On Tuesday, February 12, 2019 at 10:20:39 AM UTC-5, Phil Hobbs wrote:
On 2/11/19 8:05 PM, George Herold wrote:
On Monday, February 11, 2019 at 7:52:15 PM UTC-5, Lasse Langwadt Christensen wrote:
tirsdag den 12. februar 2019 kl. 01.30.35 UTC+1 skrev George Herold:
On Monday, February 11, 2019 at 7:02:02 PM UTC-5, George Herold wrote:
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"
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
On Thursday, February 7, 2019 at 8:12:05 PM UTC-5, Jeff Liebermann
On Thu, 7 Feb 2019 06:33:34 -0800 (PST), George Herold
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.
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
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
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
I designed the controller for a big FTMS.
The preamp was designed by some chemists,
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.
I think it would need to flow very slow because of waters long T1?
Oh a flowing thing would be all fun... how does the
magnetization change if you send the flow around a corner?
180 and back through some coil?
(The answer will depend on flow velocity, radius of bend,
and resonant freq. I think.)
I hadn't really thought about a flowing thing much..
but right the signal can only last as long as the
length divided by the flow velocity.
If you sent it along a helix, you might get topological phase too.
Works for photons anyway.
Not sure what topological phase is.
Spins will tend to 'follow' the field, as long as the
field doesn't change too fast*. 'too fast' is ~ set by
the resonance/ precession freq. The problem with
my bending tube thing, is that unless the field is
perfectly homogeneous, the spins all go at different
rates. T2 time- dephasing. And if I move the spins to some new
spot... well I can't do the echo things.. that rephases
the spins when they stay in the same field.
I guess you could move the water a little and measure the
*The first optical pumping (wow, my vote for
worst wiki article)
was seen by flipping the spins in a 60 Hz field.
If you try and reproduce it, you'll find that
you need some perpendicular component to the flipped
field, else the spins get stuck at zero field.
Playing around near zero is fun... there's a news letter
I've got less hair now and much more grey. :^)
You and me both. ;)
non-planar path. Polarization shift in a system with mirrors is the