote:

On 3/10/2013 1:32 AM, Jeff Liebermann wrote:

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?

Turning off the house was easier than finding the multiple sources of
noise at 60KHz. What drove me nuts for about an hour was that much of
the noise was coming from my bench oscilloscope. Argh.

Yes, like I said, not much in this house and there is not much near it.
I have a laptop and my roommate (when he is here) uses one along with
an iPhone. I suppose they might generate some noise, but he turns off
his laptop at night I'm sure. Otherwise, there just isn't much in the
house that isn't 10 or 15 years old. I have a car radio on a linear
regulator and an electric shaver that sits charging (part of the time).
Otherwise it should be pretty quiet electrically here.


This is typical. WWVH through an active preamp showing the effect of
power line noise (probably from attached switching power supplies).
http://www.prc68.com/I/Images/AMRAD110.GIF
and after adding some better line filtering:
http://www.prc68.com/I/Images/AMRAD_BT.GIF
Main page:
http://www.prc68.com/I/LF-Ant.shtml

I didn't log the setup or take pictures. So, let's do the math and
guesswork.
http://vk1od.net/calc/FS2RPCalc.htm
I plugged in some guesses and recollections as to what the antenna
(Q=30) and amp (+20dB gain) were doing and got:
http://802.11junk.com/jeffl/crud/WWVH-rx-signal-estimate.jpg
-15.8dBm or about 36mv into 50 ohms. I amplified this about 20dB with
two or three U310 JFET's (I forgot what I did) to about 3V rms on the
scope. I didn't bother with the 50 ohm to scope input Z conversion.
Most of what I saw was noise, noise, and more noise. However, if I
was patient, I could see the data fade in an out. As I vaguely
recall, it was less than 1 division or about 0.1v change.

I'm not sure why you used 1900 meters for the distance. I also don't
get why you used 5 ohms for the receiver input impedance.


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.

Sigh. Most of what I found at 60KHz was coming from lightning storms
over Florida. The local sources were all switching power supplies,
including those in my test equipment. I didn't have an CFL or LED
room lights at the time. I've recently found them to be a rather
nasty noise source. Also, the switching power supply wall warts were
rather awful. My standard test is to fire up my antique IC-735 HF
xceiver, attach a long length of RG-58c/u to the antenna with a
resonant loop at the end, tune it to 100KHz (as low as it will go),
and sniff around the house.

There's just not much of that in this house.


What you'll see on a spectrum analyzer.
http://www.prc68.com/I/Spec_0002.shtml
If you're thinking of removing all that junk with a 5Hz wide digital
filter in software, please note that you'll need to have the input A/D
handle the total power of almost all that junk. Also, the amplifier
that you're trying to avoid between the antenna and A/D will also need
to be rather linear, and therefore rather high power, in order to
avoid producing more spurious junk via intermodulation products.

What A/D? Oh, you mean the LVDS input. How do you saturate a 1 bit ADC?


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.

So that's how they change daylight savings time. If I had known, I
would have stayed and watched. Thanks for the tip.

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...

The depth of the notch seems to be less as the antenna shrinks in
size. I'm not sure about this as I haven't attempted to recently
model a 60KHz magnetic loop with 4NEC2, but that's what my tinkering
shows. If there were a deep notch, most of the home "atomic clock"
receivers would be orientation sensitive and I would expect warnings
in the docs.

I wouldn't say the *depth* of the null depends on the size of the loop.
I think it is a null with a Q, much like a resonance peak, but a null
of course. The smaller the loop, the sharper the null like a high Q
resonance, so the orientation becomes very critical. In theory at
least, the null is perfect, 0 signal.


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.

Well, you have to start somewhere, and an ideal receiver is a good
place to start. The advantage is that reality only makes everything
worse, never better. You should be able to build the BPSK
demodulator, and then use a PC to decode the data. I've seen several
such programs that do not require I/Q outputs. Here's one based on
FreeBSD intended to sync the system clock to WWV/WWVH:
http://docs.freebsd.org/doc/4.0-RELEASE/usr/share/doc/ntp/driver36.htm
I'm sure there are others.

PC?!!! We don't need no stinking PCs! The demodulator is simple. The
signal is beat with a quadrature reference which will bring it down to 0
Hz. This gives two values, a sin and a cos signal. Take the ratio and
do an arcTan. This is a simple table lookup made simpler by some
convenient math relations. For example, the table only needs to cover
0° to 45° since the ratio can be swapped for 45° to 90° and the other
three quadrants distinguished by the sign bits.

Some folks would like you to think this has to be done like a high
fidelity receiver, but it only has to pull the signal out of the noise.


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.

Gain at 60KHz is very cheap. Watch out for overload issues. If you
design it to work at full scale with whatever you get at 50uV/m, and
the signal climbs to 100uV/m, your input A/D isn't going to be very
happy. AGC will help, but I don't think it will be needed if you
calculate your signal levels so that the A/D input amp isn't clipping.

If this is *signal* strength then it won't matter. If this is noise you
are talking about, I'm not sure it will be a problem, the signal will
still be able to be dug out with enough processing gain.


Out of service for a day. It seems that about 30 years of chemistry
experiments has finally destroyed much of the kitchen sink plumbing. I
hate plumbing.

Not much fun, but then what it if you have to work on your knees and get
dirty? I don't enjoy working on my car anymore either.

--

Rick

On 3/11/2013 5:54 AM, Rob wrote:
wrote:
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.

In Europe we have DCF-77 which is at 77.5 kHz.
The trouble receiving it is similar to WWVB.
I have several clocks in the house but some of them have only very
weak sync. Also, to save battery they only sync once every 12 hours
or so. At DST change, they may display the wrong time for a couple
of days, especially the one in the kitchen. I need to relocate it
to a place where I know there is better signal.

Yes, noise can be a problem I understand. I am hoping to get the
bandwidth down much more than most receivers so the noise won't be so
big a factor. With a signal bandwidth of a handful of Hz, it should be
possible.


The problem is (harmonics of) switching power supplies here as well.
Once I had a big open-frame SMPS that I used to power my radio equipment
and that switched around 25 kHz. Under the right circumstances, the
3rd harmonic wiped away all DCF-77 receiving within 5 meters or so.
Old CRT computer monitors also were problematic.

I presume you have some specific needs, low power being among them,
to stay focussed on WWVB for your clock sync. Most computer users
would use GPS now, or simply sync via the internet. GPS has a different
receiving conditions problem, but at least it isn't so much affected
by prominently present local interference.

Yes, this is actually a demo to illustrate how low power an FPGA can be.
An FPGA will run both the clock and the receiver and use power from
the environment rather than batteries.


Of course a GPS receiver requires a lot more power than a WWVB receiver,
although this has come down over the years. Using some sort of on/off
switching (syncing with the received time and then having it run free
for some time) may help a bit, the battery powered radio clocks do
that as well.

Yes, the receiver itself only has to run part of the time, 10% perhaps.
The clock has to run 100% obviously. Interesting enough, the FPGA has
a base power consumption (0 Hz) of nearly 50% of the power budget and I
am confident it will still make the goal.

--

Rick

On Mon, 11 Mar 2013 18:10:23 -0400, rickman wrote:

So, let's do the math and
guesswork.
http://vk1od.net/calc/FS2RPCalc.htm
I plugged in some guesses and recollections as to what the antenna
(Q=30) and amp (+20dB gain) were doing and got:
http://802.11junk.com/jeffl/crud/WWVH-rx-signal-estimate.jpg
-15.8dBm or about 36mv into 50 ohms. I amplified this about 20dB with
two or three U310 JFET's (I forgot what I did) to about 3V rms on the
scope. I didn't bother with the 50 ohm to scope input Z conversion.
Most of what I saw was noise, noise, and more noise. However, if I
was patient, I could see the data fade in an out. As I vaguely
recall, it was less than 1 division or about 0.1v change.

I'm not sure why you used 1900 meters for the distance. I also don't
get why you used 5 ohms for the receiver input impedance.

(Quick reply... still working on my expanding plumbing problem).

The 1900 meters is because I screwed up. It should be about 1900Km
from San Francisco to Denver. However, any distance greater than zero
will suffice for this calculation. The controlling numbers are the
100uV/m field strength, the -3dB antenna gain, and the receiver
bandwidth (5Hz). All of the other numbers can change without having
any effect on the recovered power. The 5 ohms rx input Z was because
the original antenna that I used, was a base loaded 100ft "whip"
antenna with a rather impedance. I couldn't decide if the field
strength to receive power form wanted the antenna impedance before the
50 ohm matching network, or if it treated the matching as part of the
antenna. I flipped a coin and chose 5 ohms. I guess for a loop,
100-200 ohms would be more appropriate. Again, the value makes no
difference in the calculations.

(Back to plumbing and fixing the 48" farm jack that my neighbor
borrowed and returned looking like a pretzel).




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

Jeff Liebermann wrote:

(Back to plumbing and fixing the 48" farm jack that my neighbor
borrowed and returned looking like a pretzel).


It sounds like your neighbor doesn't know jacks. ;-)

On Mon, 11 Mar 2013, rickman wrote:

Yes, this is actually a demo to illustrate how low power an FPGA can be. An
FPGA will run both the clock and the receiver and use power from the
environment rather than batteries.

But this is all relative. My first "atomic clock" ran on the same set of
AA batteries for five or six year, and the second set is now four years
old. The Centrios "atomic" wall clock (digital) uses one AA battery, I'm
not sure how long that's been in. My watch is a Casio Waveceptor, 3 or 4
years old, which has a solar cell to refresh the battery and it's never
been less than fully charged.

Fice or six years seems almost as good as 'shelf life" and while I had a
small LCD clock (not "atomic") that seemed to run a long time on an AA
cell, I'm not sure it was all that lower current than the 'atomic clock".

I'm more impressed by the clock running off 1.5v than that the batteries
last reasonably long.

These things are amazing, considering the effort people used to put into
making WWVB receivers, admittedly the "atomic clock" craze has very much
benefitted from the power increase at the station.

I do orient them, but here in Montreal it's the rare night that they don't
sync up, and I don't think since I've had more than one that they all miss
the sync.

Michael

On Mon, 11 Mar 2013 07:41:41 +0000, Jef wrote:


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.


What you are describing is a difference in the source NOT a difference
in the transformer!

The transformer behaves exactly the same in both cases, the inversion is
merely down to ohms law.

A transformer is a transformer. The only time it gets tricky is when the
core and saturation etc etc comes into play.

Jeff

Very true. A transformer is a transformer and all obey the same
equations. But that doesn't prevent real transformers from being called
voltage or current transformers. And it doesn't prevent them from being
constructed differently or their outputs behaving differently.

Part of the problem is that real-world AC current sources are rather
rare. So current transformers are seldom connected as shown in the
example that was given. They are usually connected in series between a
voltage source and some other load, frequently for the purpose of
measuring the current. This is what gives rise to the different
constructions.

Voltage transformers have to withstand the source voltage across their
primaries so they have to have enough turns to prevent the core from
saturating. The actual number of turns depends on the voltage,
frequency, and core size and material.

Current transformers are generally constructed to have minimum primary
voltage drop so they have few turns on the primary, frequently only one.
Considerations of frequency, core size and material still apply, of
course. The differences in the name and construction are determined by
the intended use. Many current transformers consist of a secondary
winding on a toroidal core. The primary is supplied by the user by
passing a wire through the hole in the core; thus, one turn.

For a loop antenna with a secondary, there is voltage across the antenna
winding and current through it. Which is the secondary responding to?
The current, since this is what produces the magnetic field. But, since
this current is flowing through some number of turns and developing a
voltage, it could also be considered a voltage transformer, which is why
I said the distinction is blurred.

--
Jim Mueller

To get my real email address, replace wrongname with dadoheadman.
Then replace nospam with fastmail. Lastly, replace com with us.

"Jef" wrote in message
...
For a loop antenna with a secondary, there is voltage across the
antenna
winding and current through it. Which is the secondary responding to?


Since you can't have a current thorough the antenna without a voltage
the answer is both, It is a chicken and an egg situation tied together
by Ohm's Law.

That said, low impedances look like constant voltage sources or shorts,
and carry a lot of current, while high impedances look like constant
current sources or opens, and develop a lot of voltage. It's plenty of an
approximation, but when you call an SWR 1.2 a good match, it doesn't
take much in either direction to produce that.

This is relative to system impedance (50 ohms, say), or if not in a system
with characteristic impedances, then some factor of Zo (377 ohms). At
"lumped constant" frequencies, Zo and SWR and stuff aren't all that
relevant anymore, but the concepts always apply.

Tim

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