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George Herold
Guest

Sun Feb 10, 2019 12:45 am   



On Saturday, February 9, 2019 at 3:36:42 AM UTC-5, 69883925...@nospam.org wrote:
Quote:
George Herold wrote:

So my boss asked today if we could make a cheap kit/ gizmo
that would let people 'see' proton nmr, pulse a coil,
look at the ring down. Insert water sample into coil,
see ring down change.

So (to have fun) the first question is how big is the
sample/magnetic field? At ~2.5kHz you can see nmr in
a beer can size of water in the Earth's field ~0.6 Gauss

In a permanent magnet ~0.5 Tesla (5 k Gauss) you
can see a signal from .. well a few good sized
drops of water. at ~20 MHz.

(proton gyromagnetic ratio is ~42 MHz/ T
4.2 kHz/ Gauss.)

I've got some 8" Helmholtz coils that do
~100 Gauss at 3A. But lotsa turns and spendy.

Some permanent magnets and a yoke? The field
homogeneity of little permanent magnetics is
going to suck. Field homogeneity is either
less volume or less Q, T2 broadening in the
nmr lingo.

You mention 'yoke', and I though of the old CRT deflection yokes.
There are many kinds of those, for different defection angles,
some older ones had permanent magnet focus.
The field homogeneity in the deflection coils should be OK.
But small space inside, enough for a glass test tube though.
I have quite a bit of experience driving those, think of 10A currents,
many turns.
Should be available for next to nothing?
The older ones with magnetic focus should have a nice field too over at least some length,
Would that work?


I was just thinking about the yoke as that big hunk of iron that
closes the magnetic loop around the pole area.
like this.. my first google hit.
https://www.lakeshore.com/products/electromagnets/models/pages/overview.aspx

GH

George Herold
Guest

Sun Feb 10, 2019 12:45 am   



On Saturday, February 9, 2019 at 12:15:23 AM UTC-5, Carl wrote:
Quote:
On 9/2/19 12:55 pm, George Herold wrote:
On Friday, February 8, 2019 at 7:13:52 PM UTC-5, John Larkin wrote:
On Fri, 8 Feb 2019 06:05:42 -0800 (PST), George Herold
gherold_at_teachspin.com> wrote:

On Thursday, February 7, 2019 at 8:12:05 PM UTC-5, Jeff Liebermann
wrote:
On Thu, 7 Feb 2019 06:33:34 -0800 (PST), George Herold
gherold_at_teachspin.com> wrote:

Hi all, A (mostly clueless) physics type wants to do
some RF stuff. (measuring nmr signals.. protons,
~5-25 MHz range) He's got something working, but
construction is ugly.

The uglier the antenna, construction, or layout, the better it works.
https://hackadaycom.files.wordpress.com/2016/05/williams-workbench.jpg
http://hephaestusaudio.com/media/2009/05/bob-pease-breadboard.jpg

I was thinking of recommending an ARRL handbook, is
any version better than others?

I don't know. I have some old ARRL Handbook issues at home that I
rarely read or use.

5-25 MHz is practically DC as far as breadboarding is concerned.

Right... but of concern is not just bread boarding, but in nmr you
typically have a coil down the end of a probe/ transmission line and
there is some thought needed to get good coupling into the coil.

There's usually a tuning network and a low-noise (sometimes cryo, a
"chilly probe") preamp in a box just outside the main field. Some sort
of t/r switching, too, since there's only one sample coil for transmit
and receive.

So we've done things a few different ways. (Our latest nmr is all
done by Norman, and I'm mostly clueless on the details.)
Tuning near the coil is what we do now. But the first one
had a coil on a fixed length of coax, (less than 1/4 wavelength)
and a tuned series inductor for the transmitter tuning.
(I'm not sure how the input was tuned, parallel C I assume.
Norman did the hard parts here too. :^) You've got to have
some Q on the input.

So my boss asked today if we could make a cheap kit/ gizmo
that would let people 'see' proton nmr, pulse a coil,
look at the ring down. Insert water sample into coil,
see ring down change.

So (to have fun) the first question is how big is the
sample/magnetic field? At ~2.5kHz you can see nmr in
a beer can size of water in the Earth's field ~0.6 Gauss

In a permanent magnet ~0.5 Tesla (5 k Gauss) you
can see a signal from .. well a few good sized
drops of water. at ~20 MHz.

(proton gyromagnetic ratio is ~42 MHz/ T
4.2 kHz/ Gauss.)

I've got some 8" Helmholtz coils that do
~100 Gauss at 3A. But lotsa turns and spendy.

Some permanent magnets and a yoke? The field
homogeneity of little permanent magnetics is
going to suck. Field homogeneity is either
less volume or less Q, T2 broadening in the
nmr lingo.

Two disc magnets spaced a distance of half the diameter (just like a
Helmholtz coil) gives the best homogeneity at the center of the gap.
Assuming perfect uniformity of the magnets themselves, at a guess a pair of
3" discs would give a volume with a homogeneity of maybe 0.1 to 1 part in
1000 over 1 cubic centimeter volume at a field of maybe 0.2-0.4 T depending
on the NdFeB grade. If you don't already have it grab a copy of FEMM and
run some simulations. Not great compared to 1 part in 10^8 or 9 like a high
resolution supercon NMR, but maybe enough to give a recognizable signal for
under $500 for the magnet. It's been a couple of years since I did a fair
number of sims with larger discs, playing around to see what useable volume
I could get with 6" discs for a tiny FTMS so I could be off an order of
magnitude here or there Smile.

--
Regards,
Carl Ijames


Yeah something like that could work.

The signal is proportional to the polarizing
field, so the bigger the better.

Thinking more.. How about we figure many users
will have their own B-field. And we make a cheap
gizmo to stick in the field and see an FID*. I was thinking
I can drive it with a Sig gen in burst mode...
maybe without an intermediate amp? And then
I have to amplify the ~1uV signal up to scope levels.

George H.

*free induction decay

George Herold
Guest

Sun Feb 10, 2019 1:45 am   



On Saturday, February 9, 2019 at 10:04:00 AM UTC-5, Lasse Langwadt Christensen wrote:
Quote:
lørdag den 9. februar 2019 kl. 02.55.29 UTC+1 skrev George Herold:
On Friday, February 8, 2019 at 7:13:52 PM UTC-5, John Larkin wrote:
On Fri, 8 Feb 2019 06:05:42 -0800 (PST), George Herold
gherold_at_teachspin.com> wrote:

On Thursday, February 7, 2019 at 8:12:05 PM UTC-5, Jeff Liebermann wrote:
On Thu, 7 Feb 2019 06:33:34 -0800 (PST), George Herold
gherold_at_teachspin.com> wrote:

Hi all, A (mostly clueless) physics type wants to do
some RF stuff. (measuring nmr signals.. protons,
~5-25 MHz range) He's got something working, but
construction is ugly.

The uglier the antenna, construction, or layout, the better it works.
https://hackadaycom.files.wordpress.com/2016/05/williams-workbench.jpg
http://hephaestusaudio.com/media/2009/05/bob-pease-breadboard.jpg

I was thinking of recommending an ARRL handbook, is
any version better than others?

I don't know. I have some old ARRL Handbook issues at home that I
rarely read or use.

5-25 MHz is practically DC as far as breadboarding is concerned.

Right... but of concern is not just bread boarding, but in nmr you
typically have a coil down the end of a probe/ transmission line and
there is some thought needed to get good coupling into the coil.

There's usually a tuning network and a low-noise (sometimes cryo, a
"chilly probe") preamp in a box just outside the main field. Some sort
of t/r switching, too, since there's only one sample coil for transmit
and receive.

So we've done things a few different ways. (Our latest nmr is all
done by Norman, and I'm mostly clueless on the details.)

Tuning near the coil is what we do now. But the first one
had a coil on a fixed length of coax, (less than 1/4 wavelength)
and a tuned series inductor for the transmitter tuning.
(I'm not sure how the input was tuned, parallel C I assume.
Norman did the hard parts here too. :^) You've got to have
some Q on the input.

So my boss asked today if we could make a cheap kit/ gizmo
that would let people 'see' proton nmr, pulse a coil,
look at the ring down. Insert water sample into coil,
see ring down change.

So (to have fun) the first question is how big is the
sample/magnetic field? At ~2.5kHz you can see nmr in
a beer can size of water in the Earth's field ~0.6 Gauss

In a permanent magnet ~0.5 Tesla (5 k Gauss) you
can see a signal from .. well a few good sized
drops of water. at ~20 MHz.

(proton gyromagnetic ratio is ~42 MHz/ T
4.2 kHz/ Gauss.)

I've got some 8" Helmholtz coils that do
~100 Gauss at 3A. But lotsa turns and spendy.

Some permanent magnets and a yoke? The field
homogeneity of little permanent magnetics is
going to suck. Field homogeneity is either
less volume or less Q, T2 broadening in the
nmr lingo.

George H.


something like this? https://www.thingiverse.com/thing:942257


I don't think so... but could be wrong. The figure of merit is
field homogeneity over field strength.
There are electromagnetic solutions for like a cylinder cut into
magnetic material... Which implies I could give up total
field for homogeneity.

George H.

George Herold
Guest

Sun Feb 10, 2019 2:45 am   



On Saturday, February 9, 2019 at 11:12:23 AM UTC-5, John Larkin wrote:
Quote:
On Fri, 8 Feb 2019 17:55:24 -0800 (PST), George Herold
gherold_at_teachspin.com> wrote:

On Friday, February 8, 2019 at 7:13:52 PM UTC-5, John Larkin wrote:
On Fri, 8 Feb 2019 06:05:42 -0800 (PST), George Herold
gherold_at_teachspin.com> wrote:

On Thursday, February 7, 2019 at 8:12:05 PM UTC-5, Jeff Liebermann wrote:
On Thu, 7 Feb 2019 06:33:34 -0800 (PST), George Herold
gherold_at_teachspin.com> wrote:

Hi all, A (mostly clueless) physics type wants to do
some RF stuff. (measuring nmr signals.. protons,
~5-25 MHz range) He's got something working, but
construction is ugly.

The uglier the antenna, construction, or layout, the better it works.
https://hackadaycom.files.wordpress.com/2016/05/williams-workbench.jpg
http://hephaestusaudio.com/media/2009/05/bob-pease-breadboard.jpg

I was thinking of recommending an ARRL handbook, is
any version better than others?

I don't know. I have some old ARRL Handbook issues at home that I
rarely read or use.

5-25 MHz is practically DC as far as breadboarding is concerned.

Right... but of concern is not just bread boarding, but in nmr you
typically have a coil down the end of a probe/ transmission line and
there is some thought needed to get good coupling into the coil.

There's usually a tuning network and a low-noise (sometimes cryo, a
"chilly probe") preamp in a box just outside the main field. Some sort
of t/r switching, too, since there's only one sample coil for transmit
and receive.

So we've done things a few different ways. (Our latest nmr is all
done by Norman, and I'm mostly clueless on the details.)

Tuning near the coil is what we do now. But the first one
had a coil on a fixed length of coax, (less than 1/4 wavelength)
and a tuned series inductor for the transmitter tuning.
(I'm not sure how the input was tuned, parallel C I assume.
Norman did the hard parts here too. :^) You've got to have
some Q on the input.

So my boss asked today if we could make a cheap kit/ gizmo
that would let people 'see' proton nmr, pulse a coil,
look at the ring down. Insert water sample into coil,
see ring down change.

So (to have fun) the first question is how big is the
sample/magnetic field? At ~2.5kHz you can see nmr in
a beer can size of water in the Earth's field ~0.6 Gauss

In a permanent magnet ~0.5 Tesla (5 k Gauss) you
can see a signal from .. well a few good sized
drops of water. at ~20 MHz.

(proton gyromagnetic ratio is ~42 MHz/ T
4.2 kHz/ Gauss.)

I've got some 8" Helmholtz coils that do
~100 Gauss at 3A. But lotsa turns and spendy.

Some permanent magnets and a yoke? The field
homogeneity of little permanent magnetics is
going to suck. Field homogeneity is either
less volume or less Q, T2 broadening in the
nmr lingo.


A good NMR system has hydrogen line Qs of 1e9. To get that, the mag
field has to be uniform across the sample to a part per billion. That
leads to the arcane craft of magnet shimming.

The superconductive magnets have a few superconductive shim coils, and
a mess of (as in 10 to maybe 40) ruum temp shims, each with its own
programmable power supply.

If the sample is long and skinny, the field is expressed as a
polynomial on Z, the long axis. Z0 is the main field, and there are
shim coils to null the higher order terms. I sold a lot of gradient
coil drivers, which apply a pulsed Z1 field during the
transmit/receive sequances to do some sort of quantum mechanical
tricks.

Just the alloy and plating of the antenna coil was a trade secret, to
avoid field distortions. Screws were of exotic alloys and the hand
selected to be magnetically neutral. Tiny amounts of iron dissolved in
a sample were bad. One could buy solvents made of specific isotopes.

*Everything* is magnetic at 1 PPB.

I think NMR has been largely replaced by other technologies. It's just
a big deal to buy and install and feed a megabuck superconductive
magnet.



--

John Larkin Highland Technology, Inc

lunatic fringe electronics


Right... So I want to see a signal on a 'scope.
Say a 10 MHz freq. with a field homogeneity of
1ppt 0.1% over the sample I'd get 10 kHz...
uggh! I guess I'd like the signal to last
~100 times longer than the Q of the circuit.
(So it's harder confuse ringing with signal.)

George H.

George Herold
Guest

Sun Feb 10, 2019 2:45 am   



On Saturday, February 9, 2019 at 12:28:36 PM UTC-5, John Larkin wrote:
Quote:
On Sat, 9 Feb 2019 12:12:53 -0500, "Carl"
carl.ijamesXYZ_at_ZYXverizon.net> wrote:

"Lasse Langwadt Christensen" wrote in message
news:0f82c6e6-90eb-41ca-bca3-daf0581aa4f4_at_googlegroups.com...

lørdag den 9. februar 2019 kl. 02.55.29 UTC+1 skrev George Herold:
On Friday, February 8, 2019 at 7:13:52 PM UTC-5, John Larkin wrote:
On Fri, 8 Feb 2019 06:05:42 -0800 (PST), George Herold
gherold_at_teachspin.com> wrote:

On Thursday, February 7, 2019 at 8:12:05 PM UTC-5, Jeff Liebermann
wrote:
On Thu, 7 Feb 2019 06:33:34 -0800 (PST), George Herold
gherold_at_teachspin.com> wrote:

Hi all, A (mostly clueless) physics type wants to do
some RF stuff. (measuring nmr signals.. protons,
~5-25 MHz range) He's got something working, but
construction is ugly.

The uglier the antenna, construction, or layout, the better it
works.
https://hackadaycom.files.wordpress.com/2016/05/williams-workbench.jpg
http://hephaestusaudio.com/media/2009/05/bob-pease-breadboard.jpg

I was thinking of recommending an ARRL handbook, is
any version better than others?

I don't know. I have some old ARRL Handbook issues at home that I
rarely read or use.

5-25 MHz is practically DC as far as breadboarding is concerned.

Right... but of concern is not just bread boarding, but in nmr you
typically have a coil down the end of a probe/ transmission line and
there is some thought needed to get good coupling into the coil.

There's usually a tuning network and a low-noise (sometimes cryo, a
"chilly probe") preamp in a box just outside the main field. Some sort
of t/r switching, too, since there's only one sample coil for transmit
and receive.

So we've done things a few different ways. (Our latest nmr is all
done by Norman, and I'm mostly clueless on the details.)

Tuning near the coil is what we do now. But the first one
had a coil on a fixed length of coax, (less than 1/4 wavelength)
and a tuned series inductor for the transmitter tuning.
(I'm not sure how the input was tuned, parallel C I assume.
Norman did the hard parts here too. :^) You've got to have
some Q on the input.

So my boss asked today if we could make a cheap kit/ gizmo
that would let people 'see' proton nmr, pulse a coil,
look at the ring down. Insert water sample into coil,
see ring down change.

So (to have fun) the first question is how big is the
sample/magnetic field? At ~2.5kHz you can see nmr in
a beer can size of water in the Earth's field ~0.6 Gauss

In a permanent magnet ~0.5 Tesla (5 k Gauss) you
can see a signal from .. well a few good sized
drops of water. at ~20 MHz.

(proton gyromagnetic ratio is ~42 MHz/ T
4.2 kHz/ Gauss.)

I've got some 8" Helmholtz coils that do
~100 Gauss at 3A. But lotsa turns and spendy.

Some permanent magnets and a yoke? The field
homogeneity of little permanent magnetics is
going to suck. Field homogeneity is either
less volume or less Q, T2 broadening in the
nmr lingo.

George H.


something like this? https://www.thingiverse.com/thing:942257

It looks cute but the pictures show a lot of slop in the magnet position so
I doubt it will achieve anywhere near that field map. Not that I can
eyeball sub-part per thousand differences from a map like that but I've
spent some time playing with arrays just like that and testing small
position and tilt errors. Also, given the size of the "bore" opening
compared to the length of the magnets pretty much only a sub, sub millimeter
thick slice across the center will be homogeneous. The distance from the
closest side of the homogenous region needs to be at least two (three is
better) bore diameters from the end of the magnet segments to limit the
effect of field falloff as you approach the ends. Again, percent is easy,
sub part per thousand gets hard. You could probably shorten the magnet
length with some form of end cap but then you lose the clear bore access..
After doing a lot of comparisons I decided that just using two discs and
having side access perpendicular to the magnetic field gave the same or
greater field strength and homogeneity over an appreciable volume for the
same cost and much easier construction. I'm not a magnet physicist, this is
worth what you are paying, YMMV, etc., etc. Smile. Other things I played
with: making the discs thicker doesn't get that much more field, using a
yoke of flat plates across the outer faces of the discs with one or two side
panels gave more field but added a lot of weight and didn't really change
the simulated homogeneity so it was a wash compared to just buying thicker
discs, and trying a few simple pole cap shapes helped the homogeneity but
lowered the field since the magnets were now further apart for the same
clear gap. I was bored, curious, and had a pc and FEMM so I explored Smile.

Maybe one could solder a bunch of unshielded drum core inductors to a
PC board and glue a couple of them to the front or back of your disk
magnets. Make an active shim system. That would allow George's system
to be a shimming trainer too.

This is probably Gauss's theorem #8, but I find that you can
cancel any inhomogeneity to first order, by adding the right dipole
field (magnitude and direction.) In practice this means you can wave
a magnetic around in space an find one of the two sweet spots for it.
(given you have some signal that changes with homogeneity.)

George H.
Quote:

--

John Larkin Highland Technology, Inc

lunatic fringe electronics


nuny@bid.nes
Guest

Sun Feb 10, 2019 9:45 am   



On Saturday, February 9, 2019 at 3:39:19 PM UTC-8, George Herold wrote:
Quote:
On Saturday, February 9, 2019 at 5:01:01 AM UTC-5, John Miles, KE5FX wrote:
On Friday, February 8, 2019 at 5:55:29 PM UTC-8, George Herold wrote:
So (to have fun) the first question is how big is the
sample/magnetic field? At ~2.5kHz you can see nmr in
a beer can size of water in the Earth's field ~0.6 Gauss


You might consider offering a kit like Joe Geller's, which I
don't believe he sells anymore:

https://www.edn.com/design/analog/4392193/Geomagnetic-observatory
http://www.ke5fx.com/Geller_proton_magnetometer.pdf

In other words, instead of using specific magnets and shields to
achieve a consistent result, set up the instrument to live with (and
measure) the local field.

-- john, KE5FX

Nice! I like the side by side 180 out of phase samples!
(the 180 out of phase cancels the interference noise.)
We sell an Earth Fields NMR
https://www.teachspin.com/earths-field-nmr
Instead of side by side coils it uses one
good sample coil, and a bigger, (but of the same
turns area) co-axial cancellation coil. seen in pic.
The sample is a plastic bottle, beer can size.


The Geller kit is inspired by the proton precession magnetometer article in the Amateur Scientist section of Scientific American of years gone by.

Apparently it managed 20 nT resolution, and used tubes.

If your physics type is interested it's in the Amateur Scientist CD:

https://www.surplusshed.com/pages/item/M2071.html

I think this is the book the CD is based on- it's searchable online:

https://archive.org/details/TheAmateurScientist/page/n171

...or you can download it, which I just did.


Mark L. Fergerson

Lasse Langwadt Christense
Guest

Sun Feb 10, 2019 3:45 pm   



søndag den 10. februar 2019 kl. 00.19.41 UTC+1 skrev George Herold:
Quote:
On Saturday, February 9, 2019 at 12:15:23 AM UTC-5, Carl wrote:
On 9/2/19 12:55 pm, George Herold wrote:
On Friday, February 8, 2019 at 7:13:52 PM UTC-5, John Larkin wrote:
On Fri, 8 Feb 2019 06:05:42 -0800 (PST), George Herold
gherold_at_teachspin.com> wrote:

On Thursday, February 7, 2019 at 8:12:05 PM UTC-5, Jeff Liebermann
wrote:
On Thu, 7 Feb 2019 06:33:34 -0800 (PST), George Herold
gherold_at_teachspin.com> wrote:

Hi all, A (mostly clueless) physics type wants to do
some RF stuff. (measuring nmr signals.. protons,
~5-25 MHz range) He's got something working, but
construction is ugly.

The uglier the antenna, construction, or layout, the better it works.
https://hackadaycom.files.wordpress.com/2016/05/williams-workbench.jpg
http://hephaestusaudio.com/media/2009/05/bob-pease-breadboard.jpg

I was thinking of recommending an ARRL handbook, is
any version better than others?

I don't know. I have some old ARRL Handbook issues at home that I
rarely read or use.

5-25 MHz is practically DC as far as breadboarding is concerned.

Right... but of concern is not just bread boarding, but in nmr you
typically have a coil down the end of a probe/ transmission line and
there is some thought needed to get good coupling into the coil.

There's usually a tuning network and a low-noise (sometimes cryo, a
"chilly probe") preamp in a box just outside the main field. Some sort
of t/r switching, too, since there's only one sample coil for transmit
and receive.

So we've done things a few different ways. (Our latest nmr is all
done by Norman, and I'm mostly clueless on the details.)
Tuning near the coil is what we do now. But the first one
had a coil on a fixed length of coax, (less than 1/4 wavelength)
and a tuned series inductor for the transmitter tuning.
(I'm not sure how the input was tuned, parallel C I assume.
Norman did the hard parts here too. :^) You've got to have
some Q on the input.

So my boss asked today if we could make a cheap kit/ gizmo
that would let people 'see' proton nmr, pulse a coil,
look at the ring down. Insert water sample into coil,
see ring down change.

So (to have fun) the first question is how big is the
sample/magnetic field? At ~2.5kHz you can see nmr in
a beer can size of water in the Earth's field ~0.6 Gauss

In a permanent magnet ~0.5 Tesla (5 k Gauss) you
can see a signal from .. well a few good sized
drops of water. at ~20 MHz.

(proton gyromagnetic ratio is ~42 MHz/ T
4.2 kHz/ Gauss.)

I've got some 8" Helmholtz coils that do
~100 Gauss at 3A. But lotsa turns and spendy.

Some permanent magnets and a yoke? The field
homogeneity of little permanent magnetics is
going to suck. Field homogeneity is either
less volume or less Q, T2 broadening in the
nmr lingo.

Two disc magnets spaced a distance of half the diameter (just like a
Helmholtz coil) gives the best homogeneity at the center of the gap.
Assuming perfect uniformity of the magnets themselves, at a guess a pair of
3" discs would give a volume with a homogeneity of maybe 0.1 to 1 part in
1000 over 1 cubic centimeter volume at a field of maybe 0.2-0.4 T depending
on the NdFeB grade. If you don't already have it grab a copy of FEMM and
run some simulations. Not great compared to 1 part in 10^8 or 9 like a high
resolution supercon NMR, but maybe enough to give a recognizable signal for
under $500 for the magnet. It's been a couple of years since I did a fair
number of sims with larger discs, playing around to see what useable volume
I could get with 6" discs for a tiny FTMS so I could be off an order of
magnitude here or there Smile.

--
Regards,
Carl Ijames

Yeah something like that could work.

The signal is proportional to the polarizing
field, so the bigger the better.

Thinking more.. How about we figure many users
will have their own B-field. And we make a cheap
gizmo to stick in the field and see an FID*. I was thinking
I can drive it with a Sig gen in burst mode...
maybe without an intermediate amp? And then
I have to amplify the ~1uV signal up to scope levels.


but then you'd need to support multiple frequencies

John Larkin
Guest

Sun Feb 10, 2019 5:45 pm   



On Sat, 9 Feb 2019 00:14:24 -0500, "Carl"
<carl.ijamesXYZ_at_ZYXverizon.net> wrote:

Quote:
On 9/2/19 12:55 pm, George Herold wrote:
On Friday, February 8, 2019 at 7:13:52 PM UTC-5, John Larkin wrote:
On Fri, 8 Feb 2019 06:05:42 -0800 (PST), George Herold
gherold_at_teachspin.com> wrote:

On Thursday, February 7, 2019 at 8:12:05 PM UTC-5, Jeff Liebermann
wrote:
On Thu, 7 Feb 2019 06:33:34 -0800 (PST), George Herold
gherold_at_teachspin.com> wrote:

Hi all, A (mostly clueless) physics type wants to do
some RF stuff. (measuring nmr signals.. protons,
~5-25 MHz range) He's got something working, but
construction is ugly.

The uglier the antenna, construction, or layout, the better it works.
https://hackadaycom.files.wordpress.com/2016/05/williams-workbench.jpg
http://hephaestusaudio.com/media/2009/05/bob-pease-breadboard.jpg

I was thinking of recommending an ARRL handbook, is
any version better than others?

I don't know. I have some old ARRL Handbook issues at home that I
rarely read or use.

5-25 MHz is practically DC as far as breadboarding is concerned.

Right... but of concern is not just bread boarding, but in nmr you
typically have a coil down the end of a probe/ transmission line and
there is some thought needed to get good coupling into the coil.

There's usually a tuning network and a low-noise (sometimes cryo, a
"chilly probe") preamp in a box just outside the main field. Some sort
of t/r switching, too, since there's only one sample coil for transmit
and receive.

So we've done things a few different ways. (Our latest nmr is all
done by Norman, and I'm mostly clueless on the details.)
Tuning near the coil is what we do now. But the first one
had a coil on a fixed length of coax, (less than 1/4 wavelength)
and a tuned series inductor for the transmitter tuning.
(I'm not sure how the input was tuned, parallel C I assume.
Norman did the hard parts here too. :^) You've got to have
some Q on the input.

So my boss asked today if we could make a cheap kit/ gizmo
that would let people 'see' proton nmr, pulse a coil,
look at the ring down. Insert water sample into coil,
see ring down change.

So (to have fun) the first question is how big is the
sample/magnetic field? At ~2.5kHz you can see nmr in
a beer can size of water in the Earth's field ~0.6 Gauss

In a permanent magnet ~0.5 Tesla (5 k Gauss) you
can see a signal from .. well a few good sized
drops of water. at ~20 MHz.

(proton gyromagnetic ratio is ~42 MHz/ T
4.2 kHz/ Gauss.)

I've got some 8" Helmholtz coils that do
~100 Gauss at 3A. But lotsa turns and spendy.

Some permanent magnets and a yoke? The field
homogeneity of little permanent magnetics is
going to suck. Field homogeneity is either
less volume or less Q, T2 broadening in the
nmr lingo.

Two disc magnets spaced a distance of half the diameter (just like a
Helmholtz coil) gives the best homogeneity at the center of the gap.
Assuming perfect uniformity of the magnets themselves, at a guess a pair of
3" discs would give a volume with a homogeneity of maybe 0.1 to 1 part in
1000 over 1 cubic centimeter volume at a field of maybe 0.2-0.4 T depending
on the NdFeB grade. If you don't already have it grab a copy of FEMM and
run some simulations. Not great compared to 1 part in 10^8 or 9 like a high
resolution supercon NMR, but maybe enough to give a recognizable signal for
under $500 for the magnet. It's been a couple of years since I did a fair
number of sims with larger discs, playing around to see what useable volume
I could get with 6" discs for a tiny FTMS so I could be off an order of
magnitude here or there Smile.


I designed the controller for a big FTMS.

https://www.dropbox.com/s/1pvax542n1erhfz/IMG_0304.JPG?dl=0

https://www.dropbox.com/s/79dqhigfza4hnda/IMG_9667.JPG?dl=0

https://www.dropbox.com/s/rqkojfxw8svc4mx/IMG_9697.JPG?dl=0

https://www.dropbox.com/s/k9tny9da3sikgcw/IMG_9717.JPG?dl=0

https://www.dropbox.com/s/0e71x4r2a72y86m/FTMS_Cell_top.jpg?dl=0


The preamp was designed by some chemists,

https://www.dropbox.com/s/sm2x1az6193y5fx/FTMS_Preamp.jpg?dl=0

and they gave up 30 dB of s/n or so. It was ghastly.

I thought it might be possible to detect a single ion in orbit, but we
never got to try.

Varian acquired Ion Spec, then Agilent acquired Varian, then Agilent
kill off the NMR and FTMS product lines.

I have some planned preamp schematics around here somewhere.





--

John Larkin Highland Technology, Inc

lunatic fringe electronics


Guest

Sun Feb 10, 2019 6:45 pm   



On Thursday, February 7, 2019 at 9:33:41 AM UTC-5, George Herold wrote:
Quote:
Hi all, A (mostly clueless) physics type wants to do some RF stuff. (measuring nmr signals.. protons, ~5-25 MHz range) He's got something
working, but construction is ugly. I was thinking of recommending an ARRL handbook, is any version better than others?


This is pretty good:
http://home.sandiego.edu/~ekim/otherjunk/rf_proto.pdf

sdy
Guest

Sun Feb 10, 2019 8:45 pm   



On Thursday, February 7, 2019 at 9:33:41 AM UTC-5, George Herold wrote:
Quote:
Hi all, A (mostly clueless) physics type wants to do some RF stuff. (measuring nmr signals.. protons, ~5-25 MHz range) He's got something
working, but construction is ugly. I was thinking of recommending an ARRL handbook, is any version better than others?


Very nice write-up! I will distribute it to my RF Lab class.

Lasse Langwadt Christense
Guest

Tue Feb 12, 2019 1:45 am   



tirsdag den 12. februar 2019 kl. 01.02.02 UTC+1 skrev George Herold:
Quote:
On Sunday, February 10, 2019 at 10:52:28 AM UTC-5, John Larkin wrote:
On Sat, 9 Feb 2019 00:14:24 -0500, "Carl"
carl.ijamesXYZ_at_ZYXverizon.net> wrote:

On 9/2/19 12:55 pm, George Herold wrote:
On Friday, February 8, 2019 at 7:13:52 PM UTC-5, John Larkin wrote:
On Fri, 8 Feb 2019 06:05:42 -0800 (PST), George Herold
gherold_at_teachspin.com> wrote:

On Thursday, February 7, 2019 at 8:12:05 PM UTC-5, Jeff Liebermann
wrote:
On Thu, 7 Feb 2019 06:33:34 -0800 (PST), George Herold
gherold_at_teachspin.com> wrote:

Hi all, A (mostly clueless) physics type wants to do
some RF stuff. (measuring nmr signals.. protons,
~5-25 MHz range) He's got something working, but
construction is ugly.

The uglier the antenna, construction, or layout, the better it works.
https://hackadaycom.files.wordpress.com/2016/05/williams-workbench.jpg
http://hephaestusaudio.com/media/2009/05/bob-pease-breadboard.jpg

I was thinking of recommending an ARRL handbook, is
any version better than others?

I don't know. I have some old ARRL Handbook issues at home that I
rarely read or use.

5-25 MHz is practically DC as far as breadboarding is concerned.

Right... but of concern is not just bread boarding, but in nmr you
typically have a coil down the end of a probe/ transmission line and
there is some thought needed to get good coupling into the coil.

There's usually a tuning network and a low-noise (sometimes cryo, a
"chilly probe") preamp in a box just outside the main field. Some sort
of t/r switching, too, since there's only one sample coil for transmit
and receive.

So we've done things a few different ways. (Our latest nmr is all
done by Norman, and I'm mostly clueless on the details.)
Tuning near the coil is what we do now. But the first one
had a coil on a fixed length of coax, (less than 1/4 wavelength)
and a tuned series inductor for the transmitter tuning.
(I'm not sure how the input was tuned, parallel C I assume.
Norman did the hard parts here too. :^) You've got to have
some Q on the input.

So my boss asked today if we could make a cheap kit/ gizmo
that would let people 'see' proton nmr, pulse a coil,
look at the ring down. Insert water sample into coil,
see ring down change.

So (to have fun) the first question is how big is the
sample/magnetic field? At ~2.5kHz you can see nmr in
a beer can size of water in the Earth's field ~0.6 Gauss

In a permanent magnet ~0.5 Tesla (5 k Gauss) you
can see a signal from .. well a few good sized
drops of water. at ~20 MHz.

(proton gyromagnetic ratio is ~42 MHz/ T
4.2 kHz/ Gauss.)

I've got some 8" Helmholtz coils that do
~100 Gauss at 3A. But lotsa turns and spendy.

Some permanent magnets and a yoke? The field
homogeneity of little permanent magnetics is
going to suck. Field homogeneity is either
less volume or less Q, T2 broadening in the
nmr lingo.

Two disc magnets spaced a distance of half the diameter (just like a
Helmholtz coil) gives the best homogeneity at the center of the gap.
Assuming perfect uniformity of the magnets themselves, at a guess a pair of
3" discs would give a volume with a homogeneity of maybe 0.1 to 1 part in
1000 over 1 cubic centimeter volume at a field of maybe 0.2-0.4 T depending
on the NdFeB grade. If you don't already have it grab a copy of FEMM and
run some simulations. Not great compared to 1 part in 10^8 or 9 like a high
resolution supercon NMR, but maybe enough to give a recognizable signal for
under $500 for the magnet. It's been a couple of years since I did a fair
number of sims with larger discs, playing around to see what useable volume
I could get with 6" discs for a tiny FTMS so I could be off an order of
magnitude here or there Smile.

I designed the controller for a big FTMS.

https://www.dropbox.com/s/1pvax542n1erhfz/IMG_0304.JPG?dl=0

https://www.dropbox.com/s/79dqhigfza4hnda/IMG_9667.JPG?dl=0

https://www.dropbox.com/s/rqkojfxw8svc4mx/IMG_9697.JPG?dl=0

https://www.dropbox.com/s/k9tny9da3sikgcw/IMG_9717.JPG?dl=0

https://www.dropbox.com/s/0e71x4r2a72y86m/FTMS_Cell_top.jpg?dl=0


The preamp was designed by some chemists,

https://www.dropbox.com/s/sm2x1az6193y5fx/FTMS_Preamp.jpg?dl=0

and they gave up 30 dB of s/n or so. It was ghastly.

I thought it might be possible to detect a single ion in orbit, but we
never got to try.

Varian acquired Ion Spec, then Agilent acquired Varian, then Agilent
kill off the NMR and FTMS product lines.

I have some planned preamp schematics around here somewhere.
I didn't look at all the pics.

I've been 1/2 thinking about this.
For a single coil system, I'm thinking I need some sort
of switching between the two circuits, flip the spins and
then see the signal. Which is doable, but another thing...
A dual cross coil system has some advantages.
Flip with one, sense with other.
The two functions are naturally apart.
That makes it easier to test when something is not working.
(which if you are contemplating some marginally functional kit
is very important.)


something like this works ok,

https://images.slideplayer.com/27/8912193/slides/slide_14.jpg

tweak Cmatch and Ctune for resonance and 50R match using a VNA

you can do the quarter-wave with a length of coax or a CLC pi section
for moderate powers 1n4148 diodes will work

George Herold
Guest

Tue Feb 12, 2019 1:45 am   



On Monday, February 11, 2019 at 7:02:02 PM UTC-5, George Herold wrote:
Quote:
On Sunday, February 10, 2019 at 10:52:28 AM UTC-5, John Larkin wrote:
On Sat, 9 Feb 2019 00:14:24 -0500, "Carl"
carl.ijamesXYZ_at_ZYXverizon.net> wrote:

On 9/2/19 12:55 pm, George Herold wrote:
On Friday, February 8, 2019 at 7:13:52 PM UTC-5, John Larkin wrote:
On Fri, 8 Feb 2019 06:05:42 -0800 (PST), George Herold
gherold_at_teachspin.com> wrote:

On Thursday, February 7, 2019 at 8:12:05 PM UTC-5, Jeff Liebermann
wrote:
On Thu, 7 Feb 2019 06:33:34 -0800 (PST), George Herold
gherold_at_teachspin.com> wrote:

Hi all, A (mostly clueless) physics type wants to do
some RF stuff. (measuring nmr signals.. protons,
~5-25 MHz range) He's got something working, but
construction is ugly.

The uglier the antenna, construction, or layout, the better it works.
https://hackadaycom.files.wordpress.com/2016/05/williams-workbench.jpg
http://hephaestusaudio.com/media/2009/05/bob-pease-breadboard.jpg

I was thinking of recommending an ARRL handbook, is
any version better than others?

I don't know. I have some old ARRL Handbook issues at home that I
rarely read or use.

5-25 MHz is practically DC as far as breadboarding is concerned.

Right... but of concern is not just bread boarding, but in nmr you
typically have a coil down the end of a probe/ transmission line and
there is some thought needed to get good coupling into the coil.

There's usually a tuning network and a low-noise (sometimes cryo, a
"chilly probe") preamp in a box just outside the main field. Some sort
of t/r switching, too, since there's only one sample coil for transmit
and receive.

So we've done things a few different ways. (Our latest nmr is all
done by Norman, and I'm mostly clueless on the details.)
Tuning near the coil is what we do now. But the first one
had a coil on a fixed length of coax, (less than 1/4 wavelength)
and a tuned series inductor for the transmitter tuning.
(I'm not sure how the input was tuned, parallel C I assume.
Norman did the hard parts here too. :^) You've got to have
some Q on the input.

So my boss asked today if we could make a cheap kit/ gizmo
that would let people 'see' proton nmr, pulse a coil,
look at the ring down. Insert water sample into coil,
see ring down change.

So (to have fun) the first question is how big is the
sample/magnetic field? At ~2.5kHz you can see nmr in
a beer can size of water in the Earth's field ~0.6 Gauss

In a permanent magnet ~0.5 Tesla (5 k Gauss) you
can see a signal from .. well a few good sized
drops of water. at ~20 MHz.

(proton gyromagnetic ratio is ~42 MHz/ T
4.2 kHz/ Gauss.)

I've got some 8" Helmholtz coils that do
~100 Gauss at 3A. But lotsa turns and spendy.

Some permanent magnets and a yoke? The field
homogeneity of little permanent magnetics is
going to suck. Field homogeneity is either
less volume or less Q, T2 broadening in the
nmr lingo.

Two disc magnets spaced a distance of half the diameter (just like a
Helmholtz coil) gives the best homogeneity at the center of the gap.
Assuming perfect uniformity of the magnets themselves, at a guess a pair of
3" discs would give a volume with a homogeneity of maybe 0.1 to 1 part in
1000 over 1 cubic centimeter volume at a field of maybe 0.2-0.4 T depending
on the NdFeB grade. If you don't already have it grab a copy of FEMM and
run some simulations. Not great compared to 1 part in 10^8 or 9 like a high
resolution supercon NMR, but maybe enough to give a recognizable signal for
under $500 for the magnet. It's been a couple of years since I did a fair
number of sims with larger discs, playing around to see what useable volume
I could get with 6" discs for a tiny FTMS so I could be off an order of
magnitude here or there Smile.

I designed the controller for a big FTMS.

https://www.dropbox.com/s/1pvax542n1erhfz/IMG_0304.JPG?dl=0

https://www.dropbox.com/s/79dqhigfza4hnda/IMG_9667.JPG?dl=0

https://www.dropbox.com/s/rqkojfxw8svc4mx/IMG_9697.JPG?dl=0

https://www.dropbox.com/s/k9tny9da3sikgcw/IMG_9717.JPG?dl=0

https://www.dropbox.com/s/0e71x4r2a72y86m/FTMS_Cell_top.jpg?dl=0


The preamp was designed by some chemists,

https://www.dropbox.com/s/sm2x1az6193y5fx/FTMS_Preamp.jpg?dl=0

and they gave up 30 dB of s/n or so. It was ghastly.

I thought it might be possible to detect a single ion in orbit, but we
never got to try.

Varian acquired Ion Spec, then Agilent acquired Varian, then Agilent
kill off the NMR and FTMS product lines.

I have some planned preamp schematics around here somewhere.
I didn't look at all the pics.

I've been 1/2 thinking about this.
For a single coil system, I'm thinking I need some sort
of switching between the two circuits, flip the spins and
then see the signal. Which is doable, but another thing...
A dual cross coil system has some advantages.
Flip with one, sense with other.
The two functions are naturally apart.
That makes it easier to test when something is not working.
(which if you are contemplating some marginally functional kit
is very important.)


The down side is inter-penetrating coils are hard.
so the flip coil is often put outside the other,
and thus has to be bigger...

You could make some water sample that flowed through
two coils. That would be fun... but more complicated.

If you are thinking of two coaxial coils, I think
it's a bad idea... there's all this 'transformer/ flux'
pickup in the sense coil*. You still have some of that in
cross coils, and back to back diodes across the input will
limit that.

*manufacture-wise two coaxial coils right on top of each other
is dirt simple. Maybe I just need the right protection diodes
across the sense input?

GH

Quote:

George H.







--

John Larkin Highland Technology, Inc

lunatic fringe electronics


George Herold
Guest

Tue Feb 12, 2019 1:45 am   



On Sunday, February 10, 2019 at 12:22:13 PM UTC-5, bloggs.fred...@gmail.com wrote:
Quote:
On Thursday, February 7, 2019 at 9:33:41 AM UTC-5, George Herold wrote:
Hi all, A (mostly clueless) physics type wants to do some RF stuff. (measuring nmr signals.. protons, ~5-25 MHz range) He's got something
working, but construction is ugly. I was thinking of recommending an ARRL handbook, is any version better than others?

This is pretty good:
http://home.sandiego.edu/~ekim/otherjunk/rf_proto.pdf


Saved, thanks.
I will not disparage your lack of electronic posts/content for X-months.

Got any other good stuff/ links?
George H.

George Herold
Guest

Tue Feb 12, 2019 1:45 am   



On Sunday, February 10, 2019 at 10:52:28 AM UTC-5, John Larkin wrote:
Quote:
On Sat, 9 Feb 2019 00:14:24 -0500, "Carl"
carl.ijamesXYZ_at_ZYXverizon.net> wrote:

On 9/2/19 12:55 pm, George Herold wrote:
On Friday, February 8, 2019 at 7:13:52 PM UTC-5, John Larkin wrote:
On Fri, 8 Feb 2019 06:05:42 -0800 (PST), George Herold
gherold_at_teachspin.com> wrote:

On Thursday, February 7, 2019 at 8:12:05 PM UTC-5, Jeff Liebermann
wrote:
On Thu, 7 Feb 2019 06:33:34 -0800 (PST), George Herold
gherold_at_teachspin.com> wrote:

Hi all, A (mostly clueless) physics type wants to do
some RF stuff. (measuring nmr signals.. protons,
~5-25 MHz range) He's got something working, but
construction is ugly.

The uglier the antenna, construction, or layout, the better it works.
https://hackadaycom.files.wordpress.com/2016/05/williams-workbench.jpg
http://hephaestusaudio.com/media/2009/05/bob-pease-breadboard.jpg

I was thinking of recommending an ARRL handbook, is
any version better than others?

I don't know. I have some old ARRL Handbook issues at home that I
rarely read or use.

5-25 MHz is practically DC as far as breadboarding is concerned.

Right... but of concern is not just bread boarding, but in nmr you
typically have a coil down the end of a probe/ transmission line and
there is some thought needed to get good coupling into the coil.

There's usually a tuning network and a low-noise (sometimes cryo, a
"chilly probe") preamp in a box just outside the main field. Some sort
of t/r switching, too, since there's only one sample coil for transmit
and receive.

So we've done things a few different ways. (Our latest nmr is all
done by Norman, and I'm mostly clueless on the details.)
Tuning near the coil is what we do now. But the first one
had a coil on a fixed length of coax, (less than 1/4 wavelength)
and a tuned series inductor for the transmitter tuning.
(I'm not sure how the input was tuned, parallel C I assume.
Norman did the hard parts here too. :^) You've got to have
some Q on the input.

So my boss asked today if we could make a cheap kit/ gizmo
that would let people 'see' proton nmr, pulse a coil,
look at the ring down. Insert water sample into coil,
see ring down change.

So (to have fun) the first question is how big is the
sample/magnetic field? At ~2.5kHz you can see nmr in
a beer can size of water in the Earth's field ~0.6 Gauss

In a permanent magnet ~0.5 Tesla (5 k Gauss) you
can see a signal from .. well a few good sized
drops of water. at ~20 MHz.

(proton gyromagnetic ratio is ~42 MHz/ T
4.2 kHz/ Gauss.)

I've got some 8" Helmholtz coils that do
~100 Gauss at 3A. But lotsa turns and spendy.

Some permanent magnets and a yoke? The field
homogeneity of little permanent magnetics is
going to suck. Field homogeneity is either
less volume or less Q, T2 broadening in the
nmr lingo.

Two disc magnets spaced a distance of half the diameter (just like a
Helmholtz coil) gives the best homogeneity at the center of the gap.
Assuming perfect uniformity of the magnets themselves, at a guess a pair of
3" discs would give a volume with a homogeneity of maybe 0.1 to 1 part in
1000 over 1 cubic centimeter volume at a field of maybe 0.2-0.4 T depending
on the NdFeB grade. If you don't already have it grab a copy of FEMM and
run some simulations. Not great compared to 1 part in 10^8 or 9 like a high
resolution supercon NMR, but maybe enough to give a recognizable signal for
under $500 for the magnet. It's been a couple of years since I did a fair
number of sims with larger discs, playing around to see what useable volume
I could get with 6" discs for a tiny FTMS so I could be off an order of
magnitude here or there Smile.

I designed the controller for a big FTMS.

https://www.dropbox.com/s/1pvax542n1erhfz/IMG_0304.JPG?dl=0

https://www.dropbox.com/s/79dqhigfza4hnda/IMG_9667.JPG?dl=0

https://www.dropbox.com/s/rqkojfxw8svc4mx/IMG_9697.JPG?dl=0

https://www.dropbox.com/s/k9tny9da3sikgcw/IMG_9717.JPG?dl=0

https://www.dropbox.com/s/0e71x4r2a72y86m/FTMS_Cell_top.jpg?dl=0


The preamp was designed by some chemists,

https://www.dropbox.com/s/sm2x1az6193y5fx/FTMS_Preamp.jpg?dl=0

and they gave up 30 dB of s/n or so. It was ghastly.

I thought it might be possible to detect a single ion in orbit, but we
never got to try.

Varian acquired Ion Spec, then Agilent acquired Varian, then Agilent
kill off the NMR and FTMS product lines.

I have some planned preamp schematics around here somewhere.
I didn't look at all the pics.


I've been 1/2 thinking about this.
For a single coil system, I'm thinking I need some sort
of switching between the two circuits, flip the spins and
then see the signal. Which is doable, but another thing...
A dual cross coil system has some advantages.
Flip with one, sense with other.
The two functions are naturally apart.
That makes it easier to test when something is not working.
(which if you are contemplating some marginally functional kit
is very important.)

George H.


Quote:





--

John Larkin Highland Technology, Inc

lunatic fringe electronics


George Herold
Guest

Tue Feb 12, 2019 2:45 am   



On Monday, February 11, 2019 at 7:27:51 PM UTC-5, Lasse Langwadt Christensen wrote:
Quote:
tirsdag den 12. februar 2019 kl. 01.02.02 UTC+1 skrev George Herold:
On Sunday, February 10, 2019 at 10:52:28 AM UTC-5, John Larkin wrote:
On Sat, 9 Feb 2019 00:14:24 -0500, "Carl"
carl.ijamesXYZ_at_ZYXverizon.net> wrote:

On 9/2/19 12:55 pm, George Herold wrote:
On Friday, February 8, 2019 at 7:13:52 PM UTC-5, John Larkin wrote:
On Fri, 8 Feb 2019 06:05:42 -0800 (PST), George Herold
gherold_at_teachspin.com> wrote:

On Thursday, February 7, 2019 at 8:12:05 PM UTC-5, Jeff Liebermann
wrote:
On Thu, 7 Feb 2019 06:33:34 -0800 (PST), George Herold
gherold_at_teachspin.com> wrote:

Hi all, A (mostly clueless) physics type wants to do
some RF stuff. (measuring nmr signals.. protons,
~5-25 MHz range) He's got something working, but
construction is ugly.

The uglier the antenna, construction, or layout, the better it works.
https://hackadaycom.files.wordpress.com/2016/05/williams-workbench.jpg
http://hephaestusaudio.com/media/2009/05/bob-pease-breadboard.jpg

I was thinking of recommending an ARRL handbook, is
any version better than others?

I don't know. I have some old ARRL Handbook issues at home that I
rarely read or use.

5-25 MHz is practically DC as far as breadboarding is concerned.

Right... but of concern is not just bread boarding, but in nmr you
typically have a coil down the end of a probe/ transmission line and
there is some thought needed to get good coupling into the coil.

There's usually a tuning network and a low-noise (sometimes cryo, a
"chilly probe") preamp in a box just outside the main field. Some sort
of t/r switching, too, since there's only one sample coil for transmit
and receive.

So we've done things a few different ways. (Our latest nmr is all
done by Norman, and I'm mostly clueless on the details.)
Tuning near the coil is what we do now. But the first one
had a coil on a fixed length of coax, (less than 1/4 wavelength)
and a tuned series inductor for the transmitter tuning.
(I'm not sure how the input was tuned, parallel C I assume.
Norman did the hard parts here too. :^) You've got to have
some Q on the input.

So my boss asked today if we could make a cheap kit/ gizmo
that would let people 'see' proton nmr, pulse a coil,
look at the ring down. Insert water sample into coil,
see ring down change.

So (to have fun) the first question is how big is the
sample/magnetic field? At ~2.5kHz you can see nmr in
a beer can size of water in the Earth's field ~0.6 Gauss

In a permanent magnet ~0.5 Tesla (5 k Gauss) you
can see a signal from .. well a few good sized
drops of water. at ~20 MHz.

(proton gyromagnetic ratio is ~42 MHz/ T
4.2 kHz/ Gauss.)

I've got some 8" Helmholtz coils that do
~100 Gauss at 3A. But lotsa turns and spendy.

Some permanent magnets and a yoke? The field
homogeneity of little permanent magnetics is
going to suck. Field homogeneity is either
less volume or less Q, T2 broadening in the
nmr lingo.

Two disc magnets spaced a distance of half the diameter (just like a
Helmholtz coil) gives the best homogeneity at the center of the gap.
Assuming perfect uniformity of the magnets themselves, at a guess a pair of
3" discs would give a volume with a homogeneity of maybe 0.1 to 1 part in
1000 over 1 cubic centimeter volume at a field of maybe 0.2-0.4 T depending
on the NdFeB grade. If you don't already have it grab a copy of FEMM and
run some simulations. Not great compared to 1 part in 10^8 or 9 like a high
resolution supercon NMR, but maybe enough to give a recognizable signal for
under $500 for the magnet. It's been a couple of years since I did a fair
number of sims with larger discs, playing around to see what useable volume
I could get with 6" discs for a tiny FTMS so I could be off an order of
magnitude here or there Smile.

I designed the controller for a big FTMS.

https://www.dropbox.com/s/1pvax542n1erhfz/IMG_0304.JPG?dl=0

https://www.dropbox.com/s/79dqhigfza4hnda/IMG_9667.JPG?dl=0

https://www.dropbox.com/s/rqkojfxw8svc4mx/IMG_9697.JPG?dl=0

https://www.dropbox.com/s/k9tny9da3sikgcw/IMG_9717.JPG?dl=0

https://www.dropbox.com/s/0e71x4r2a72y86m/FTMS_Cell_top.jpg?dl=0


The preamp was designed by some chemists,

https://www.dropbox.com/s/sm2x1az6193y5fx/FTMS_Preamp.jpg?dl=0

and they gave up 30 dB of s/n or so. It was ghastly.

I thought it might be possible to detect a single ion in orbit, but we
never got to try.

Varian acquired Ion Spec, then Agilent acquired Varian, then Agilent
kill off the NMR and FTMS product lines.

I have some planned preamp schematics around here somewhere.
I didn't look at all the pics.

I've been 1/2 thinking about this.
For a single coil system, I'm thinking I need some sort
of switching between the two circuits, flip the spins and
then see the signal. Which is doable, but another thing...
A dual cross coil system has some advantages.
Flip with one, sense with other.
The two functions are naturally apart.
That makes it easier to test when something is not working.
(which if you are contemplating some marginally functional kit
is very important.)


something like this works ok,

https://images.slideplayer.com/27/8912193/slides/slide_14.jpg


Awesome. I think that might be what Norman did on our latest nmr thing.
Norman was part of W-map at Princeton. Smart guy!

The current thing has two caps to adjust, ones called coupling and the
other tuning. Tuning it up is a skill, takes time...
and it's not made better by crappy tuning caps.

George H.




Quote:

tweak Cmatch and Ctune for resonance and 50R match using a VNA

you can do the quarter-wave with a length of coax or a CLC pi section
for moderate powers 1n4148 diodes will work


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