Polarizeing current

[Tod]

http://www.thunting.com/geotech/forum/sonar/images/Dc200zCircuitlowres.JPG


Hi all.


Well, I was able to find a LM2904 in my junk circuit board inventory. It seems to have about the same spec as the LM358, at least it goes to ground. I put it together just as you said Carl, and it worked fine. Then I integrated it into the AD605 variable gain amp and with a little trouble, I got that to work. Analog”s applications bulletin on the AD605 uses caps for AC coupling on VGN and Vin, I removed them and it seemed to work better. Now that I have all that, I’m not sure where to put it, and GREG, I don’t need an answer from you :) If looking at the schematic gives any one an idea on where to incorporate this ramp let me know. I can send a high res copy to any one that wants it, just email me and I’ll send it right out. Even the high res is not all that good because it was faxed to me from Raytheon and there copy is about 4 generations old.


Thanks,


Tod


Tcp56@optonline.net

[Carl]

Yikes! There's a lot of stuff in that schematic and not enough resolution. Could you mail me a good photocopy?

[chief]

I am trying to make something similar for use in the north when doing ice rescue for people under ice. I was looking at about the same rpms, but I haven't been able to do much besides designs and searches for schematics for controlling the systems. I'll let you know if I find or design anything more.


Chief

[ian]

hi dan
just read your message about changes ill let you know how i got on shortly

ian

[ian]

hi dan
just read your message about changes ill let you know how i got on shortly

ian

[ian]

hi dan
just read your message about changes ill let you know how i got on shortly

ian

[Carl]

Does the DD coil have 2 TX coils? Concentric coils usually have 2 TX coils but not sure why DD would.

Basically you need to make sure the TX coil(s) has the same reactance (L-C) as the stock one. That's because most detectors use a Colpitts oscillator and the coil reactance sets the frequency. Then, you need to do the same on the RX coil because most receivers (on single frequency detectors) use a bandpass input tuned to the same frequency as the TX.

It's kind of tough to calculate the inductance and capacitance of scramble-wound coils. There's some approximate equations out there (inc. one somewhere on my web site) but I don't know how good they are. So you may end up doing trial-and-error on making the individual coils. Once you get them about right, then you have to put them on a substrate and get the overlap just right.

- Carl

[Carl]

Does the DD coil have 2 TX coils? Concentric coils usually have 2 TX coils but not sure why DD would.

Basically you need to make sure the TX coil(s) has the same reactance (L-C) as the stock one. That's because most detectors use a Colpitts oscillator and the coil reactance sets the frequency. Then, you need to do the same on the RX coil because most receivers (on single frequency detectors) use a bandpass input tuned to the same frequency as the TX.

It's kind of tough to calculate the inductance and capacitance of scramble-wound coils. There's some approximate equations out there (inc. one somewhere on my web site) but I don't know how good they are. So you may end up doing trial-and-error on making the individual coils. Once you get them about right, then you have to put them on a substrate and get the overlap just right.

- Carl

[Carl]

Does the DD coil have 2 TX coils? Concentric coils usually have 2 TX coils but not sure why DD would.

Basically you need to make sure the TX coil(s) has the same reactance (L-C) as the stock one. That's because most detectors use a Colpitts oscillator and the coil reactance sets the frequency. Then, you need to do the same on the RX coil because most receivers (on single frequency detectors) use a bandpass input tuned to the same frequency as the TX.

It's kind of tough to calculate the inductance and capacitance of scramble-wound coils. There's some approximate equations out there (inc. one somewhere on my web site) but I don't know how good they are. So you may end up doing trial-and-error on making the individual coils. Once you get them about right, then you have to put them on a substrate and get the overlap just right.

- Carl

[graeme]

Dan,

That's the best post in a long, long, time. Congratulations. Burning smells do nothing for anyone, huh!

Graeme

[graeme]

Dan,

That's the best post in a long, long, time. Congratulations. Burning smells do nothing for anyone, huh!

Graeme

[graeme]

Dan,

That's the best post in a long, long, time. Congratulations. Burning smells do nothing for anyone, huh!

Graeme

[Carl]

I would guess that both impedance and resonance are important, with the resonant freq. more so, esp. on matching the RX to the TX.

Ferinstance, let's say the GM has a Colpitts osc. and you design the TX coil a little bit off, so the TX freq. is 45kHz and not 50kHz. No big deal. Also, your resonant impedance is also off some, which means you may not get as much power transfer to the coil. Again, no big deal, TX power is just a little lower.

OK, now what's critical is to get the RX resonant freq. close to 45kHz. If you err to the high side, say 55kHz, then the received signal will be greatly attenuated, depending on the Q of the RX coil. But let's say you nail the RX resonance at 45kHz, but the resonant impedance is off. Like the TX coil, this might cause a little degradation in power transfer but as long as it's not WAY off you should be OK.

[Carl]

I would guess that both impedance and resonance are important, with the resonant freq. more so, esp. on matching the RX to the TX.

Ferinstance, let's say the GM has a Colpitts osc. and you design the TX coil a little bit off, so the TX freq. is 45kHz and not 50kHz. No big deal. Also, your resonant impedance is also off some, which means you may not get as much power transfer to the coil. Again, no big deal, TX power is just a little lower.

OK, now what's critical is to get the RX resonant freq. close to 45kHz. If you err to the high side, say 55kHz, then the received signal will be greatly attenuated, depending on the Q of the RX coil. But let's say you nail the RX resonance at 45kHz, but the resonant impedance is off. Like the TX coil, this might cause a little degradation in power transfer but as long as it's not WAY off you should be OK.

[Carl]

I would guess that both impedance and resonance are important, with the resonant freq. more so, esp. on matching the RX to the TX.

Ferinstance, let's say the GM has a Colpitts osc. and you design the TX coil a little bit off, so the TX freq. is 45kHz and not 50kHz. No big deal. Also, your resonant impedance is also off some, which means you may not get as much power transfer to the coil. Again, no big deal, TX power is just a little lower.

OK, now what's critical is to get the RX resonant freq. close to 45kHz. If you err to the high side, say 55kHz, then the received signal will be greatly attenuated, depending on the Q of the RX coil. But let's say you nail the RX resonance at 45kHz, but the resonant impedance is off. Like the TX coil, this might cause a little degradation in power transfer but as long as it's not WAY off you should be OK.

[Göran]

In all the descriptions of PPM:s that I have read on the net one uses a DC puls to polarize the protons. Why isn´t an alternating current at the resonant frequency used instead, as they do in Magnetic Resonance Imaging. Wouldn´t that be more effective?

[Willy Bayot]

It is not the DC pulse which polarizes but the DC itself by the powerful magnetic field it generates through the sensor coil. This proton alignment process takes SOME times depending on the liquid put inside the sensor but for distilled water, it takes around 3 seconds. Then, the current (and the field) is cut short and we listen to the weak and short-lived sine wave signal generated by the proton precession.
However, there are indeed some commercial PPM's based on the principle that you describe (using the Overhauser principle with an adequate RF frequency polarization and a specially prepared liquid containing a small quantity of free radicals) but they are even much more delicate to build and to adjust than the traditional ones.
Willy

[Göran]

What I meant was that the DC is turned on and off in a pulslike manner. Am I right in saying that it is the (current * time) that matters. A short "puls" with a lot of current does the same job as a longer "puls" with less current?
The process is a form af energy transfer to the proton. That energy should be easier and take less energy and less time to transfer if it is given at the resonance frequency.

[Willy Bayot]

And when do you capture and measure the precession frequency generated by the release of the protons after polarization?

[Göran]

After the exciteition during the FID or maybe after refocusing the spins with a spin echo.

[Göran]

My experiance with magnetic resonance comes mainly from its medical use. We use strong (1,5T) superconductiv magnets with resonance frequency at 60MHz. The magnetic field is highly stable and the equipment needs only minor adjustment to know the exact precession frequency of the protons. The aim is to make a "picture" of the human body. To that end one "modulate" the static magnetic field with linear gradients to make the protons within the volume of interest have different frequency.
The protons are "magnetized" with a very short AC-puls. One doesn´t use the FID to collect data. Insted the precession is refocused to get an echo at witch the data is collected.

The problem is somewhat different with a magnetometer. Here the precession frequency is not known eccept for its nominal value.The task i not to make a "picture" or a spectrogram of the protons but to find the precession frequency. One goal of constructing a magnetometer could be to use as little power as possible. To that en the transfer of energy during "magnetization" should be as effektiv as possible.
So, my question was. Is it possible to make the magnetization with less power by using AC with a frequenzy at least near the expected precession frequency than with a DC puls?

[Willy Bayot]

Could you please explain these sentences in more details? What is FID? What do you call a 'spin echo'?
Willy

[Willy Bayot]

Actually, there are 4 different types of PPM's:

1. The traditional one with DC switch off only and pure proton-rich liquids
2. Polarization by DC switch off + AF excitation of free radicals based liquids.
3. Polarization by short impulses + continuous AF excitation of free radicals based liquids (GEM systems).
4. Continous AF polarization by whose frequency is controlled by feed-back of precession frequency.

I think that the one you have in mind would be the last one.
Willy

[Göran]

FID = #Free Induction Decay" That is the signal you get emediately after you turn off the polarizing current. It is the relaxation process mainly depending on the T2* when the proton precessing loses its phase coherens. But as long as the magnetization has not totaly decayed you can recover the phase coherens by applying a new puls and get a signal echo. That process can be repeted several times. Thats how it is done at higher magnet strength. I dont know if the same applies to experiment at the lewel of the earth magnetic field

[Willy Bayot]

http://www.thunting.com/geotech/forum/mag/images/mag_systems.jpg


That one seems to be the third one of my previous list.
The traditional DC approach requires a polarization field of 100 to 300 Oe while the pulse system only requires 0.2 to 1.5 Oe to get an effective value of 200 to 1500 Oe.

Willy

[Göran]

What is the HF in your exampel 2-4? Is it proton nuclear precession frequence at the earth magnetic field or is it the electron spin reconance frequency??

[Willy Bayot]

the electron spin resonance frequency.
Willy

[Göran]

Thanks for your help and valuble information. I am planning to build a PP Magnetometer just for fun and experimentaition. I am sure I will come back and ask for more help. Yours Göran

[Hubert Dong]

This figures come from the article writen by V. Sapunov, etl, Quantum Magnetometry Laboratory, Ural State Technical University ??
Can I make a overhauser senor ?
Where can I get the schematics about the senor
from a web ste like PPM ?
Thank you!

[Willy Bayot]

Yes, indeed.
Yes, I guess the easiest system is shown in the second figure. It just requires that you mix a free radical powder with the proton-rich fluid and that you excite the protons with an RF frequency during the standard polarization of the coil.

I do not know of any detailed overhauser sensor winding and mounting procedure document available on the net. You would have to make some hit-and-miss experiments, but, it is what makes the fun of this activity, is it not?

Willy

[Hubert Dong]

Thank you, Willy !
It is difficult for constructing a overhauser magnetometer, isn't ?
I havn't any reference except some articles down load from net.
I want to know:
1) the material of free radical and it ingredient;
2) the concentration of liquid;
3) the freqency of polariting and its power.

Hubert Dong

[Willy Bayot]

You could buy the 4-OXO TEMPO product at http://www.sigmaaldrich.com under the product nr 179485-1G. It costs around 40$ per gram but with that quantity you can make half a liter of solution at .01 mole/liter, which is enough for a BIG sensor.

For a solution of 0.01 mole / liter you need 0.01 mole for 1 liter. So 0.01 mole X 171.24 grams/ mole for this product = 1.7124 grams.

For the frequency, I do not know exactly but it must be a few tens of MHz and its power is very small.

With this principle, the required constant polarization current is much lower than with a standard PPM.

Willy

[sajjad]

hi sir
my final year project is magnetometer,
can you help me from the basics and afterwards
with due regards
sajjad
pat_tmc@yahoo.com
be6409@yahoo.com
http://www.thunting.com/geotech/forum/sonar/images/Dc200zCircuitlowres.JPG


Hi all.


Well, I was able to find a LM2904 in my junk circuit board inventory. It seems to have about the same spec as the LM358, at least it goes to ground. I put it together just as you said Carl, and it worked fine. Then I integrated it into the AD605 variable gain amp and with a little trouble, I got that to work. Analog”s applications bulletin on the AD605 uses caps for AC coupling on VGN and Vin, I removed them and it seemed to work better. Now that I have all that, I’m not sure where to put it, and GREG, I don’t need an answer from you :) If looking at the schematic gives any one an idea on where to incorporate this ramp let me know. I can send a high res copy to any one that wants it, just email me and I’ll send it right out. Even the high res is not all that good because it was faxed to me from Raytheon and there copy is about 4 generations old.


Thanks,


Tod


Tcp56@optonline.net