Loop Antenna at ~60 kHz
 

rickman wrote in :

The US located beacon added PSK a few years
back to make the signal easier to receive.

I went Googlong after I wrote that last one. I'd have thought PSK would be
harder to detect than AM. So much for what I know. :) I read that
wris****ches can detect the PSK signal too, so low power must have been done,
with a small antenna too, but I don't knpow what they did so I'll leave it
there.

About op-amps, I just found it hard to let go of a favourite idea. :) Too
many transistors though...

Just one thought left that might be a kernel of a new idea: if you have a
tiny resonant circuit at 60KHz, with high Q, then the change of phase ought
to make some kind of detectable upset, a spike maybe, whose polarity you can
use to determine mark or space in the signal. The combination of resonance
and short spike duration might give you a usable combination of low power and
detectable threshold.

I'll stop there because I don't think I have anything you can use.. I'm
interested in what you come up with though, especially if it avoids a large
antenna.

Loop Antenna at ~60 kHz
 

On 10/30/2014 3:20 PM, Lostgallifreyan wrote:
rickman wrote in :

The US located beacon added PSK a few years
back to make the signal easier to receive.

I went Googlong after I wrote that last one. I'd have thought PSK would be
harder to detect than AM. So much for what I know. :) I read that
wris****ches can detect the PSK signal too, so low power must have been done,
with a small antenna too, but I don't knpow what they did so I'll leave it
there.

About op-amps, I just found it hard to let go of a favourite idea. :) Too
many transistors though...

Just one thought left that might be a kernel of a new idea: if you have a
tiny resonant circuit at 60KHz, with high Q, then the change of phase ought
to make some kind of detectable upset, a spike maybe, whose polarity you can
use to determine mark or space in the signal. The combination of resonance
and short spike duration might give you a usable combination of low power and
detectable threshold.

I'll stop there because I don't think I have anything you can use.. I'm
interested in what you come up with though, especially if it avoids a large
antenna.

Define large... lol. I'm already looking at a 2 foot diameter which is
just a wee bit too large for my wrist watch.. lol. Actually I really
don't know how practical this is. The digital approach will depend
greatly on pulling a very weak signal out of the noise. Not that the
noise is in the signal on the antenna, but noise in terms of poor
detection of such a weak signal.

I will happily report back if/when I get any sort of results.

--

Rick

Loop Antenna at ~60 kHz
 

On 10/30/2014 01:27 PM, rickman wrote:
On 10/30/2014 1:02 PM, Lostgallifreyan wrote:
rickman wrote in :

Before integration comes demodulation. How would you demodulate and
integrate in the analog domain on a 100 uW power budget? The signal is
PSK. But that is not the real reason. My goal is to show it is
possible to do this entirely in the digital domain.


Low Vf diode in feedback loop of op-amp? I'm curious though, it's an
interesting thought, doing it all in digital equipment, but why? The main
drive behind me 'off-shelf' remark is that I suspect the best answer already
exists in many forms. I'm curious about what makes a need to keep searching.
:) I'm not denying it, far from it, there's usually more than one good way to
do something, I'm just not sure what the differentiating factor is in this
case.

I don't know about "best" but you can buy a time code receiver chip that
spits out a demodulated signal to be decoded by an MCU. At that point
the data rate is pretty low so an MCU can run at very low power levels,
likely dominated by the quiescent current.

When you suggest an op amp, we already covered that ground and they aren't
low power enough. I'm curious how they amplify the signal in the receiver
chip with the whole circuit drawing a very low power level.

Motorola's app notes on the old 4000 series CMOS included
various analog circuits, including use of a CMOS inverter
as an amplifier. I'm enough of a packrat that I keep those
things.

4000 series may not be useful in your case, but the circuits
or variants of them may apply in newer CMOS implementations.

'Course calling it all digital may be just a game if your
input stage is a digital circuit biased to operate in an
analog mode.

George

Loop Antenna at ~60 kHz
 

George Cornelius wrote:
Motorola's app notes on the old 4000 series CMOS included
various analog circuits, including use of a CMOS inverter
as an amplifier. I'm enough of a packrat that I keep those
things.

I'm sure this guy (who is coming back on this subject regularly) is
not going to consider that low-power. The inverter was driven into
the area between switching to '1' and to '0' by using a feedback
resistor, and so both output fets are conducting and drawing current
from Vcc to Gnd.

Loop Antenna at ~60 kHz
 

On 11/3/2014 3:08 AM, Rob wrote:
George Cornelius wrote:
Motorola's app notes on the old 4000 series CMOS included
various analog circuits, including use of a CMOS inverter
as an amplifier. I'm enough of a packrat that I keep those
things.

I'm sure this guy (who is coming back on this subject regularly) is
not going to consider that low-power. The inverter was driven into
the area between switching to '1' and to '0' by using a feedback
resistor, and so both output fets are conducting and drawing current
from Vcc to Gnd.

If you I am "the guy", whether or not this is low power enough depends
on the power. My understanding is that when operated in the linear mode
significant current can flow in a CMOS device. So likely this isn't low
enough power, no.

I'm very curious about how they do it in the commercial chips. I have
seen block diagrams and they show an amplifier as the first part of the
chip. Maybe the design really isn't all that low power. Rather than
running at low power all the time, they just limit the duty cycle of the
receiver. "Atomic" clocks don't need to monitor the signal except for a
few minutes each day.

--

Rick

Loop Antenna at ~60 kHz
 

rickman wrote:
On 11/3/2014 3:08 AM, Rob wrote:
George Cornelius wrote:
Motorola's app notes on the old 4000 series CMOS included
various analog circuits, including use of a CMOS inverter
as an amplifier. I'm enough of a packrat that I keep those
things.

I'm sure this guy (who is coming back on this subject regularly) is
not going to consider that low-power. The inverter was driven into
the area between switching to '1' and to '0' by using a feedback
resistor, and so both output fets are conducting and drawing current
from Vcc to Gnd.

If you I am "the guy", whether or not this is low power enough depends
on the power. My understanding is that when operated in the linear mode
significant current can flow in a CMOS device. So likely this isn't low
enough power, no.

I'm very curious about how they do it in the commercial chips. I have
seen block diagrams and they show an amplifier as the first part of the
chip. Maybe the design really isn't all that low power. Rather than
running at low power all the time, they just limit the duty cycle of the
receiver. "Atomic" clocks don't need to monitor the signal except for a
few minutes each day.

I have several battery-powered "atomic clocks" and all of them enable
the receiver only for a few minutes, either every hour or twice a day
depending on the particular design. The receiver I have connected to
my computer is of course enabled all the time.

Many years ago I worked on a "shop-shelf tag" system that used a low
frequency receiver in a single-chip design, and it also had a power
saving mechanism. The tags (powered by single lithium cell like those
used as a BIOS backup battery) were usually in a sleep mode only driving
the LCD, and once every so many seconds they briefly enabled the receiver.
To run an update, the controller sent a wakeup signal that lasted long
enough to get the attention of all tags, then it sent the updates
addressed to each tag, and finally an end-of-transmission signal that
put everything back into sleep mode. The lithium cell lasted several
years, I think.

Loop Antenna at ~60 kHz
 

On 11/5/2014 8:29 PM, Jeff Liebermann wrote:
Incidentally, I don't believe using a high impedance loop and amp are
good ideas. While there are benefits, my experiences from the marine
radio biz convinced me that high voltage is an invitation to problems
from condensation, salt fog, and PCB leakage. In other words, it
works on the bench, but craps out in the field. I'll probably end up
with a large high Q loop, and a separate low-Z coupling loop (i.e. a
step down xformer).

Not sure why you can't discuss this in the right thread of this group.
I've posted my reply to your post in the loop antenna thread.

First, I'm not sure what you are talking about connecting high impedance
antennas to condensation and salt fog. If you are transmitting, then
maybe you could get such high voltages as to attract microscopic
objects, but this is a receiver design.

Also, the antenna is not high impedance, just the input to the receiver.
The transformer I am looking at is a high turns ratio current sensor.
It spans the right frequency range and is a nice compact package easy
to mount on a PCB.

My main concern is lowering the Q because of the loading from the
receiver input, especially with the change in impedance as reflected
through the transformer. I think when I simulated it, I found the max
signal strength came with a 25 or 33:1 turns ratio because with higher
turns ratios the Q was spoiled enough to bring the voltage down at the
receiver input.

This simulation didn't include the effect of the radiation resistance,
so I will need to add that in. I expect this will lower the Q as a
starting point which means the affect from the receiver input loading
will not be as significant, possibly making a higher turns ratio in the
transformer more useful.

--

Rick

Loop Antenna at ~60 kHz
 

On Wed, 05 Nov 2014 20:50:31 -0500, rickman wrote:

Not sure why you can't discuss this in the right thread of this group.
I've posted my reply to your post in the loop antenna thread.

Because I prefaced my comments by mentioning that a 60 KHz loop is on
my "agenda". I guess that's a bit vague. What I meant to say was
that I'm not very well read on the technology involved, a total clutz
with LTspice, and I haven't built another loop so I can measure how it
acts. In other words, I'm not ready to discuss it (unless you can
tolerate my guesswork).

First, I'm not sure what you are talking about connecting high impedance
antennas to condensation and salt fog. If you are transmitting, then
maybe you could get such high voltages as to attract microscopic
objects, but this is a receiver design.

Well, a 33:1 turns ratio is a 1000:1 impedance ratio. Using 75 ohms
as the coax cable and the characteristic impedance, that's 75K ohms.
In general, board leakage and conduction problems start around 100K
(depending on trace spacing etc), so I suspect you can make it work,
at least on the bench. However, in the typical marine atmosphere,
with ionic crud in the water, there will be leakage issues. I don't
recall the typical sheet resistivity for a standing salt water puddle
on a PCB, but I suspect it will be a problem. Of course, you can
conformal coat the board, hermetically seal the package, wax dip it,
or pot the antenna amplifier in epoxy to avoid the problem. However,
the favored method is to design with low impedances and not create new
problems with conformal coatings and sealed boxes.

There are also some PCB layout tricks that will help. For example,
here's part of a book on PCB design issues:
http://www.analog.com/library/analogdialogue/archives/43-09/edch%2012%20pc%20issues.pdf
See Pg 12-15 to 12-19 on "Static PCB Effects" with examples of PCB
guard patterns.

Incidentally, my unofficial test for decent design was to immerse the
radio in a bucket of genuine San Francisco Bay salt water. If the
board continued to operate normally, it passes. If not, I get to
spend the evening with the bucket and a megohmmeter looking for the
culprit.

If you're building this loop as an academic exercise, you can probably
ignore all the aforementioned comments on PCB leakage. However, if
you're going to sell it, think carefully about such environment
problems.

Also, the antenna is not high impedance, just the input to the receiver.
The transformer I am looking at is a high turns ratio current sensor.
It spans the right frequency range and is a nice compact package easy
to mount on a PCB.

Why not just make it a 40 KHz tuned xformer? You get the same
impedance transformation with the added bonus of additional bandwidth
reduction (increased Q) to eliminate as much atmospheric and man made
noise as possible. It's also much less lossy than a broadband
xformer.

My main concern is lowering the Q because of the loading from the
receiver input, especially with the change in impedance as reflected
through the transformer.

Well, you're stuck with matching the loop to the receiver input
anyway, so there's no way around that with passive components. You
can insert an emitter follower to do the impedance transformation.

Incidentally, the typical loaded Q for such loops seems to be around
100. Some claim 200 or more, but for small loops, 100 seems to be the
target. At 40 KHz, that's a -3dB bandwidth of 200 Hz, which is rather
wide for a 1Hz wide WWVB signal. You could probably increase the Q
somewhat, mostly be reducing the resistive losses, but that might
create drift and tuning accuracy problems. Higher Q is possible, but
I suspect will require a much more rigid and beefy design.

I think when I simulated it, I found the max
signal strength came with a 25 or 33:1 turns ratio because with higher
turns ratios the Q was spoiled enough to bring the voltage down at the
receiver input.

This simulation didn't include the effect of the radiation resistance,
so I will need to add that in. I expect this will lower the Q as a
starting point which means the affect from the receiver input loading
will not be as significant, possibly making a higher turns ratio in the
transformer more useful.

I can't comment on that without seeing the design. Actually, I'm not
sure seeing the design will help as I need to do some more reading
before I can understand exactly how it works.

11:30PM. Time for dinner.

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

Loop Antenna at ~60 kHz
 

On Wed, 05 Nov 2014 23:23:32 -0800, Jeff Liebermann
wrote:

Why not just make it a 40 KHz tuned xformer?

Somehow, my brain converted the 60 KHz WWVB frequency, to the 40 KHz
frequency of the vehicle detection system I'm working on. The
frequencies mentioned should be 60 KHz and as usual, my arithmetic
sucks.

target. At 40 KHz, that's a -3dB bandwidth of 200 Hz, which is rather
wide for a 1Hz wide WWVB signal.

At 60 KHz, that's a -3dB bandwidth of 600 Hz, which is rather
wide for a 1Hz wide WWVB signal.

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

Loop Antenna at ~60 kHz
 

On 11/6/2014 2:23 AM, Jeff Liebermann wrote:
On Wed, 05 Nov 2014 20:50:31 -0500, rickman wrote:

Not sure why you can't discuss this in the right thread of this group.
I've posted my reply to your post in the loop antenna thread.

Because I prefaced my comments by mentioning that a 60 KHz loop is on
my "agenda". I guess that's a bit vague. What I meant to say was
that I'm not very well read on the technology involved, a total clutz
with LTspice, and I haven't built another loop so I can measure how it
acts. In other words, I'm not ready to discuss it (unless you can
tolerate my guesswork).

I'm happy bouncing things off you. I did some reading on this back a
year or so ago and feel like I got a lot, but not enough to really
optimize it for my application. One thing I was missing was an
understanding of the radiation resistance which I now have a formula for
and can include in my LTspice simulation when I get to it.

I don't quite have a feel for radiation resistance in terms of its
effect on the receive antenna, but I'm sure that will come once I look
at the equations. I expect it will be small, hopefully small compared
to the wire resistance.

One thing that gave me fits early on is the calculation of the loop
inductance. Seems there are a lot of equations out there and most of
the sources don't talk about where they got them or what they assume. I
finally got one from Lundin that seems pretty good and covers the widest
range of coils I might be using.


First, I'm not sure what you are talking about connecting high impedance
antennas to condensation and salt fog. If you are transmitting, then
maybe you could get such high voltages as to attract microscopic
objects, but this is a receiver design.

Well, a 33:1 turns ratio is a 1000:1 impedance ratio. Using 75 ohms
as the coax cable and the characteristic impedance, that's 75K ohms.

Forget 75 ohms. There is no cable. The antenna connects directly to
the receiver circuit through the transformer. The characteristics of
the antenna are defined by the inductance of the loop and the resonance
with the tuning capacitor and the Q.


In general, board leakage and conduction problems start around 100K
(depending on trace spacing etc), so I suspect you can make it work,
at least on the bench.

100k? I will be using up to 10 Megohm parts but even that is not very
sensitive to board leakage unless you leave a lot of rosin on the board
and it collects dust for a few years.


However, in the typical marine atmosphere,
with ionic crud in the water, there will be leakage issues. I don't
recall the typical sheet resistivity for a standing salt water puddle
on a PCB, but I suspect it will be a problem.

I won't be in salt spray, it will be in my living room. Still, any
aquatic electronics would be in a sealed enclosure.


Of course, you can
conformal coat the board, hermetically seal the package, wax dip it,
or pot the antenna amplifier in epoxy to avoid the problem. However,
the favored method is to design with low impedances and not create new
problems with conformal coatings and sealed boxes.

There are also some PCB layout tricks that will help. For example,
here's part of a book on PCB design issues:
http://www.analog.com/library/analogdialogue/archives/43-09/edch%2012%20pc%20issues.pdf
See Pg 12-15 to 12-19 on "Static PCB Effects" with examples of PCB
guard patterns.

I am familiar with guarding, but that is not going to be needed with an
antenna. The voltage will be very low level even when the Q is
optimized, so no appreciable leakage currents.


Incidentally, my unofficial test for decent design was to immerse the
radio in a bucket of genuine San Francisco Bay salt water. If the
board continued to operate normally, it passes. If not, I get to
spend the evening with the bucket and a megohmmeter looking for the
culprit.

If you're building this loop as an academic exercise, you can probably
ignore all the aforementioned comments on PCB leakage. However, if
you're going to sell it, think carefully about such environment
problems.

I don't think it will ever see duty on a sea vessel.


Also, the antenna is not high impedance, just the input to the receiver.
The transformer I am looking at is a high turns ratio current sensor.
It spans the right frequency range and is a nice compact package easy
to mount on a PCB.

Why not just make it a 40 KHz tuned xformer? You get the same
impedance transformation with the added bonus of additional bandwidth
reduction (increased Q) to eliminate as much atmospheric and man made
noise as possible. It's also much less lossy than a broadband
xformer.

What would that entail?


My main concern is lowering the Q because of the loading from the
receiver input, especially with the change in impedance as reflected
through the transformer.

Well, you're stuck with matching the loop to the receiver input
anyway, so there's no way around that with passive components. You
can insert an emitter follower to do the impedance transformation.

You aren't in tune with this design. The goal is to minimize power.
There won't be a preamp of any kind unless absolutely required.


Incidentally, the typical loaded Q for such loops seems to be around
100. Some claim 200 or more, but for small loops, 100 seems to be the
target. At 40 KHz, that's a -3dB bandwidth of 200 Hz, which is rather
wide for a 1Hz wide WWVB signal. You could probably increase the Q
somewhat, mostly be reducing the resistive losses, but that might
create drift and tuning accuracy problems. Higher Q is possible, but
I suspect will require a much more rigid and beefy design.

There is only so much that can be done to increase Q. The wire I am
using in the antenna is already pushing the skin effect at 1 mm
diameter. If I am reading the equations correctly increasing the number
of turns on the loop does increase Q. I am currently looking at 8 turns
(50 feet of RG-6) and may increase it to 100 feet (16 turns). But I've
already built a support and 16 turns will be hard to add without a
redesign.


I think when I simulated it, I found the max
signal strength came with a 25 or 33:1 turns ratio because with higher
turns ratios the Q was spoiled enough to bring the voltage down at the
receiver input.

This simulation didn't include the effect of the radiation resistance,
so I will need to add that in. I expect this will lower the Q as a
starting point which means the affect from the receiver input loading
will not be as significant, possibly making a higher turns ratio in the
transformer more useful.

I can't comment on that without seeing the design. Actually, I'm not
sure seeing the design will help as I need to do some more reading
before I can understand exactly how it works.

The equations are pretty simply once I found them (and could trust I had
the right ones).

Lundin's formula for inductance of a solenoid
L = N^2 * a * Correction Factor * μ0

N is the number of turns
a is the loop radius in meters
the correction factor based on the coil shape is a bit complex but comes
to 3.3 ballpark with the loop shape used.
μ0 is the permeability of free space

He is the effective height of the antenna, an expression of the
effectiveness of the antenna in converting the field into a voltage.

He = 2pi * N * A / λ, ignoring the orientation factor cos θ.

N is the number of turns
A is the loop area in meters^2
λ is the wavelength of the 60 kHz signal

Inductance and frequency get the reactance which when compared to the
total loss resistance yields the Q.

Multiply the effective height by the field strength (on the east coast
it's ~100 uV from WWVB) to get the antenna voltage. Someone was trying
to get me to use an equation based on the magnetic field but I believe
once you combine the equations you get the same calculation.

Multiply by Q and the transformer ratio and you have the voltage at the
receiver input.

Wire resistance goes up with the product of N and a, or in other words
the length of the cable. The loop inductance goes up with N^2 and a.
Effective height goes up with N and a squared (area). So a bigger loop
will get a larger signal but the same Q. Adding turns will get a larger
signal *and* a higher Q. Obviously the size of the loop has an upper
limit based on practicality, but more turns gets improved performance
with less impact on the size.

--

Rick