rickman wrote in :

The entire power
budget is a couple hundred microwatts.

There's a tiny Texas Instruments one that might do it, very cheap too.
TLV2341, uses as little as 17µA single rail supply at up to 8V. I didn't use
it because it wasn't fast enough for what I bought it for, but it might be
worth trying for MSF signals.

rickman wrote in :

The entire power
budget is a couple hundred microwatts. I have yet to find an amplifier
that will fit this power budget.

That TLV2341 will stretch to do this drawing just 17µA, UGB is only 27KHz,
but if you set it for medium bias, consuming 250µA, you'll get 300KHz. Not
sure how much gain it will let you have for 60KHz, but I think it's one to
try.

El 28-10-14 21:33, rickman escribió:
I have a project in mind that would need a very good antenna in the
frequency range of 60 kHz. Originally I looked at loop antennas and
liked the idea of a large shielded loop made of coax tuned with a
capacitor. My goal is to get as large a signal as possible from the
antenna and matching circuit to allow the use of a receiver with very
low sensitivity... in fact an all digital receiver.

I spent some time simulating antennas in spice and was able to get a
bit of a feel for the circuit, but I'm not convinced it would work the
way I want. Just before I set the project aside I was told I needed to
model the radiation resistance. That has the potential of wrecking the
Q of the circuit. I am counting on the high Q to boost the output
voltage. If the radiation resistance is at all appreciable I would
lose the high Q and need to start over.

Anyone have an idea of how to estimate the radiation resistance of a
tuned, shielded loop antenna?

The other factor I don't understand how to factor in is the
distributed capacitance of the coax. Is that a significant influence
on an antenna or is it in the noise compared to the tuning capacitor.
The coax is RG-6-Solid Coax Cable. The loop is made up from 50 feet of
this. The specs are 16.2 pf/foot and 6.5 mOhms/foot in the center
conductor, or would the resistance be a round trip measurement of both
inner conductor and shield? I assume the shield has a much lower
resistance than the inner conductor but I don't know that for sure.


To get some idea of the output voltage of a loop you need to know:

The fieldstrength of the desired signal at your area. This is probably
given in V/m (dBuV/m, etc). As a first guess use E/H = 377 Ohms to
convert this to H-field [A/m].

EMF = n*A*u0*w*H gives you the EMF for a loop with area A and n number
of turns, w = radian frequency, u0 = magn. permeability for air.

The EMF is boosted with the Q-factor of your tuned loop. Guessing the
Q is the difficult part. You can't just use resistive loss (even
when corrected for skin effect). As you have a multi-turn loop there
is an eddy current loss due to proximity of the turns (the so-called
proximity loss). At these frequencies loss due to radiation is
negligible, unless you make very large coils.

Practically spoken you can't model the proximity loss in spice. In my
opinion you should measure the Q of your loop, or do some search on
Q-factor of VLF/MF coils for your coil geometry. That result you can
put into spice together with the induced EMF.

At these frequencies, external (induced) noise is the dominant factor,
think of man made noise. Only the resistive loss part of the capacitor
generates thermal noise. Using a coaxial cable as tuning capacitance
will not give the highest Q as you have a long/thin conductor. A
parallel plate capacitor has less resistive loss.

Are you able to use good quality RG58? As far as I know RG6 for
consumer CATV has low copper content and may have a CCS center conductor.



--
Wim
PA3DJS
Please remove abc first in case of PM

On 10/29/2014 6:45 AM, Wimpie wrote:
El 28-10-14 21:33, rickman escribió:
I have a project in mind that would need a very good antenna in the
frequency range of 60 kHz. Originally I looked at loop antennas and
liked the idea of a large shielded loop made of coax tuned with a
capacitor. My goal is to get as large a signal as possible from the
antenna and matching circuit to allow the use of a receiver with very
low sensitivity... in fact an all digital receiver.

I spent some time simulating antennas in spice and was able to get a
bit of a feel for the circuit, but I'm not convinced it would work the
way I want. Just before I set the project aside I was told I needed to
model the radiation resistance. That has the potential of wrecking the
Q of the circuit. I am counting on the high Q to boost the output
voltage. If the radiation resistance is at all appreciable I would
lose the high Q and need to start over.

Anyone have an idea of how to estimate the radiation resistance of a
tuned, shielded loop antenna?

The other factor I don't understand how to factor in is the
distributed capacitance of the coax. Is that a significant influence
on an antenna or is it in the noise compared to the tuning capacitor.
The coax is RG-6-Solid Coax Cable. The loop is made up from 50 feet of
this. The specs are 16.2 pf/foot and 6.5 mOhms/foot in the center
conductor, or would the resistance be a round trip measurement of both
inner conductor and shield? I assume the shield has a much lower
resistance than the inner conductor but I don't know that for sure.


To get some idea of the output voltage of a loop you need to know:

The fieldstrength of the desired signal at your area. This is probably
given in V/m (dBuV/m, etc). As a first guess use E/H = 377 Ohms to
convert this to H-field [A/m].

EMF = n*A*u0*w*H gives you the EMF for a loop with area A and n number
of turns, w = radian frequency, u0 = magn. permeability for air.

The EMF is boosted with the Q-factor of your tuned loop. Guessing the Q
is the difficult part. You can't just use resistive loss (even when
corrected for skin effect). As you have a multi-turn loop there is an
eddy current loss due to proximity of the turns (the so-called proximity
loss). At these frequencies loss due to radiation is negligible, unless
you make very large coils.

Practically spoken you can't model the proximity loss in spice. In my
opinion you should measure the Q of your loop, or do some search on
Q-factor of VLF/MF coils for your coil geometry. That result you can put
into spice together with the induced EMF.

At these frequencies, external (induced) noise is the dominant factor,
think of man made noise. Only the resistive loss part of the capacitor
generates thermal noise. Using a coaxial cable as tuning capacitance
will not give the highest Q as you have a long/thin conductor. A
parallel plate capacitor has less resistive loss.

Are you able to use good quality RG58? As far as I know RG6 for consumer
CATV has low copper content and may have a CCS center conductor.

It is good to hear from you again, Wim. I have missed your very
knowledgeable posts.

John KD5YI

On 10/29/2014 6:53 AM, Lostgallifreyan wrote:
rickman wrote in :

MSF time signals? Just a thought... If you're interfacing an analog
signal to digital, one trick I used (for audio but it ought to help
here too) is a CA3140 with a bit of positive feedback through a few
Mohms for hysteresis to clean the signal a bit. The resulting Schmitt
trigger, powered by about 5 or 6V, could be sensitive to take a lot of
strain off your antenna. Whether this alone gives you enough gain I
don't know, but it is cheap to try.

Thanks for the suggestion. I'm not sure this would be any better than
feeding it directly into my digital input. That is a differential input
and I expect to use feedback to overcome the residual input offset. So
the input will be pretty sensitive

Well, try it.

Yes, easier said than done. The receiver isn't built yet, I am
currently looking at the antenna design again and wish to improve my
simulation by adding the radiation resistance. If the antenna will only
put out microvolts even after tuning I will need to figure out how to
add the amp without having to double or quadruple the power budget.


If it works then inputs are better these days. Or at least,
more sensitive to small changes. As far as I know, digital inputs are usually
specified with a wide dead band for levels, amounting to HUGE hysteresis and
a need for a lot of gain first sp you already ned an op-amp stage no matter
what unless your digital inputs have hair triggers at exactly the threshold
you wanr.

This is a differential input which is not far from an analog input.
Actually even single ended digital inputs don't have much hysteresis
unless they are designed for that. But there is always some because of
the parasitic capacitance between the input and output of the buffer.


The thing about the CA3140 is that with just three passive parts: M-ohmage of
positive feedback, input series capacitance, and input ground resistor after
the cap, you can empirically set some very nice signal preconditioning as
well as raw gain, all on a very convenient single rail supply at 5V.

This design won't have a 5 volt rail. Most of the design will run on
1.2~1.8 volts with some I/O at 3.3 volts to drive an LCD. It's very low
power, remember?

--

Rick

On 10/29/2014 7:05 AM, Lostgallifreyan wrote:
rickman wrote in :

The entire power
budget is a couple hundred microwatts.

There's a tiny Texas Instruments one that might do it, very cheap too.
TLV2341, uses as little as 17µA single rail supply at up to 8V. I didn't use
it because it wasn't fast enough for what I bought it for, but it might be
worth trying for MSF signals.

GBW is only 0.79 MHz @ 3V Vdd, so I could only get a gain of... well not
much at 60 kHz. For an opamp to work as an opamp it needs to have
significant gain over the BW in use. I suppose I could use it open
loop, but then it would act as a low pass filter with a high gain and a
very low corner frequency.

--

Rick

rickman wrote in :

Actually even single ended digital inputs don't have much hysteresis
unless they are designed for that.

Well, as a proportion if they only go high above soem fairly close approach
to V+, then low when close to 0V, then the dead band could be wide, the aim
was to eliminate false states so they ARE usually designed for it. I take
your point on very low volt systems, if the actual difference is small even
though proportionally it may not be.

Anyway, now I know that the supply is so small, your suggestion of discrete
transistors is almost certainly the way to go, unless there is enough similar
demand out there to have cause an off-shelf part to be made.

Normally I'd just look at how others are solving similar problems, so I guess
the question I can ask is: what is the signficant difference in this case
that prevents the nearest off-shelf answer from working?

rickman wrote in :

GBW is only 0.79 MHz @ 3V Vdd, so I could only get a gain of... well not
much at 60 kHz.

True, I looked at it more earlier this evening, at 3V supply you'd be lucky
to get much more than a gain of 40 I think, so some specific and discrete
transistor fix might be best.

rickman wrote in :

For an opamp to work as an opamp it needs to have
significant gain over the BW in use.

Ok, how about just enough gsain to get a buffered output of some oomph to
survive integration to slow clean pulses? That might not take so much to do,
and if it works, it really takes the strain off the real gain stage
which follows it because that will be operating pretty much at DC capability.

On 10/29/2014 7:45 AM, Wimpie wrote:
El 28-10-14 21:33, rickman escribió:
I have a project in mind that would need a very good antenna in the
frequency range of 60 kHz. Originally I looked at loop antennas and
liked the idea of a large shielded loop made of coax tuned with a
capacitor. My goal is to get as large a signal as possible from the
antenna and matching circuit to allow the use of a receiver with very
low sensitivity... in fact an all digital receiver.

I spent some time simulating antennas in spice and was able to get a
bit of a feel for the circuit, but I'm not convinced it would work the
way I want. Just before I set the project aside I was told I needed to
model the radiation resistance. That has the potential of wrecking the
Q of the circuit. I am counting on the high Q to boost the output
voltage. If the radiation resistance is at all appreciable I would
lose the high Q and need to start over.

Anyone have an idea of how to estimate the radiation resistance of a
tuned, shielded loop antenna?

The other factor I don't understand how to factor in is the
distributed capacitance of the coax. Is that a significant influence
on an antenna or is it in the noise compared to the tuning capacitor.
The coax is RG-6-Solid Coax Cable. The loop is made up from 50 feet of
this. The specs are 16.2 pf/foot and 6.5 mOhms/foot in the center
conductor, or would the resistance be a round trip measurement of both
inner conductor and shield? I assume the shield has a much lower
resistance than the inner conductor but I don't know that for sure.


To get some idea of the output voltage of a loop you need to know:

The fieldstrength of the desired signal at your area. This is probably
given in V/m (dBuV/m, etc). As a first guess use E/H = 377 Ohms to
convert this to H-field [A/m].

EMF = n*A*u0*w*H gives you the EMF for a loop with area A and n number
of turns, w = radian frequency, u0 = magn. permeability for air.

This is new to me. I guess I have been mistakenly using the E field
formula. The field strength at optimum times is estimated at 100 uV/m
at my location which is at the weak end of the CONUS map. I will plug
the numbers into your H field version of the equation.


The EMF is boosted with the Q-factor of your tuned loop. Guessing the Q
is the difficult part. You can't just use resistive loss (even when
corrected for skin effect). As you have a multi-turn loop there is an
eddy current loss due to proximity of the turns (the so-called proximity
loss). At these frequencies loss due to radiation is negligible, unless
you make very large coils.

I have not seen the proximity effect taken into account in any
calculations for similar antenna, so I assumed it was also not
appreciable at this frequency. I'm not at all sure about the radiation
resistance. I will be plugging the numbers into the equation I have. I
assume this resistance would be in parallel with the inductor so a high
value is better. Or would it appear in series with the inductor and a
low value is better?


Practically spoken you can't model the proximity loss in spice. In my
opinion you should measure the Q of your loop, or do some search on
Q-factor of VLF/MF coils for your coil geometry. That result you can put
into spice together with the induced EMF.

I'm surprised you feel the Q can't be calculated. When originally
digging into this I found that the calculation of inductance is an
amazingly complex thing. There are lots of equations out there each of
which simplifies some aspect of the phenomenon and have different
applications. I would not expect the proximity effect to be any more
complex.


At these frequencies, external (induced) noise is the dominant factor,
think of man made noise. Only the resistive loss part of the capacitor
generates thermal noise. Using a coaxial cable as tuning capacitance
will not give the highest Q as you have a long/thin conductor. A
parallel plate capacitor has less resistive loss.

Q is important, but not the only factor. The coax was chosen to be
inexpensive and easy to work with. RG-6 with an 18 ga solid center
conductor is just slightly bigger than the skin effect and so is about
as usefully large a conductor without it being hollow. So I'm not sure
what might be better. I suppose Litz wire could improve the Q, but I'm
already looking at a Q of ball park 100 or more. Once you get a very
high Q it become hard to use the device without ruining the Q.


Are you able to use good quality RG58? As far as I know RG6 for consumer
CATV has low copper content and may have a CCS center conductor.

I picked an RG-6 with a solid center conductor. The specified
resistance is 6.5 mohm per foot. Funny, I'm sure most RG-6 is used for
cable TV where the center conductor is steel for strength with copper
plating for conductivity at high frequencies. One vendor argued with me
that solid copper cores were not available in RG-6. lol

BTW, I measured the resistance of my 50 foot of cable and it is in the
right ball park for 6.5 mohm/foot. The shield measured in the same
range as well. I thought the shield might have had a lower resistance
because it would amount to a larger cross section, but I guess not. I
don't think the shield resistance factors into the Q, but I'm not
certain of that.

--

Rick