Design, Care and Feeding of NMR Probes: A Tutorial 2011 Experimental NMR Conference, Asilomar CA.
-
Upload
spencer-fowle -
Category
Documents
-
view
223 -
download
1
Transcript of Design, Care and Feeding of NMR Probes: A Tutorial 2011 Experimental NMR Conference, Asilomar CA.
![Page 1: Design, Care and Feeding of NMR Probes: A Tutorial 2011 Experimental NMR Conference, Asilomar CA.](https://reader036.fdocuments.us/reader036/viewer/2022062417/5518a38c550346a61f8b4982/html5/thumbnails/1.jpg)
Design, Care and Feeding of NMR Probes: A Tutorial
2011 Experimental NMR Conference, Asilomar CA
![Page 2: Design, Care and Feeding of NMR Probes: A Tutorial 2011 Experimental NMR Conference, Asilomar CA.](https://reader036.fdocuments.us/reader036/viewer/2022062417/5518a38c550346a61f8b4982/html5/thumbnails/2.jpg)
sources of inspiration....
Don AldermanMark ConradiDavid HoultBob McKayEichii FukuskimaDavid DotyToby ZensFrank EngelkeAllen PalmerJohn Stringer....
and many, many, many others too numerous to mention ....
Don Alderman
Mark ConradiDavid Hoult
Bob McKayEichii Fukushima
David Doty
Toby Zens
![Page 3: Design, Care and Feeding of NMR Probes: A Tutorial 2011 Experimental NMR Conference, Asilomar CA.](https://reader036.fdocuments.us/reader036/viewer/2022062417/5518a38c550346a61f8b4982/html5/thumbnails/3.jpg)
Design, Care and Feeding of NMR Probes: A Tutorial
Part I
What is a tank circuit?
Impedance matchingtransmission linesthe Smith chartexperimental techniques
Circuit resonanceQ and voltage risearcing and avoiding arcing
Part II
Signal to noiseRf efficiencycomparing specifications
Circuit balancing and multiple resonance circuits
Finite element field simulations
Tuning tube and transmission line probes
![Page 4: Design, Care and Feeding of NMR Probes: A Tutorial 2011 Experimental NMR Conference, Asilomar CA.](https://reader036.fdocuments.us/reader036/viewer/2022062417/5518a38c550346a61f8b4982/html5/thumbnails/4.jpg)
Disclaimer: semi-professional driver on a closed track. Do not attempt at home. The "management" of ENC is not responsible for damages as a result. Opinions of the speaker do not reflect those of the ENC unless you happen to like them, and in that instance ENC takes full credit..
"it is easier to repent than it is to get permission" - D. M. Grant
![Page 5: Design, Care and Feeding of NMR Probes: A Tutorial 2011 Experimental NMR Conference, Asilomar CA.](https://reader036.fdocuments.us/reader036/viewer/2022062417/5518a38c550346a61f8b4982/html5/thumbnails/5.jpg)
NMR probe basics
An NMR probe is a tank circuit used to excite NMR signals and detect them by Faraday induction
CT L
CM
Z O
resonator
RF input
impedance matching network
+-
v ( t )
L
signal emf = -
s
1V
d(B M(t))dV
dt
![Page 6: Design, Care and Feeding of NMR Probes: A Tutorial 2011 Experimental NMR Conference, Asilomar CA.](https://reader036.fdocuments.us/reader036/viewer/2022062417/5518a38c550346a61f8b4982/html5/thumbnails/6.jpg)
Solenoidal coil
B1(t)
Bo
Saddle coil
slotted tube
scrollcoil
Zensresonator
birdcage
all resonators are inductive
elements
Resonators...type usually dictated by sample geometry and frequency
N
S
![Page 7: Design, Care and Feeding of NMR Probes: A Tutorial 2011 Experimental NMR Conference, Asilomar CA.](https://reader036.fdocuments.us/reader036/viewer/2022062417/5518a38c550346a61f8b4982/html5/thumbnails/7.jpg)
transmission lineRF
inputresonator
primitive probe
very inefficient because of impedance mismatch
RF signals have wave character and require phase matching
![Page 8: Design, Care and Feeding of NMR Probes: A Tutorial 2011 Experimental NMR Conference, Asilomar CA.](https://reader036.fdocuments.us/reader036/viewer/2022062417/5518a38c550346a61f8b4982/html5/thumbnails/8.jpg)
index of refraction n1
index of refraction n2
for p-polarized light the transmission coefficient depends on n1, n2 and the incident angle θ.
Impedance matching of light waves
R = 0 and T = 1 when
1 1
B2
nn
-θ tan
n1 n2
![Page 9: Design, Care and Feeding of NMR Probes: A Tutorial 2011 Experimental NMR Conference, Asilomar CA.](https://reader036.fdocuments.us/reader036/viewer/2022062417/5518a38c550346a61f8b4982/html5/thumbnails/9.jpg)
DC electronics basics – Ohm's Law
V
V = IR
V
dVI =C
dt dIV =L
dt
RF electronics – Ohm's law works using Impedance
V(t) = Vocos(ωt + φ) or V(t) = Re(ej(ωt +φ))
VR C L
![Page 10: Design, Care and Feeding of NMR Probes: A Tutorial 2011 Experimental NMR Conference, Asilomar CA.](https://reader036.fdocuments.us/reader036/viewer/2022062417/5518a38c550346a61f8b4982/html5/thumbnails/10.jpg)
RF electronics basics – Impedance and Reactance
V
V = IZR
V
from V(t) = ej(ωt +φ)
V = IZC
V = IZL
ZR = R
Z = R + jX
Define a complex
impedance Z
ZC = 1/jωC
ZL = jωL
X = Reactance
VR C L
j = 1
![Page 11: Design, Care and Feeding of NMR Probes: A Tutorial 2011 Experimental NMR Conference, Asilomar CA.](https://reader036.fdocuments.us/reader036/viewer/2022062417/5518a38c550346a61f8b4982/html5/thumbnails/11.jpg)
Transmission lines and characteristic impedance
Can't just connect a load to a source with a wire – it will radiate the power
Dipole antenna radiates power from transmitter into space
To transmit RF to a load need a structure to contain the EM waves
![Page 12: Design, Care and Feeding of NMR Probes: A Tutorial 2011 Experimental NMR Conference, Asilomar CA.](https://reader036.fdocuments.us/reader036/viewer/2022062417/5518a38c550346a61f8b4982/html5/thumbnails/12.jpg)
Transmission lines and characteristic impedance
twin-line
coaxial line
characteristic impedance Zo is the equivalent of a refractive index
L o
in oo L
Z jZ tan(βl)Z (l) Z
Z jZ tan(βl)
length l
2πβ
λ
μη
ε for dielectric
o
ηZ ln(b/ a)
2π
b
do
ηZ ln(d/ a)
2π
Minimum attenuation cable: if εR = 1.0, Zo = 75Ω; εR = 2.0, Zo = 51Ω
![Page 13: Design, Care and Feeding of NMR Probes: A Tutorial 2011 Experimental NMR Conference, Asilomar CA.](https://reader036.fdocuments.us/reader036/viewer/2022062417/5518a38c550346a61f8b4982/html5/thumbnails/13.jpg)
ZL = 50 Ω
refl ected L s
L sincident
V Z ZΓ
V Z Zreflection coefficient
VincVinc
Vinc Vinc
VrefVref
Vref out of phase
180o for short
Vref in phase
for open
ZL = 25 Ω
if ZS = Zo = 50 Ω
VSWR = Vmax/Vmin
= (1 + |ΓL|)/(1 − |ΓL|)
VSWR = 1
VSWR = ∞ VSWR = ∞
VSWR = 2
![Page 14: Design, Care and Feeding of NMR Probes: A Tutorial 2011 Experimental NMR Conference, Asilomar CA.](https://reader036.fdocuments.us/reader036/viewer/2022062417/5518a38c550346a61f8b4982/html5/thumbnails/14.jpg)
Impedance matching the coil
in o m
T
1Z Z jωC R' jX
1jωL R
jωC
CT L
CM
Z O
R
set X = 0 to findallowed ω
there will be 2 !
Then choose CT/CM
to render R' = Zo
resonant frequency ω ~ T1/ LC
![Page 15: Design, Care and Feeding of NMR Probes: A Tutorial 2011 Experimental NMR Conference, Asilomar CA.](https://reader036.fdocuments.us/reader036/viewer/2022062417/5518a38c550346a61f8b4982/html5/thumbnails/15.jpg)
easier to use a Smith chart
VSWR = 0
50Ω
200Ω
L
C
load 200Ω
Example: match a 200Ω load
![Page 16: Design, Care and Feeding of NMR Probes: A Tutorial 2011 Experimental NMR Conference, Asilomar CA.](https://reader036.fdocuments.us/reader036/viewer/2022062417/5518a38c550346a61f8b4982/html5/thumbnails/16.jpg)
Measuring
Spectrum analyzer + tracking generator
RF outputRF receiver
directional coupler
Forward wave
DUT
reflected wave
50Ω
frequency
matched
Γ
Γ
0
1
![Page 17: Design, Care and Feeding of NMR Probes: A Tutorial 2011 Experimental NMR Conference, Asilomar CA.](https://reader036.fdocuments.us/reader036/viewer/2022062417/5518a38c550346a61f8b4982/html5/thumbnails/17.jpg)
Resonator inductance
for solenoid length , diameter d, n turns
22 d
L 250n (nH)4.5d 10
usually more practical to measure L
dimensions in inches
![Page 18: Design, Care and Feeding of NMR Probes: A Tutorial 2011 Experimental NMR Conference, Asilomar CA.](https://reader036.fdocuments.us/reader036/viewer/2022062417/5518a38c550346a61f8b4982/html5/thumbnails/18.jpg)
Resonant RLC circuits
when ω2 = 1/LC the circuit is "resonant"
RF energy at ω is stored alternately in C and L
Circuit can be excited by an antenna or other
source
C
L
R
![Page 19: Design, Care and Feeding of NMR Probes: A Tutorial 2011 Experimental NMR Conference, Asilomar CA.](https://reader036.fdocuments.us/reader036/viewer/2022062417/5518a38c550346a61f8b4982/html5/thumbnails/19.jpg)
Zo
Zo
Zo
Using resonance to span a break in a transmission line circuit
![Page 20: Design, Care and Feeding of NMR Probes: A Tutorial 2011 Experimental NMR Conference, Asilomar CA.](https://reader036.fdocuments.us/reader036/viewer/2022062417/5518a38c550346a61f8b4982/html5/thumbnails/20.jpg)
Zo
Zo
Zo
Using resonance to span a break in a circuit
C
L
2
1plot vs. C
ω
![Page 21: Design, Care and Feeding of NMR Probes: A Tutorial 2011 Experimental NMR Conference, Asilomar CA.](https://reader036.fdocuments.us/reader036/viewer/2022062417/5518a38c550346a61f8b4982/html5/thumbnails/21.jpg)
Impedance matching the coil
in o m
T
1Z Z jωC R' jX
1iωL R
jωC
CT L
CM
Z O
R
set X = 0 to findallowed ω
there will be 2 !
Then choose CT/CM
to render R' = Zo
resonant frequency ω ~ 1/ LC
![Page 22: Design, Care and Feeding of NMR Probes: A Tutorial 2011 Experimental NMR Conference, Asilomar CA.](https://reader036.fdocuments.us/reader036/viewer/2022062417/5518a38c550346a61f8b4982/html5/thumbnails/22.jpg)
Measuring
Spectrum analyzer + tracking generator
RF outputRF receiver
directional coupler
Forward wave
DUT
reflected wave
50Ω
frequency
Γ
![Page 23: Design, Care and Feeding of NMR Probes: A Tutorial 2011 Experimental NMR Conference, Asilomar CA.](https://reader036.fdocuments.us/reader036/viewer/2022062417/5518a38c550346a61f8b4982/html5/thumbnails/23.jpg)
Voltage at circuit resonance
C LV in
V H R
in
1Z = +jωL+R
jωC
j = - +jωL+R
ωC
when ω2 = 1/LC Zin = R and |XC| = |XL| I = Vin/Zin = Vin/R
What is the voltage VH – Vin across C?
L inc in inH in c in
| Z | V| Z | V ωLVV V | Z | I QV
R R R
![Page 24: Design, Care and Feeding of NMR Probes: A Tutorial 2011 Experimental NMR Conference, Asilomar CA.](https://reader036.fdocuments.us/reader036/viewer/2022062417/5518a38c550346a61f8b4982/html5/thumbnails/24.jpg)
Coil quality factor Q =ωL/R = E stored/E dissipated per cycle = ω/3dB bandwidth
typical Q ~ 100 or more
Input pulse power
2 2 22
ptp ptpRMSptp
o o
(V / 2 2) VV100 Watts V 200V
Z Z 400
Vptp
Peak V across C ~ 10,000 V !!!
![Page 25: Design, Care and Feeding of NMR Probes: A Tutorial 2011 Experimental NMR Conference, Asilomar CA.](https://reader036.fdocuments.us/reader036/viewer/2022062417/5518a38c550346a61f8b4982/html5/thumbnails/25.jpg)
Conditions for arcing through air
![Page 26: Design, Care and Feeding of NMR Probes: A Tutorial 2011 Experimental NMR Conference, Asilomar CA.](https://reader036.fdocuments.us/reader036/viewer/2022062417/5518a38c550346a61f8b4982/html5/thumbnails/26.jpg)
Non-magnetic fixed and variable capacitors
20 kV
10 kV
500V
2.5 kV
2.5 kV
Vacuum variable
Teflon variable
Sapphire trimmer
RF transmittingceramic chips
![Page 27: Design, Care and Feeding of NMR Probes: A Tutorial 2011 Experimental NMR Conference, Asilomar CA.](https://reader036.fdocuments.us/reader036/viewer/2022062417/5518a38c550346a61f8b4982/html5/thumbnails/27.jpg)
Probe testing under power
Power amplifier
Directional coupler output
forward reflected
![Page 28: Design, Care and Feeding of NMR Probes: A Tutorial 2011 Experimental NMR Conference, Asilomar CA.](https://reader036.fdocuments.us/reader036/viewer/2022062417/5518a38c550346a61f8b4982/html5/thumbnails/28.jpg)
Probe testing under power
Power amplifier
Directional coupler output
forward
arcing
reflected
![Page 29: Design, Care and Feeding of NMR Probes: A Tutorial 2011 Experimental NMR Conference, Asilomar CA.](https://reader036.fdocuments.us/reader036/viewer/2022062417/5518a38c550346a61f8b4982/html5/thumbnails/29.jpg)
Arc testing
•tune and match at low power
•increase drive 1 dB at a time while watching reflected power
•operate 2 dB lower than initial arc
•avoid hard sustained arcing – carbonized capacitors do not heal
•don't try and rematch to compensate for arcing
![Page 30: Design, Care and Feeding of NMR Probes: A Tutorial 2011 Experimental NMR Conference, Asilomar CA.](https://reader036.fdocuments.us/reader036/viewer/2022062417/5518a38c550346a61f8b4982/html5/thumbnails/30.jpg)
Always make sure you have air flow into the probe
![Page 31: Design, Care and Feeding of NMR Probes: A Tutorial 2011 Experimental NMR Conference, Asilomar CA.](https://reader036.fdocuments.us/reader036/viewer/2022062417/5518a38c550346a61f8b4982/html5/thumbnails/31.jpg)
Dewar's frequently provide sources of arcing
If the probe only arcs in the magnet suspect the dewar
![Page 32: Design, Care and Feeding of NMR Probes: A Tutorial 2011 Experimental NMR Conference, Asilomar CA.](https://reader036.fdocuments.us/reader036/viewer/2022062417/5518a38c550346a61f8b4982/html5/thumbnails/32.jpg)
Optimizing RF efficiency and S/N
signal emf = -
s
1V
d(B M(t))dV
dt
1B= B1 from unit current
"principle of reciprocity"
Excitation and detection are equally efficient
o 1xy o s oemf ωB M V (cosω t)
2 2o o sM Nγ I (I 1)B / 3kT
2o
1
1
emf B
emf B / unit current
B / power
Part II.......maybe for next year
![Page 33: Design, Care and Feeding of NMR Probes: A Tutorial 2011 Experimental NMR Conference, Asilomar CA.](https://reader036.fdocuments.us/reader036/viewer/2022062417/5518a38c550346a61f8b4982/html5/thumbnails/33.jpg)
Acknowledgements
For inspiration and graphics
A. AbragamD. VanderHartM. ConradiT. and H. BarbaraT. ZensD. AldermanB. MckayA. PalmerJ. StringerF. EngelkeG. FaceyJ. DanielsE. FukushimaD. Doty
L. PageS. BrinS. WolframJ. WalesJ. HornakR. Schurko
For support
NSFExxonMobilAgilentYale University
all of my present and former students
![Page 34: Design, Care and Feeding of NMR Probes: A Tutorial 2011 Experimental NMR Conference, Asilomar CA.](https://reader036.fdocuments.us/reader036/viewer/2022062417/5518a38c550346a61f8b4982/html5/thumbnails/34.jpg)
Disclaimer: semi-professional driver on a closed track. Do not attempt at home. The "management" of ENC is not responsible for damages as a result. Opinions of the speaker do not reflect those of the ENC unless you happen to like them, and in that instance ENC takes full credit..
"your students only learn your worst habits" - C. P. Slichter
![Page 35: Design, Care and Feeding of NMR Probes: A Tutorial 2011 Experimental NMR Conference, Asilomar CA.](https://reader036.fdocuments.us/reader036/viewer/2022062417/5518a38c550346a61f8b4982/html5/thumbnails/35.jpg)