1. NMR Hardware - Global NMR Discussion Meeting
14 September, 2021
Mark S. Conradi
ABQMR, Inc.
Albuquerque, NM 87108 USA
msc@wuphys.wustl.edu
ABQMR – contract NMR research and specialty equipment, to the
government and industry. Lately, MRI of sorghum plant roots
undisturbed, in the ground (and greenhouse).
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2. Target Audience
Solid state NMR scientists
< mid-career
Now in research group with technically-
knowledgable people
Thinking “what do I do when I am my own
assistant professor?”
Curious “how does it work?”
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3. Today’s topics
Diagnosis of
Spectrometer
Probe
Test gear – scopes, signal generators …
Build a simple probe
Most of us will not build our own spectrometer. But
we will need to de-bug it when there is
No More Resonance. So:
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5. No signal – Is it Tx? Rcvr? Probe?
If it does not pulse and run, suspect computer control.
Check that magnet is ON, with a steel tool.
Transmitter – look for rf pulse with an oscilloscope.
Receiver – check that Rcvr noise looks normal. Turn off Tx power amp,
inject signal at resonance freq where probe normally connects. Use
1 mWt level and 60 dB atten. Look for signal.
Check triggering of scope
5
6. Looking at the High Power Tx
Usual Attenuators (½ watt, 20, 30, 40 dB) are OK.
Except at first position. Use attenuator built for high peak power.
20 dB atten if each R is
½ watt, 120 ohm,
carbon composition
(Stackpole, from
DigiKey), in small alum
box.
►If you look directly at rf, need rf scope ($$$).
►Or, insert a diode detector in front of scope (at “here”), so you see
pulse envelope.
►Can also look at input to Tx power amp (level is ≅ 1 mW, no
attenuators needed). 6
7. Testing the Receiver
NMR receivers can be narrow-band (1-100 kHz
wide).
Get signal generator onto desired frequency
1 mW = 0 dBm = 0.22 Vrms = 0.63 VP-P
You should be able to “see” 1 µV rms (= -107 dB
from 1 mW)
Can check “after” T-R switch.
Can check “after” preamp.
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8. What is dB?
dB Power Ratio Volt Ratio
0 1 1
3 2 1.41
6 4 2
10 10 3.2
20 100 10
40 10,000 100
60 1M 1000
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9. Testing the Probe
Check tuning with tune-up bridge. Maybe coil is open?
This is low power.
If you have in-line forward / reflected power meter, what
does it say? A big reflection suggests bad arcing
(breakdown) or other.
Put in big signal sample (water, D2O, 13C-labelled ethylene
glycol). Even if probe arcs, will see a signal.
Listen (ear) for breakdown (snap, snap, …). Use dark
room and look (eye) for blue light. Go to 200 µs pulse,
every 0.1 sec.
Note – probe will de-tune if you remove cover. Need to
re-tune. 9
10. Replace Probe with a Crystal
Turn power down to 1 watt and look for “FID” from crystal. No
echoes! See Sam Emery and MSC, JMR 242, 1-3 (2014).
Again, crystals are narrow-band but are usually within 500 ppm of the
frequency stamped on the case.
OR build a simple test probe. Use it to check Tx (is 90o length
changed?) and Rcvr (is received FID same as it was when everything
was working?)
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12. What and Where to Buy – Test Equipment
Borrow what you can
Try eBay or equivalent. Remember, you will only use occasionally.
Oscilloscopes
If you need to see 500 MHz, ≥ $10,000
If you only need to see diode detected envelope, ≤$600.
Tektronix, Rhode-Schwarz, Rigol, …
Diode detectors, SMA connecters = zero-bias Schottky,
Pasternack PE8012 $550
Fairview Microwave SMD 0102 $550
Digikey CPDETLS-4000 $34
Careful, diode detectors need peak inputs of around 10 mW, = +10 dBm
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15. Signal generators. HP, Tektronix, Agilent, Keysight, Rhode-Schwarz …
Try eBay
Most VNA (vector network analyzers) can be put into CW signal generator
mode. My $60 VNA-H4 is CW if you reduce SPAN to 1K (1 kHz) or less.
VNA Can be pricy
Or, try VNWA 3 from SDRKits.com $500. Needs a computer (Windows) to
drive it.
Or, Nano VNA (VNA-H4) from AURSINCO $60 from Amazon
Take the VNA to an rf-person to set yours up! Keep the NanoVNA flow
chart to help you change it later.
VNWA 3 uses computer keyboard, needs computer.
NanoVNA is battery-operated, hand held, but has awkward touch screen
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19. Attenuators, Filters
Mini Circuits Laboratories
JFW
Crystek, sold by DigiKey
Cinch, sold by DigiKey
Bird
EME Precisions 7020/30 A, needs diode detectors
and scope
In-line Directional Wattmeters
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21. Old Man Advice: Try it first
(when it is all working)!!
Don’t wait until you need it to get used to your
oscilloscope, VNA, signal generator, …
“Why can’t I get this ___ ___ scope to trigger?”
Always use external trigger
Write it All Down
As you disconnect cables, change settings, lengthen rf
pulses, make a record so you can get back to start
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22. Build a Simple Probe – not rocket science
The only way to learn is to do.
Keep it simple (not triple resonance)
Read Wheeler and Conradi, Concepts in Magnetic
Resonance, part A, 40A, pp1-13 (2012).
I can send you a copy.
You need a shielding aluminum enclosure, a box or old
probe body.
Figure out the mechanical aspects first.
Find a small permanent magnet (5 cm3) to look at all
parts for magnetism.
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23. A Suitable Coil, Resonating it.
Resonance: ω2LC = 1, ω = 2πf
Estimate solenoidℒ =
𝑛2𝑟2
9𝑟+10𝑙
ℒ in μH, r and l in inches (sorry).
Typical ℒ has ZL = ωℒ = 50-100 Ω
So C obeys 𝑍𝐶 =
1
𝜔𝐶
= 𝑍𝐿, too.
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24. Finding L-C Resonance Frequency
Adjust C to desired freq.
Kong – good way to
bench-tune a trap for a
new freq, in double or
triple-resonance probe
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25. What is Tuning and Matching for?
Typically 𝑍𝑐𝑜𝑖𝑙 = 𝑖100 + 1
So 𝑄 =
𝜔𝐿
𝑟
= 100
𝑍𝑐𝑜𝑖𝑙 = 𝑖𝜔𝐿 + 𝑟
* You want to transform this Zcoil to i0+50 (50 ohms
resistive).
So you add C to cancel i𝜔L, leaving r alone, provided:
𝑖𝜔𝐿 +
−𝑖
𝜔𝐶
= 0, or 𝜔2𝐿𝐶 = 1
Then you need to step-up the impedance to 50 ohm.
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27. This is a Current Divider
to 50 Ω
spectr.
Impedance matched iff dissipation internally (in r) matches dissipation
externally (in 50 Ω ext)
𝐼𝐿
2
∙ 𝑟 = 𝐼50
2
∙ 50 =>
𝐼50
𝐼𝐿
2
=
𝑟
50
or I stepdown
𝐼50
𝐼𝐿
=
C𝑐
𝐶𝑐+𝐶𝑡
=
𝑟
50
*You could just add the impedances of L, Ct, Cc in series and parallel,
following the usual rules. But it is a mess and will carry no take-home
meaning.
Instead, use 𝜔2𝐿(𝐶𝑐 + 𝐶𝑡)=1 and
C𝑐
𝐶𝑐+𝐶𝑡
=
𝑟
50
=
1
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