You'll be much better off simply using the conventional radio approach
than trying to simulate everything, especially when circuit equivalents
are nebulous like this.

After all, if you can't quite tell what it *should* look like, how would
you know if you could implement your model once you've found a
satisfactory result?

What kind of antenna are you looking at, loop? The first thing to know
about a loop is, if it's a very small loop (I'm guessing, at this
frequency, it is), its radiation resistance is very low, meaning, you can
treat it as a nearly pure inductance (Q 10 I think is typical), and its
bandwidth (even with a matched load) will be correspondingly narrow.

The nature of the incoming signal could be modeled as a voltage or current
source; how doesn't really matter, because it isn't really either, it's a
power source that couples in. Again, you don't have voltage without
current and vice versa, it's all about power flow, and the matching that
allows the power to flow.

Since the loop is inductive, your first priority is to resonate it with a
capacitor at the desired frequency. This will require a very precise
value, and even for a single frequency, may require a variable capacitor
to account for manufacturing tolerances. In the AM BCB, a Q of 10 gets
you 50-160kHz bandwidth, so you only get a few channels for any given
tuning position. And if the Q is higher, you get even fewer.

Now that you've got a high Q resonant tank, you can do two things: couple
into the voltage across the capacitor, or the current through the
inductor. You need only a small fraction of either, because the Q is
still going to be large. This can be arranged with a voltage divider
(usually the capacitor is split into a huge hunk and a small variable
part, e.g., 300pF variable + 10nF, output from across the 10nF), a
transformer (a potential transformer across the cap, or a current
transformer in series with the inductor), an inductive pickup (the big
loop carries lots of volts, but you only need a few, so a much smaller
loop can be placed inside the big loop), an impractically large inductor
(like in my example circuit, which models radiation resistance as a
parallel equivalent), etc. Whatever the case, you need to match
transmission line impedance (e.g., 50 ohms) to radiation resistance
(whichever series or parallel equivalent you have).

Once you get the signal into a transmission line, with a reasonable match
(Z ~= Z_line, or alternately, SWR ~= 1), you can do whatever you want with
it. Put it into an amplifier (don't forget to match it, too), etc. Yes,
you're going to have funny behavior at other frequencies, and if you're
concerned about those frequencies, you'll have to choose the coupling
circuit and adjustable (or selectable) components accordingly. But for
the most part, you completely ignore any frequency that you aren't tuning
for, usually enforcing that concept by inserting filters to reject any
stragglers.

Example: suppose you have a loop of 5uH and need to tune it to 500kHz. It
has a reactance of 15.7 ohms. Suppose further it has Q = 20. The ESR
(not counting DCR and skin effect) is X_L / Q, or 0.78 ohms; alternately,
the EPR is X_L * Q, or 314 ohms. The capacitor required is 20.3nF. If we
use a current transformer to match to a 50 ohm line, it needs an impedance
ratio of 1:64, or a turns ratio of 1:8. If we use a voltage transformer,
it's of course 8:1. (A capacitor divider is unsuitable for resonant
impedances less than line impedance, since it can only divide the
impedance down. If the inductance were a lot larger, it could be used.)
To a rough approximation, a smaller inductive loop, of 1/8 diameter of the
larger, I think, would also work.

Tim

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

"rickman" wrote in message
...
On 2/28/2013 6:40 PM, Tim Williams wrote:
wrote in message
...
A higher frequency would imply a smaller L and/or C. How do you
combine
them to produce that? Consider the two caps to be in series???

Sure. If you bring the 10p over to the primary, it looks like 10p *
(30m
/ 5u), or whatever the ratio was (I don't have it in front of me now),
in
parallel with the primary. (I misspoke earlier, you can safely ignore
Ls,
because k = 1. There's no flux which is not common to both windings.)

Reflecting the capacitance through the transformer changes it by the
square of the turns ratio assuming the coupling coefficient is
sufficiently high. I am simulating K at 1.

This is also true for the inductance, but in the opposite manner. So
going from the 25 turn side to the 1 turn side, the effective
capacitance is multiplied by 625 and the effective inductance (or
resistance) is divided by 625. In fact, in LTspice you indicate the
turns ratio by setting the inductance of the two coils by this ratio.

I see now that the reflected secondary capacitance is in parallel with
the primary, rather than in parallel with the primary capacitor. That
explains a lot... I'll have to hit the books to see how to calculate
this new arrangement. I found a very similar circuit in the Radiotron
Designer's Handbook. In section 4.6(iv)E on page 152 they show a
series-parallel combination that only differs in the placement of the
resistance in the parallel circuit. It need to be placed inline with
the inductor... or is placing it parallel correct since this is the
reflected resistance of the secondary? I'll have to cogitate on that a
bit. I'm thinking it would be properly placed inline with the capacitor
in the reflection since it is essentially inline in the secondary.
Either way I expect it will have little impact on the resonant frequency
and I can just toss all the resistances simplifying the math.

I do see one thing immediately. The null in Vcap I see is explained by
the parallel resonance of the secondary cap with the secondary inductor.
If you reflect that cap back to the primary in parallel with the primary
inductor (resonating at the same frequency) it explains the null in the
capacitor C1 voltage I see. C2' (reflected) and L1 make a parallel
resonance with a high impedance dropping the primary cap current and
voltage to a null. This null is calculated accurately.

What I need to do is change the impedance equation from Radiotron to one
indicating the voltage at Vout relative to the input signal. I think I
can do that by treating the circuit as a voltage divider taking the
ratio of the impedance at the input versus the impedance at the primary
coil. No?


Inductors effectively in parallel also increase the expected resonant
frequency. If you have this,

. L1
. +-----UUU--+------+------+
. | + | | |
. ( Vsrc ) === C R 3 L2
. | - | 3
. | | | |
. +----------+------+------+
. _|_ GND

You might expect the resonant frequency is L2 + C, but it's actually
(L1
|| L2) = Leq. If L1 is not substantially larger than L2, the resonant
frequency will be pulled higher.

I see, L1 and L2 are in parallel because the impedance of Vsrc is very
low. That is not the circuit I am simulating however. The loop of the
antenna and the loop of the inductor are in series along with the
primary capacitor. I'm not sure what the resistor is intended to
represent, perhaps transformer losses? The resistance of L1 was added
to the simulation model along with the resistance of the secondary coil
which you have not shown... I think. It seems to me you have left out
the tuning capacitor on the primary.


Incidentally, don't forget to include loss components. I didn't see
any
explict R on the schematic. I didn't check if you set the LTSpice
default
parasitic ESR (cap), or DCR or EPR (coil) on the components. Besides
parasitic losses, your signal is going *somewhere*, and that "where"
consumes power!

The actual transmitter is most certainly not a perfect current source
inductor, nor is the receiver lossless. This simulation has no
expression
for radiation in any direction that's not directly between the two
antennas: if all the power transmitted by the current source is
reflected
back, even though it's through a 0.1% coupling coefficient, it has to
go
somewhere. If it's coming back out the antenna, and it's not being
burned
in the "transformer", it's coming back into the transmitter. This is
at
odds with reality, where a 100% reflective antenna doesn't magically
smoke
a distant transmitter, it simply reflects 99.9% back into space. The
transmitter hardly knows.

Interesting point. My primary goal with this is to simulate the
resonance of the tuning so I can understand how to best tune the
circuit. In many of the simulations I run the Q ends up being high
enough that a very small drift in the parasitic capacitance on the
secondary detunes the antenna and drops the signal level. It sounds
like there are other losses that will bring the Q much lower.

I would also like to have some idea of the signal strength to expect. My
understanding is that the radiation resistance of loop antennas is
pretty low. So not much energy will be radiated out. No?

You make it sound as if in the simulation, even with a small coupling
coefficient all the energy from antenna inductor will still couple back
into the transmitter inductor regardless of the K value. Do I
misunderstand you? It seems to result in the opposite, minimizing this
back coupling. Or are you saying that the simulation needs to simulate
the radiation resistance to show radiated losses?


In this example, if you set R very large, you'll see ever more voltage
on
the output, and ever more current draw from Vsrc. You can mitigate
this
by increasing L1 still further, but the point is, if the source and
load
(R) aren't matched in some fashion, the power will reflect back to the
transmitter and cause problems (in this case, power reflected back
in-phase causes excessive current draw; in the CCS case, reflected
power
in-phase causes minimal voltage generation and little power
transmission).

Power is always coming and going somewhere, and if you happen to forget
this fact, it'll reflect back and zap you in the butt sooner or later!

Tim

Actually, my goal was to build the receiver and I realized that my
design would require the largest signal I could get from the antenna. I
never realized I would end up having to learn quite so much about
antenna design.

I've been planning to create a PCB with lots of options so I can test a
number of configurations. Nothing about the simulation makes me doubt
the utility of this idea.

One thing that continues to bug me is that nothing I have seen gives me
a hint on how to factor in the distributed capacitance of the antenna
shield. I am using RG6 with 16 pF/Ft and likely will end up with 100
foot of coax total. At some point I'll just have to make some
measurements and see what the real world does.

--

Rick

On 3/1/2013 8:53 PM, Tim Williams wrote:
You'll be much better off simply using the conventional radio approach
than trying to simulate everything, especially when circuit equivalents
are nebulous like this.

I don't know what you mean by the "conventional radio approach".


After all, if you can't quite tell what it *should* look like, how would
you know if you could implement your model once you've found a
satisfactory result?

I was simulating a specific circuit for a specific purpose. I got the
answer I was looking for.


What kind of antenna are you looking at, loop? The first thing to know
about a loop is, if it's a very small loop (I'm guessing, at this
frequency, it is), its radiation resistance is very low, meaning, you can
treat it as a nearly pure inductance (Q 10 I think is typical), and its
bandwidth (even with a matched load) will be correspondingly narrow.

Yes, I plan to use a shielded loop. I have found some contradictory
info on the effectiveness of the "shield". One reference seems to have
measurements that show it is primarily E-field coupled in the longer
distance portion of the near-field.

I am aware of the low radiation resistance and have not included that
factor in my simulation. The Q of just the antenna loop is around 100
as calculated from the ratio of reactance to resistance.


The nature of the incoming signal could be modeled as a voltage or current
source; how doesn't really matter, because it isn't really either, it's a
power source that couples in. Again, you don't have voltage without
current and vice versa, it's all about power flow, and the matching that
allows the power to flow.

A friend in a loop antenna Yahoo group suggested the use of the
transformer coupling with a low k to model the signal reception.


Since the loop is inductive, your first priority is to resonate it with a
capacitor at the desired frequency. This will require a very precise
value, and even for a single frequency, may require a variable capacitor
to account for manufacturing tolerances. In the AM BCB, a Q of 10 gets
you 50-160kHz bandwidth, so you only get a few channels for any given
tuning position. And if the Q is higher, you get even fewer.

Yes, that is loop antenna 101 I think. It was when I added a coupling
transformer with 100:1 turns ratio that I was told I needed to consider
the parasitics. I have found it is not useful to go much above 25 or
33:1 on the turns ratio. I am receiving a single frequency, 60 kHz.
There is no need for a wide bandwidth. Ultimately, I prefer a Q of
100 for the higher gain. If it gets too high, the off tuning by
variations (drift) in the parasitic capacitance affects the antenna gain
appreciably.


Now that you've got a high Q resonant tank, you can do two things: couple
into the voltage across the capacitor, or the current through the
inductor. You need only a small fraction of either, because the Q is
still going to be large. This can be arranged with a voltage divider
(usually the capacitor is split into a huge hunk and a small variable
part, e.g., 300pF variable + 10nF, output from across the 10nF), a
transformer (a potential transformer across the cap, or a current
transformer in series with the inductor), an inductive pickup (the big
loop carries lots of volts, but you only need a few, so a much smaller
loop can be placed inside the big loop), an impractically large inductor
(like in my example circuit, which models radiation resistance as a
parallel equivalent), etc. Whatever the case, you need to match
transmission line impedance (e.g., 50 ohms) to radiation resistance
(whichever series or parallel equivalent you have).

Transmission line? What transmission line? The antenna is directly
connected to the receiver which has a very high input impedance. Why do
I need to consider radiation resistance? I have not read that anywhere.


Once you get the signal into a transmission line, with a reasonable match
(Z ~= Z_line, or alternately, SWR ~= 1), you can do whatever you want with
it. Put it into an amplifier (don't forget to match it, too), etc. Yes,
you're going to have funny behavior at other frequencies, and if you're
concerned about those frequencies, you'll have to choose the coupling
circuit and adjustable (or selectable) components accordingly. But for
the most part, you completely ignore any frequency that you aren't tuning
for, usually enforcing that concept by inserting filters to reject any
stragglers.

Example: suppose you have a loop of 5uH and need to tune it to 500kHz. It
has a reactance of 15.7 ohms. Suppose further it has Q = 20. The ESR
(not counting DCR and skin effect) is X_L / Q, or 0.78 ohms; alternately,
the EPR is X_L * Q, or 314 ohms. The capacitor required is 20.3nF. If we
use a current transformer to match to a 50 ohm line, it needs an impedance
ratio of 1:64, or a turns ratio of 1:8. If we use a voltage transformer,
it's of course 8:1. (A capacitor divider is unsuitable for resonant
impedances less than line impedance, since it can only divide the
impedance down. If the inductance were a lot larger, it could be used.)
To a rough approximation, a smaller inductive loop, of 1/8 diameter of the
larger, I think, would also work.

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

How did you get the 1:64 impedance ratio and the 1:8 turns ratio? I
don't follow that. Are you saying the line impedance should match the
ESR? Why exactly would it need to match the ESR?

--

Rick

"rickman" wrote in message
...
Yes, I plan to use a shielded loop. I have found some contradictory
info on the effectiveness of the "shield". One reference seems to have
measurements that show it is primarily E-field coupled in the longer
distance portion of the near-field.

I trust this resource:
http://vk1od.net/antenna/shieldedloop/
He's got gobs of analytical articles.

Yes, that is loop antenna 101 I think. It was when I added a coupling
transformer with 100:1 turns ratio that I was told I needed to consider
the parasitics. I have found it is not useful to go much above 25 or
33:1 on the turns ratio. I am receiving a single frequency, 60 kHz.
There is no need for a wide bandwidth. Ultimately, I prefer a Q of
100 for the higher gain. If it gets too high, the off tuning by
variations (drift) in the parasitic capacitance affects the antenna gain
appreciably.

High Q isn't the goal, high radiation resistance is -- the bigger the
loop, the better it couples with free space, until it's a wave length
around.

You can go ahead and make a teeny coil out of polished silver litz wire,
and push the Q up into the hundreds, but all you'll see is internal
resistance, hardly anything attributable to actual radiation. Since the
losses dominate over radiation, it makes a crappy antenna. But you know
that from looking at it -- it's a tiny lump, of course it's not going to
see the outside world.

It is true, however, that a small coil, with low losses, will have low
noise. AM radios rely on this, which is how they get away with tiny hunks
of ferrite for picking up radio.

Of course, it doesn't hurt that AM stations are 50kW or so, to push over
atmospheric noise.

Transmission line? What transmission line? The antenna is directly
connected to the receiver which has a very high input impedance. Why do
I need to consider radiation resistance? I have not read that
anywhere.

Ok, then you can merge the matching transformer, transmission line and
receiver input transformer into one -- an even larger stepup into whatever
impedance it's looking at (what's "very high", kohms? Mohms?) will get you
that much more SNR.

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).

How did you get the 1:64 impedance ratio and the 1:8 turns ratio? I
don't follow that. Are you saying the line impedance should match the
ESR? Why exactly would it need to match the ESR?

ESR (and Q) measured on the coil corresponds to radiation resistance
(series equivalent) *plus* internal losses (also series equivalent). You
can't separate the two components, so you can only get the best power
match by the good old impedance theorem.

~1:64 is 50 ohm / 0.78 ohm, and N2/N1 = sqrt(Z2/Z1), or 8:1 turns ratio.

Tim

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

On 3/6/2013 8:13 PM, Tim Williams wrote:
wrote in message
...
Yes, I plan to use a shielded loop. I have found some contradictory
info on the effectiveness of the "shield". One reference seems to have
measurements that show it is primarily E-field coupled in the longer
distance portion of the near-field.

I trust this resource:
http://vk1od.net/antenna/shieldedloop/
He's got gobs of analytical articles.

Yes, I've seen this page. Thanks.


Yes, that is loop antenna 101 I think. It was when I added a coupling
transformer with 100:1 turns ratio that I was told I needed to consider
the parasitics. I have found it is not useful to go much above 25 or
33:1 on the turns ratio. I am receiving a single frequency, 60 kHz.
There is no need for a wide bandwidth. Ultimately, I prefer a Q of
100 for the higher gain. If it gets too high, the off tuning by
variations (drift) in the parasitic capacitance affects the antenna gain
appreciably.

High Q isn't the goal, high radiation resistance is -- the bigger the
loop, the better it couples with free space, until it's a wave length
around.

I'm not clear on why you keep referring to radiation resistance for a
receiving antenna. Does this result in a larger received signal? I am
concerned with maximizing the voltage at the input to the receiver.


You can go ahead and make a teeny coil out of polished silver litz wire,
and push the Q up into the hundreds, but all you'll see is internal
resistance, hardly anything attributable to actual radiation. Since the
losses dominate over radiation, it makes a crappy antenna. But you know
that from looking at it -- it's a tiny lump, of course it's not going to
see the outside world.

I have no idea why you are talking about Litz wire and tiny coils. I
never said I was looking to maximize the Q. I said I wanted a Q of over
100. I should have said, slightly over 100. A higher Q clearly does
increase the voltage on the input in my simulations. Is there something
wrong with my simulations?


It is true, however, that a small coil, with low losses, will have low
noise. AM radios rely on this, which is how they get away with tiny hunks
of ferrite for picking up radio.

Of course, it doesn't hurt that AM stations are 50kW or so, to push over
atmospheric noise.

Transmission line? What transmission line? The antenna is directly
connected to the receiver which has a very high input impedance. Why do
I need to consider radiation resistance? I have not read that
anywhere.

Ok, then you can merge the matching transformer, transmission line and
receiver input transformer into one -- an even larger stepup into whatever
impedance it's looking at (what's "very high", kohms? Mohms?) will get you
that much more SNR.

Yes, a higher stepup ratio gets larger signal up to a point. That point
is determined by the parasitic capacitance of the receiver input. That
capacitance is reflected back through the transformer and affects the
antenna tuning. In my simulations it creates a filter with two resonances.


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. I have trouble
with series and parallel resonance, but I'm starting to get the concept.
Sometimes it is hard to tell how a circuit is being stimulated.


How did you get the 1:64 impedance ratio and the 1:8 turns ratio? I
don't follow that. Are you saying the line impedance should match the
ESR? Why exactly would it need to match the ESR?

ESR (and Q) measured on the coil corresponds to radiation resistance
(series equivalent) *plus* internal losses (also series equivalent). You
can't separate the two components, so you can only get the best power
match by the good old impedance theorem.

Internal losses of what? How do you determine the internal losses?


~1:64 is 50 ohm / 0.78 ohm, and N2/N1 = sqrt(Z2/Z1), or 8:1 turns ratio.

Ok, so you were matching the hypothetical ESR to the hypothetical line
impedance.

--

Rick

"rickman" wrote in message
...
High Q isn't the goal, high radiation resistance is -- the bigger the
loop, the better it couples with free space, until it's a wave length
around.

I'm not clear on why you keep referring to radiation resistance for a
receiving antenna. Does this result in a larger received signal? I am
concerned with maximizing the voltage at the input to the receiver.

You're also not concerned about that -- you're concerned about maximizing
SNR at the receiver.

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.

An antenna with high (expressed as ESR) radiation resistance might have a
modest Q, but gives far better SNR because it couples to free space.

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

Yes, a higher stepup ratio gets larger signal up to a point. That point
is determined by the parasitic capacitance of the receiver input. That
capacitance is reflected back through the transformer and affects the
antenna tuning. In my simulations it creates a filter with two
resonances.

Oooh, capacitance! I like capacitance. Capacitance is easy to
cancel...inductors are good at that.

What's a nearby inductor working against that capacitance? The current
transformer in your simulation, if its inductance can be controlled, would
be an excellent candidate. The circuit effectively becomes a double tuned
interstage transformer, like,

http://www.jrmagnetics.com/rf/doubtune/doubccl_c.php
This is two resonators coupled with a cap, but any coupling method will
do. Capacitive, magnetic (putting the coils end-to-end) or
electromagnetic (coils side-by-side) coupling does equally well; normal
arrangements have them all in phase, so in practice, unshielded coils will
need smaller coupling capacitance than designed, etc.

If you line up that 10p resonance with the operating frequency, you should
get gobs more gain. In fact, because the reactances cancel, the driven
impedance will be much higher than you were expecting, and so will the
gain. The CT might go from, say, 1:8 up to, who knows, 1:20? 1:100?

The bandwidth of that coupling (not necessarily of the antenna itself, so
they should be similar bandwidths) is determined by the coupling
coefficient (in the coupled-inductors case, simply k) and Q of the
components.

If your receiver datasheet specifies an equivalent input circuit, you
might be able to estimate the equivalent loss and optimize gain.

Tim

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

On 3/7/2013 5:14 PM, Tim Williams wrote:
wrote in message
...
High Q isn't the goal, high radiation resistance is -- the bigger the
loop, the better it couples with free space, until it's a wave length
around.

I'm not clear on why you keep referring to radiation resistance for a
receiving antenna. Does this result in a larger received signal? I am
concerned with maximizing the voltage at the input to the receiver.

You're also not concerned about that -- you're concerned about maximizing
SNR at the receiver.

SNR would be good, but I am concerned with maximizing the signal actually.


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.


An antenna with high (expressed as ESR) radiation resistance might have a
modest Q, but gives far better SNR because it couples to free space.

I have not found anything to indicate this produces a better receive
antenna. I have a formula for the effective height of a loop antenna
which is what determines the received signal strength at the antenna. It
does not calculate the radiation resistance, it uses the coil parameters
and the wire resistance. Is that a wrong formula?


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.


Yes, a higher stepup ratio gets larger signal up to a point. That point
is determined by the parasitic capacitance of the receiver input. That
capacitance is reflected back through the transformer and affects the
antenna tuning. In my simulations it creates a filter with two
resonances.

Oooh, capacitance! I like capacitance. Capacitance is easy to
cancel...inductors are good at that.

What's a nearby inductor working against that capacitance? The current
transformer in your simulation, if its inductance can be controlled, would
be an excellent candidate. The circuit effectively becomes a double tuned
interstage transformer, like,

http://www.jrmagnetics.com/rf/doubtune/doubccl_c.php
This is two resonators coupled with a cap, but any coupling method will
do. Capacitive, magnetic (putting the coils end-to-end) or
electromagnetic (coils side-by-side) coupling does equally well; normal
arrangements have them all in phase, so in practice, unshielded coils will
need smaller coupling capacitance than designed, etc.

If you line up that 10p resonance with the operating frequency, you should
get gobs more gain. In fact, because the reactances cancel, the driven
impedance will be much higher than you were expecting, and so will the
gain. The CT might go from, say, 1:8 up to, who knows, 1:20? 1:100?

The bandwidth of that coupling (not necessarily of the antenna itself, so
they should be similar bandwidths) is determined by the coupling
coefficient (in the coupled-inductors case, simply k) and Q of the
components.

If your receiver datasheet specifies an equivalent input circuit, you
might be able to estimate the equivalent loss and optimize gain.

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

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

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?

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?

--

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

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.


--
Jim Mueller

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

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
...
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