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Account/Revue
Designing and building a low-cost portable FT-NMR
spectrometer in 2019: A modern challenge
Concevoir et construire un spectrom
etre FT-RMN portable
a bas coût en
2019 : un d
efimoderne
Alain Louis-Joseph
a
,
c
,
*
, Philippe Lesot
b
,
c
a
D
epartement de physique,
Ecole polytechnique, Laboratoire de physique de la mati
ere condens
ee (PMC),
Ecole polytechnique, Centre
national de la recherche scientifique (CNRS), IP Paris, 91128 Palaiseau cedex, France
b
Equipe de RMN en milieu orient
e, Universit
e de Paris-Sud/Paris-Saclay, Institut de chimie mol
eculaire et des mat
eriaux d’Orsay
(ICMMO), UMR CNRS 8182, B^
at. 410, 91405 Orsay cedex, France
c
Centre national de la recherche scientifique (CNRS), 3, rue Michel-Ange, 75016, Paris, France
article info
Article history:
Received 8 April 2019
Accepted 1 July 2019
Available online 4 September 2019
Keywords:
FT-NMR
Home-built
Benchtop
Low cost
Electronics
abstract
High-field Fourier-Transform Nuclear Magnetic Resonance (FT-NMR) spectroscopy is a
high performance spectroscopic technique that is essential in many analytical fields. The
non-destructive nature of NMR makes it a preferred means of analyzing chemical and
biological environments. Compact benchtop NMR spectrometers are low-cost alternatives
to conventional high-field and high-resolution spectrometers. A research laboratory may
want to develop its own compact FT-NMR spectrometer ("home-built benchtop NMR")
with a very low financial cost (~10 kV). But why? First of all, to use it punctually as an
additional channel (nucleus X) to a high-resolution spectrometer, and also to be able to
couple it with complementary physics instruments such as an optical microscope to study
spin diffusion in semiconductors, for instance. In addition, a "home-built"NMR spec-
trometer can be used with a low-field permanent magnet for the quantification of species
that does not necessarily require high-resolution, avoiding the need for weekly and
expensive cryogenic services. Outside the research laboratory, this portable NMR can be
used for the in situ analysis of outdoor natural environments. Finally, this compact spec-
trometer is naturally dedicated to the teaching of NMR technique and is open to the study
of the basic electronic functions that constitute an NMR spectrometer. The main question
then arises: how to build a robust "Home-Built”NMR? In this article, we describe the
realization of an NMR instrument based on electronic components and boards (LNA, ADC,
FPGA, ARM, DDS…) easily commercially available, allowing one to obtain a benchtop NMR
instrument presenting both a high acquisition dynamics and a good signal-to-noise ratio.
©2019 Académie des sciences. Publishedby Elsevier Masson SAS. Thisis an open access article
under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
*Corresponding author. Departement de physique,
Ecole polytechnique, Laboratoire de physique de la mati
ere condens
ee (PMC),
Ecole polytechnique,
Centre national de la recherche scientifique (CNRS), IP Paris, 91128 Palaiseau cedex, France.
E-mail address: alain.louis-joseph@polytechnique.edu (A. Louis-Joseph).
Contents lists available at ScienceDirect
Comptes Rendus Chimie
www.sciencedirect.com
https://doi.org/10.1016/j.crci.2019.07.001
1631-0748/©2019 Académie des sciences. Published by Elsevier Masson SAS. This is an open access article under the CC BY-NC-ND license (http://
creativecommons.org/licenses/by-nc-nd/4.0/).
C. R. Chimie 22 (2019) 695e711
Mots cl
es:
RMN
aTF
Benchtop
Bas coût
Electronique
résumé
La RMN
a transform
ee de Fourier (RMN FT) et
a hauts champs est une technique spec-
troscopique tr
es performante et incontournable dans de nombreux domaines analytiques.
Le caract
ere non destructif de la RMN en fait un moyen privil
egi
e pour analyser les milieux
en chimie et en biologie. Les spectrom
etres RMN
a TF compacts, dits aussi de paillasse, sont
des alternatives,
a bas coût, aux spectrom
etres classiques hauts champs et haute
r
esolution. Un laboratoire de recherche peut vouloir d
evelopper son propre spectrom
etre
RMN compact (home-built benchtop NMR)
a un coût tr
es r
eduit (~10 kV). Mais pourquoi ?
Tout d'abord, pour l'utiliser de façon ponctuelle comme canal suppl
ementaire (noyau X)
a
un spectrom
etre haute r
esolution, mais aussi pour pouvoir le coupler
a d'autres
instruments de physique, comme par exemple
a un microscope optique, pour l’
etude de la
diffusion de spin dans les semi-conducteurs. Par ailleurs, une RMN home-built peut ^
etre
utilis
ee avec un aimant permanent bas champ pour la quantification d'esp
eces qui ne
n
ecessitent pas n
ecessairement une haute r
esolution,
evitant le recours
a des services
cryog
eniques hebdomadaires et coûteux. Hors laboratoires de recherche, cette RMN
portable peut ^
etre utilis
ee pour l'analyse in situ de milieux naturels en ext
erieur. Enfin, ce
spectrom
etre compact est naturellement d
edi
e
a l'enseignement de la spectroscopie RMN,
en
etant ouvert
al’
etude des fonctions
electroniques de base constituant l'instrument. La
question principale qui se pose alors est : comment construire aujourd'hui un spec-
trom
etre «maison »? Nous d
ecrivons dans cet article la r
ealisation d'un spectrom
etre RMN
bas
e sur des composants et des cartes
electroniques (LNA, ADC, FPGA, ARM, DDS…) ac-
cessibles commercialement et permettant d'obtenir une RMN de paillasse pr
esentant
ala
fois une grande dynamique d'acquisition et de bons rapports signal/bruit.
©2019 Académie des sciences. Publishedby Elsevier Masson SAS. Thisis an open access article
under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
1. Introduction
Conceived and explored in 1945 in physics laboratories
[1e5], Nuclear Magnetic Resonance (NMR) spectroscopy is
arguably nowadays one of the most essential tools for
analyzing the structure of mineral, organic, or biological
molecular systems. This fantastic achievement was
possible with the development of Fourier-Transform NMR
(FT-NMR) proposed by R.R. Ernst [6], that in turn opened
the door to first NMR experiments based on sophisticated
multipulse sequences [7,8]. Concomitantly, the application
of NMR principles/concepts has also offered fundamental
opportunities for medical imaging for robust diagnostic
purposes. Behind the vast practical aspects of analysis of
matter, NMR basics underlie varied and sophisticated
theoretical concepts in physics (data sampling), mathe-
matics (Fourier Transform (FT) principle), electronics
(resonating circuit), or quantum mechanics (Hamiltonian,
operator density…), as nicely described in many theoret-
ical/practical books that are references for any NMR spec-
troscopists or experienced users [9e16].
As a brief reminder, Table 1 provides a chronological
summary of the main technological/methodological ad-
vances from the 1950s to the present days, in conjunction
with the magnetic field strength for a continuous enhanced
spectral resolution and sensitivity.
Unfortunately, the increasing "race of Tesla"has a sig-
nificant financial cost (purchase) and periodic maintenance
(cryofluids), and hence the need for cryofluid-free, FT-NMR
spectrometers is becoming acute again. In addition,
nowadays, the availability of rather small permanent
magnets (soft iron) with a good magnetic homogeneity (up
to 10
4
) and stability and the significant improvement in
electronic performance make it possible to offer very
compact, versatile benchtop FT-NMR spectrometers at a
reduced cost, making again this new generation of instru-
ment very attractive for various applications in chemistry
[17].
In this article, we propose to revisit the development
and assembly of a very low-cost (~10 Keuros), portable
(benchtop) FT-NMR spectrometer by integrating magnetic
and electronic classical elements. For this, the essential
units required will be described, detailing their respective
role in the excitation/detection process of signal, for
instance. In a memorial approach, in particular, toward
chemists who have generally forgotten the NMR principles,
we will start from some (important) basic physical princi-
ples of NMR to organize and choose the key block elements
necessary for the realization of this "artisanal"or home-
made (as anglicists would say) spectrometer. Interest-
ingly, such a project is possible with laboratory instru-
mentation (function generator, oscilloscope…) accessible
at a lower cost. All requested, specific electronic compo-
nents/devices with their current nomenclature are listed in
Appendix and are commercially available.
Proposing an inexpensive, home-made FT-NMR instru-
ment is above all an exciting task of engineering integrated
into an educational project, for instance. It obviously cannot
be considered as a competition with the new generation of
benchtop FT-NMR spectrometers available in the market
today from different manufacturers (Magritek, Oxford,
Thermofisher, Nanalysis…), which integrates many
optional modules (locking, sample spinning…)[17].
Moreover, although the overall cost (material/electronic
A. Louis-Joseph, P. Lesot / C. R. Chimie 22 (2019) 695e711696
supplies) of the instrument seems attractive at first sight, it
does not include the actual human costs (total cost), in
particular in view of its pedagogical dimensions in Aca-
demic Education. This home-built NMR spectrometer can
be seen as an “alive”support instrument for teaching NMR.
Finally, this type of mobile NMR apparatus is well suited for
scientific research by coupling NMR and another instru-
ment (such as a microscope).
To have an idea, the time needed by a single engineer to
build a one-channel mobile NMR may be estimated as two
months, including the tests (and assuming that all elec-
tronic hardware components have been already
purchased).
2. Contribution of modern electronics
Electronics has developed considerably in recent de-
cades, making it possible to build high-performance sys-
tems with high functional integration. To illustrate the
evolution of the power of electronic component, we have
included the number of transistors integrated on the same
chip over the years in Table 1. For example in 1965, a
microprocessor integrated nearly 2000 transistors on the
semiconductor chip. The density of the components has
increased by a factor of 10
6
in 45 years (1965e2010),
thereby reducing the size of the electronic circuits.
Currently, a simple integrated circuit is capable of per-
forming signal acquisition, processing, and control func-
tions. The modern smartphone is a blatant illustration of
the high-tech concentrate contained in such a small vol-
ume. As the density of the components increases, the
operating frequency of the hardware circuits has also
increased considerably. Hardware can now operate at fre-
quencies of several gigahertz compared to a few hundred
kilohertz in the 1970s. These high working frequencies
allow complex processes to be carried out with extremely
short times.
Another huge leap in the field of electronic equipment is
the memory capacity of components. It is now possible to
store and manipulate memory spaces of more than 64
gigabytes. Data storage is no longer a real limitation and it
is possible not only to acquire large amounts of data, but
also to store the ever more sophisticated programs to
control and process these data.
The frequency synthesis is now digitally based on DDS
(Direct Digital Synthesis) cards over a very wide range of
frequencies with very high accuracy. In Table A1 (nomen-
clature of components) presented in Appendix, we list the
references of a synthesis unit capable of generating fre-
quencies from a few kHz to more than 300 MHz. Analogi-
cally, the sizes and costs of low-noise preamplifier
components, mixers, and other hardware have been
reduced. For example, the low-noise preamplifiers in the
acquisition chain can be made with a few circuits for a
hundred euros.
Finally, an important point to keep in mind is the
reduction of the energy consumption of modern compo-
nents, allowing complete battery-powered systems to be
powered, making them autonomous and therefore
portable. This aspect was unthinkable in the 1980s. In
summary, the performance of electronic components has
increased while sizes and costs have been reduced, so the
Table 1
Historical overview of main instrumental/methodological achievements.
NMR
methodology
RMN-TFRMN-WC
1D 2D
homonuclear
2D heteronuclear
gradients
3D, 4D, TROSY
…
FAST NMR
Supraconductor
magnet
Ultra shielded magnet, cryogenic probe
Size of analyzable
molecules (kDa) < 10 25 35 50 100
B0 (T) 0.70 1.41 2.11 4.22 6.34 9.39 11.74 14.09 17.61 18.78 21.13 22.32 23.48
1H Larmor (MHz) 30 60 90 180 270 400 500 600 750 800 900 950 1000
Microprocessors
Intel family name
4004 8080 80490 Pentium 4 Intel
Core i7
Number of
transistors in
microprocessors
(Intel family)
2.3 u 103 6 u 103 1.2 u 1069.5 u 10642 u 106291 u 106 1.1 u 109
Years
1965 1975 1985 1995 2005
1950 1960 1970 1980 1990 2000 2010
A. Louis-Joseph, P. Lesot / C. R. Chimie 22 (2019) 695e711 697
realization of complex units such as a small NMR spec-
trometer is possible in research laboratories.
3. Some specificities of a benchtop FT-NMR
spectrometer
Classical or benchtop/mobile FT-NMR spectrometer is a
complex instrument, but it can be easily divided into
distinct and clearly identified functional blocks [18].In
practice, this instrumentation involves many domains of
electronics such as radiofrequency (RF), audio chain, power
electronics, low-noise preamplification, and transmission
lines. In this first decade of the 21
st
century, FT-NMR
spectrometers are fully digital and multi-nucleus. Gradu-
ally, they are connected by internet, and therefore they can
be remotely controlled and (unfortunately for experi-
menters) fully automated. The two operating modes of FT-
NMR spectrometers (excitation and detection steps) are
performed by fast microprocessors and executed by suit-
able softwares.
As a matter of principle, the channels (
1
H,
13
C, X…)of
spectrometers are similar but independent of each other, all
of them are synchronized by a master clock; a dedicated
pulse sequencer controlling the complex multi-channel
pulse sequences of the spectrometer. This sequencer
takes into account the different transmission delays and
hardware dead times (such as the pre-scan delay) to pro-
duce a reliable and precise pulse timing controlled by the
software. The duration of the (hard or soft) electromagnetic
pulses (
m
s to ms) and the delays between events (
m
stos)
are those actually achieved by the electronics, without the
need to add small compensation delays in the pulse pro-
grams. These specificities are made possible, thanks to the
very high speed and stability of modern electronics.
The assembly of mobile NMR consists of three distinct
parts: the magnet, probe, and data acquisition and pro-
cessing spectrometer. The magnet and probe depend on the
chosen application: low field or high field. We discuss these
aspects later in this article.
The electronics of our mobile NMR spectrometer is
designed to acquire very low value signals (less than
m
V)
over a high dynamic range (the noise factorof the detection
chain is less than 2 dB), and the sampling is adapted to the
required dynamics (8,12, 16, or 24 bits). The same applies to
frequency synthesis and quadratic demodulation, which
can be adapted to a very wide range of frequencies. Note
that all the electronic hardware components (RF power
amplifier, mixer, preamplifier…) have been chosen to work
in a wide frequency range (from 1 to 400 MHz). This allows
the portable FT-NMR spectrometer to work at low field
(with a static permanent magnet) or at high frequency
(with a superconducting magnet).
4. Sensitivity and resolution
The homogeneity and strength of the magnetic field of a
permanent magnet define the sensitivity and spectral res-
olution of any NMR experiment. The temporal magnetic
stability of a magnet obviously limits the duration of the
measurements. Homogeneity and magnetic stability must
allow a coherent accumulation of signals in phase.
NMR is often defined as rather insensitive and high-field
magnets are needed to achieve robust sensitivity and res-
olution to analyze complex molecules. In high-resolution
NMR, chemical shifts must be measured with a resolution
of about 0.01 ppm to solve the J couplings of a few Hertz.
Magnetic fields are of the order of several Tesla and are
generated by superconducting magnets cooled by helium
and liquid nitrogen. A system of compensation for in-
homogeneity (shims) is essential to obtain the required
resolution.
Permanent magnets with low field (<0.94 T or 40 MHz
for
1
H) are called low resolutions due to their low sensi-
tivity and homogeneity. In an inhomogeneous field, the
magnetic strength varies in relation of the spatial position
in the magnet. Such effect leads to significant broadening
or/and overlapping of resonance peaks on spectra, thus
preventing exploitable spectral resolution. Nevertheless,
the measurement of distribution of relaxation times or
diffusion coefficients of molecules is accessible at low
fields. Quantitative analysises are also possible. The char-
acterization of materials (liquid, powder) can be carried out
at low resolution with a home-made spectrometer that is
much less cumbersome, less expensive in cryogenic
maintenance, and accessible because it is close to the
synthetic bench of molecules. The latter reduces the cost
and workflow of any research activity. The mobile NMR
spectrometer we are describing is part of this approach: it
is compact and can be used with a low-field magnet at
Larmor frequencies ranging from 1 to 15 MHz in the set
presented.
Although intended for low-field, low-frequency appli-
cations, as mentioned above, the design and electronic
components of our mobile NMR spectrometer can operate
at frequencies up to 300 MHz (see the Table in the
Appendix). As a result, this mobile NMR spectrometer can
also be used, without major modifications, at high field
using the magnet and the probe part of the industrial
spectrometer. In this type of application, the interest is
methodological: development of a specific auxiliary chan-
nel, magnetization manipulation to control radiation
damping, and development of new strategies (multiple
acquisitions).
5. Back to the principles of NMR and practical
consequences
The nucleus of atoms (
1
H,
13
C,
15
N,
19
F,
31
P…) is charac-
terized by a quantum quantity, the spin, which is specificto
each nucleus and determines its magnetic properties. The
magnetic moment,
m
, of a nucleus is directly related to Iby
its gyromagnetic ratio,
g
, which is a constant depending on
the nature of the nucleus. The magnetic moment being
quantified, the energy states are degenerated.
When this nucleus is immersed inside the static mag-
netic field, B
0
, the degeneration of energy level is lifted,
leading to several energy levels according to the spin value
(Zeeman effect). If the nucleus is then subjected to a
radiofrequency wave at a specific frequency (Larmor fre-
quency), the atomic nucleus will absorb the energy of this
radiation and release it during relaxation mechanisms. This
process at the origin of the NMR phenomenon depends on
A. Louis-Joseph, P. Lesot / C. R. Chimie 22 (2019) 695e711698
the magnetic field and the properties of the molecules. The
study and measurement of an isolated atom is not possible
due to a lack of sensitivity of NMR; the result is that we are
still dealing with a very large set of nuclear spins, the
macroscopic result of which we will measure: magnetiza-
tion. In the presence of a magnetic field B
0
, the resultant of
macroscopic magnetization is collinear to B
0
at equilibrium
and proportional to the spin populations (number of
nuclei).
Under the action of B
0
, the magnetic moment,
m
,ofany
magnetically active nucleus (Is0) is subjected to a force,
C¼
m
!^B
!
0;(1)
which causes a spontaneous precessional movement
around the zaxis (// to the B
0
axis) at the Larmor frequency
(resonance condition) (see Fig. 1),
n
Larmor ¼
n
0¼
g
B0=2
p
(2)
where
g
is the gyromagnetic ratio of the considered
nucleus.
At equilibrium, all nuclei of a sample are subjected to B
0
.
The resulting macroscopic magnetization (M
0
) is oriented
along the Ozaxis. The first step of any FT-NMR experiment
is to excite the spin systemwith a (generally short) RF pulse
to remove it from its equilibrium state by tilting the M
0
magnetization in the xy plane (see Fig. 1). The return of M
0
to equilibrium (denoted Free Induction Decay (FID)) is then
detected and processed by FT to obtain a frequency spec-
trum in Hertz or expressed in parts per million (ppm). The
magnetization, M
0
, observable for a set of nuclei of spin I, is
given by the following law:
M0¼N
g
2ZIðIþ1Þ
3kT B0(3)
where Tis the sample temperature, Nis the number of
nuclei per unit volume, Iis the spin of the nucleus, kis the
Boltzmann constant, and B
0
is the static magnetic field.
In practice, the magnetization, and therefore the sensi-
tivity of a NMR experiment, is directly dependent on the
value of B
0
. The exact magnetic-field dependence of the
NMR sensitivity (considering the detection coil) can be
written as the energy (Ein Joule) detected in a coil of vol-
ume, V, namely:
E¼M0·B0·V(4)
Fig. 1. Pulse-acquisition 1D NMR experiment. The macroscopic magnetization is aligned along the B
0
axis (Oz) at equilibrium state, ideally at the end of the
relaxation delay. After an RF excitation pulse, M
0
is flipped in the Oxy plane. The precession of this magnetization gives the impulse response (or FID) which is
detected.
A. Louis-Joseph, P. Lesot / C. R. Chimie 22 (2019) 695e711 699
where B
0
and M
0
are the static magnetic field and the
magnetization per unit of volume, respectively.
A priori, it is therefore preferable to work with high
magnetic fields, which are also at the origin of high-
resolution NMR in liquids. For instance, at room tempera-
ture (300 K) and for a B
0
field of 11.7 T (500 MHz), the
detectable magnetization is about 3 10
6
(in SI unit), or
an energy of about 10
4
m
J for a sample volume Vof
1cm
3
(E¼M
0
$B
0
$V). This corresponds to a reception power
of about 1
m
W for an acquisition time of 0.1 ms. As seen, the
strength of detected signal is therefore very low. De facto,
using a so-called low-field NMR, where the B
0
strength
varies between 0.15 and 1.5 T, makes the experimental
NMR detection much more critical.
From an electronic viewpoint, the voltage of detected
signal (before its amplification) is of low amplitude, with
an order of microvolt (
m
v). In practice, the experimental
conditions as well as the instrumentation electronics
must therefore be optimized to reach acceptable signal-
to-noise ratios (SNR). This challenge can be met, thanks
to the performance of modern electronic components. In
addition, the transmission power required to energize a
frequency band of about 10 kHz (for the
1
Hnucleus)fora
1cm
3
volume sample is about 11 W (@500 MHz or 11.7 T).
For a low-field spectrometer, an excitation power of about
5 W for a pulse duration of about 20
m
sissufficient to
generate an instantaneous tilting of 90
of M
0
,i.e. in the xy
plane.
6. NMR instrumentation
6.1. The magnet
In the applications of this study, we use a permanent
magnet that is sufficiently homogeneous and stable to
provide consistent NMR measurements. Considering the
analytical purposes of low-field NMR and its trans-
portability, the weight of the permanent magnet to be used
must be reasonable (<80 kg), and generate at least a field of
0.15 T (1500 Gauss), leading to a resonant frequency for
protons around 6.4 MHz. The axis of the magnet is hori-
zontal and perpendicular to the probe (coil), which consists
of a vertical solenoid, and in which the sample is positioned
(5-mm tube) (see Fig. 2).
The NMR probe (the antenna) is carefully placed at the
magnetic center of the magnet to both excite (with a
perpendicular field, B
1
) and detect the signal along one of
the four axes perpendicular to B
0
(x,x,y,y). The probe
includes its frequency and impedance system (tuning and
Fig. 3. Simplified "block"diagram of an NMR spectrometer.
Fig. 2. Permanent horizontal magnet and its probe placed at the center of
the magnet gap with a sample tube of 5 mm, inside.
A. Louis-Joseph, P. Lesot / C. R. Chimie 22 (2019) 695e711700
matching) that is connected to the electronic chain of the
equipment. Note that it is easy to add two external coils on
either side of the magnet to produce a field gradient that
can potentially be used for possible monodimensional im-
aging applications. As the static magnetic field is not ho-
mogeneous over all the surface of the magnet, it is
therefore necessary to accurately measure the value of B
0
for determining the Larmor frequency (
n
0
). To this aim, a
field map can be made within the permanent magnet to
determine the exact value using a gaussmeter (Teslameter)
that is slowly moved into the air gap of the magnet. In our
case, the value measured at the center of the air gap is1492
Gauss, which corresponds to a resonance frequency,
n
0
,of
6.28 MHz for
1
H.
6.2. Excitation vs detection
To perform FT-NMR experiments, three steps are
necessary: i) create a homogeneous magnetic field, B
0
; ii)
excite the spin system to tilt it from its equilibrium state by
means of an excitation coil generating a B
1
field perpen-
dicular to B
0
; and iii) detect the FID signal by means of a
receiver coil (which is also the same generating the B
1
field
excitation), the whole satisfying the Larmor's resonance
condition,
n
exc
¼
n
d
ec
z
n
0
.
The "block"diagram setting up the main parts of an
NMR spectrometer is shown in Fig. 3, which shows the
permanent magnet and the NMR probe inside the magnet,
an electronic "transmitter"block responsible for exciting
the spin system with RF powers of several dozen watts, and
a second "receiver/detection"block that amplifies the FID
signal of very low power (
m
W) and demodulates the signal
in two components of low frequency (separate operations).
These two steps can be controlled by a "programmable
sequencer"that generates the multiple RF pulses trains and
controls the delays during the experiment. For very simple
experiments (pulse-acquisition), a laboratory pulse gener-
ator is sufficient. Finally, data processing and visualization
can be performed using a simple oscilloscope, but in a more
modern way using a computer (PC) with its software
interface. It should be noted that the probe is connected to
the excitation and reception blocks via a unit called
duplexer. This element can be seen as a signal "switcher"
that aims to protect the preamplifiers of the detection
channel from high RF excitation power. The duplexer di-
rects all RF power to the probe during the excitation step
(by closing access to the receiving channel) first, and then
redirects the detected NMR signal to the preamplification/
demodulation chain.
For a spectrometer operating at frequencies of the order
of 15 MHz and with relatively low powers, a simple struc-
ture based on a
p
quarter-wave filter can be used to
perform this function. It is a design strategy for a very low-
cost mobile NMR. First, the
p
filter requires only a few
electronic components at a very low cost (self and capac-
ities), capable of withstanding high powers (several Watts).
In addition, it works automatically and there is no need to
control it during acquisition, simplifying any pulse pro-
grams. The realization of the circuit is simple and fast. We
will describe how to make a quarter-wave duplexer in
Section 8.4 for a frequency around 6.4 MHz. Because the
p
filter (or quarter-wave filter) is a circuit tuned to the Larmor
frequency of the nucleus under consideration, its use is
limited to a small frequency band.
Fig. 4. Schematic description of the NMR spectrometer blocks along with the visualization of some of key signals obtained. Note the presence of the duplexer.
A. Louis-Joseph, P. Lesot / C. R. Chimie 22 (2019) 695e711 701
For wide band applications (i.e. frequencies from MHz to
a few hundred MHz), a duplexer based on fast RF electronic
switches with fast diodes is mandatory. This type of
duplexer is much more complex to implement because a
pulse program within a microprocessor must control it. The
description of such a circuit is out of scope for the version of
the mobile NMR spectrometer we present in this article.
We are targeting here a low-cost mobile NMR, with labo-
ratory instrumentation, a duplexer
p
filter, and a low-field
magnet operating at 0.15 T (or 6.4 MHz for
1
H).
6.3. The lock system
An electronic locking system may be easily added to
make the mobile NMR device capable to continously con-
trol the stability of the magnet, to avoid a slow shift of the
magnetic field over time [see for instance, pp. 44 to 50 of
ref. 19]. The lack of lock control can be particularly trou-
blesome when working at high magnetic fields, and/or
when recording long-term acquisition NMR experiments.
Considering the aims (pedagogical aspects), application
domains (1D NMR, detection of abundant nuclei), and the
cost of this home-made instrument, no locking system has
been integrated into the instrument.
7. NMR device synoptic
For clarity, the general electronic diagram of a NMR
spectrometer is schematically proposed in Fig. 4. As seen,
the attenuators and tuning/matching circuits are present at
the input and output of the power amplifier to avoid excess
power that can be harmful to the detection chain. The
excitation step generates one or more RF pulses at the
Larmor frequency. These pulses are directed via the
duplexer to the NMR probe to tilt M
0
perpendicularly to the
zaxis. The high-frequency resonance free induction decay
signal (HFFID) is then routed via the duplexer to the
detection circuit. The detection circuit includes at least two
very low-noise RF preamplifiers to amplify this HFFID
signal. The signal is then demodulated with an intermedi-
ate frequency, in quadrature (quadratic detection leading to
a90
phase shift of the signal), to generate a real (R) and an
imaginary (I) component accordingly to Eq. 5,
SðtÞ¼Afcosð2
pn
itþfÞþi sinð2
pn
itþfÞg (5)
where A is the amplitude and
f
is the phase of the detected
signal. This signal is then filtered to eliminate unwanted
frequency components. At the output of the active "low-
pass"filter, the signal is lowered to the 10-kHz range (often
wrongly called «audio filter »). The Rand Icomponents of
the signal can then be observed and checked on an oscil-
loscope or digitized via an analog-to-digital converter
(ADC) for further processing.
Although the spectrometer is optimized to work around
the 6.4 MHz frequency, it can easily operate efficiently over
a wider band. All the electronic components required for
the construction of a low-cost NMR spectrometer are listed
in Table A1, with the adequate associated nomenclature.
The choice of the bandwidths of the various elements
present in the nomenclature makes it possible to work up
to frequencies around 500 MHz. The assembly of the
various blocks can be made entirely on a printed circuit
board or more simply by connexion components (BNC or
SMA outputs).
8. Key electronic elements of the spectrometer
Let us now review the key elements of the equipment
(probe, switch, RF preamplifier…) in the order of the syn-
optic block diagram presented in Fig. 4.
8.1. RF pulse generator
The first important function of an NMR spectrometer is
the generation of the basic frequencies (BF) necessary for
excitation/reception of signal. The RF excitation pulse is
realized by means of a switch connected to a frequency
synthesizer and a pulse generator. However, to perform a
synchronous detection (resonance), part of this frequency
must be directed to the detection circuit.
The generation of RF pulses can be performed in
different ways. The first one consists of using a stan-
dard frequency and pulse generator present in any
physicist laboratory. To ensure sufficient stability and
accuracy for NMR, the resolution of the function
generator must be at least 10
6
. Typically, an arbitrary
function generator of type HP33120A or Agilent 33220A
operating up to 15 and 20 MHz, respectively, with a
power of 10 dBm is perfectly suited for a low-field NMR
spectrometer. For pulses, the pulse generator can also
be used to produce square-shaped pulses. For pulses of
the order of 10
m
s, a pulse generator with a resolution
oflessthan100nsissufficient (e.g., Keysight 33220A).
The time and frequency parameters are adjusted from
the control panels of these laboratory instruments. In
this case, only very simple single-pulse sequences are
possible: 90
pulse - acquisition; 180
inversion pulse -
acquisition, etc. The primary advantage of laboratory
equipment is their availability at a lower cost, their
speed, and simplicity of implementation (no software
development for pulse programs). The second way to
generate pulse sequences is to use a board based on an
Advanced Reduced instruction set computing Machines
(ARM) microcontroller, a Field Programmable Gate Area
(FPGA) unit, and a Direct Digital Synthesis AD9959
(DDS), which allow digital frequencies up to 300 MHz
to be synthesized. The advantage in this case is to have
a very compact and powerful unit to generate the basic
pulses and frequencies. It can be developed, for
example, from an NXPLPC1768 ARM, an Altera Cyclone
II FPGA [20, 21], and a DDS. This board is capable of
sequencing four fully independent channels. The FPGA
allows the most complex pulse sequences to be per-
formed, and the ARM component then manages both
the system and data processing. This option allows
stand-alone battery operation for outdoor (off-labora-
tory) use.
A. Louis-Joseph, P. Lesot / C. R. Chimie 22 (2019) 695e711702
8.2. The switch
The role of this circuit is to switch the RF signal for a
controlled pulse duration. This element can be realized by
means of four fast diodes (type 1N4148) mounted in head
to tail or with a PS1211 mini-circuit component (switch),
which offers fast switching times (<500 ns) and very low
channel imbalance (see Fig. 5).
8.3. The RF power amplifier
As our mobile NMR is aimed to work from low to high
field, it is necessary to have a broadband RF excitation
pulse. The excitation provided to the spin system must be
sufficiently high, with relatively short RF pulses (10e100
m
s) to excite a bandwidth covering the spectral width of an
observed nucleus. The selected power amplifier must be
able to operate in pulse mode but also in continuous wave
(CW), if nuclear decoupling or any other type of low power
excitation needs to be achieved. It is imperative that the
Fig. 6. Electrical diagram of the duplexer, including two diode arrays and the
p
filter. The
p
filter has a minimum RF signal at the receiver (port C) when there is a
maximum on the probe (port B). The "noise gate 100 isolates any signal feedback from the probe to the power amplifier to minimize losses during detection. The
"noise gate 200 protects the detection chain from residual voltages eliminated by the
p
filter.
Fig. 7. (a) Example of a probe made of glass with its integrated capacitive devices C
m
and C
t
. (b) NMR probe wiring diagram: C
m
and C
t
are adjustable capacitances
from 5 pF to 500 pF, inductance coil, L¼2e3
m
H.
Fig. 5. (On Left) Structure of an RF switch based on fast diodes. (On Right) Example of input and output signals from an RF switch. Here, the RF pulse has a length
of 80
m
s.
A. Louis-Joseph, P. Lesot / C. R. Chimie 22 (2019) 695e711 703
amplifier has a good linearity to adjust the RF pulse accu-
rately. A power amplifier generates a high noise level at the
output even at rest. It is therefore necessarily not activated.
We used for our prototype a ZHL-5W-1 unit operating in a
frequency range from 1 to 500 MHz.
8.4. The duplexer
As already mentioned above, the duplexer has a very
important role, in particular, to protect, during reception,
the low-noise preamplifier chain from the high RF power
emitted during excitation. It is a three-port system A, B, and
C (see Fig. 6), where the first port (A) is connected to the
power amplifier, the port (B) is connected to the probe, and
the output (C) is directed to the low-noise detection pre-
amplifier chain. For broadband systems, it is preferable to
use a switch network electronically controlled by the pulse
sequencer. In low-frequency (low-field) NMR applications,
a structure based on diode arrays and a quarter-wave filter
can be used; the diagram is shown in Fig. 6.Afirst network
of four head-to-tail diodes (noise gate 1) acts as an auto-
matic switch: these diodes are blocked (open circuit) for
small amplitudes, thus preventing any return of the signal
from the probe to the power amplifier. The quarter-wave
system, referred to as the
p
filter, has an input (B)/output
(C) phase shift of 90
. It is called a quarter wave because it
has a phase shift optimized at 90
around a given fre-
quency, in our case, around 6.28 MHz. This filter consists of
a choke and two capacitors. The filter input is connected to
the probe (port B) and the output (port C) to the low-noise
preamplifier chain. For a specific frequency, a quarter-wave
filter has a minimum amplitude at its output (C) when its
input (B) is maximum. To reinforce the protection of the
preamplifiers, a second network of head-to-tail diodes can
be incorporated to protect the detection circuit by short-
circuiting (at the threshold of the diodes) any residual
voltages. The filter setting must combine input and output
impedance matching, as well as phase shift and attenuation
at the resonance frequency. The characteristics of the
quarter-wave filter were adjusted to the network analyzer
with the Smith diagrams of the input (S11) and output
(S22) impedances as a function of frequency. The input
(S11) and output (S22) impedances have been adjusted to
have an actual value as close as possible to 50
U
(in practice
44.5
U
at the resonance frequency of 6.28 MHz), while
maintaining a 90
phase shift between the input and
output.
8.5. The NMR probe
Different probe body shapes (solenoid, saddle coil) can
be produced and used depending on the adaptability to the
magnet. The most simple detection antenna consists of a
solenoid coil. A probe body made entirely of glass (boro-
silicate) can be used (see Fig. 7a). For a 10-mm probe, the
solenoid is wound on the glass tube with an inner diameter
Fig. 8. Picture showing the measurement of the frequency tuning and impedance matching curve (top) and on the Smith chart (bottom) on a network analyzer.
A. Louis-Joseph, P. Lesot / C. R. Chimie 22 (2019) 695e711704
of 10 mm, thus allowing the detection of a large amount of
analyte, but small diameter (5 mm) could be used. The
capacitive network is integrated into the probe. The sole-
noid is made of rigid copper wire with a diameter of
1e1.5 mm and includes about 20 turns with a diameter of
10 mm over a length of 20 mm; the better the quality of the
winding, the smaller the artefacts will be. The elementary
structure of the frequency tuning and impedance matching
system is described in Fig. 7b. A network of capacitors
adjustable from 5 to 500 pF allows frequency tuning (C
t
)
and impedance matching (C
m
) for a coil of the order of
2.2
m
H. The tuning allows the probe to be adjusted to the
resonance frequency (
n
0
z
n
1
¼1/2
p
√LC). Impedance
matching is performed at this frequency to adjust the probe
circuit to 50
U
and minimize the energy losses.
Frequency tuning and impedance matching are per-
formed using a network analyzer to accurately position the
probe's attenuation peak at the desired resonance frequency
while observing the impedance evolution on the Smith chart
(see Fig. 8). Impedance matching consists of refining and
lowering the resonance peak as low as possible to absorb all
the energy in the coil. In our case, the resonance is adjusted
with the network analyzer at 6.284 MHz, the attenuation
obtained is 50.816 dB with a very good impedance
matching (49.72
U
), and consequently a good detection
sensitivity is obtained for a low-field spectrometer. The half-
height width obtained at this frequency is 35 kHz, giving a
quality coefficient Qof 179, a very acceptable value.
Once the frequency tuning has been achieved, the sec-
ond important parameter to be estimated is the dead time
(or response time) of the probe (see Fig. 9). The dead time is
the time required for the probe to dissipate the energy
stored during excitation. Some of the excitation energy is
sent to the probe via the duplexer, but once the RF pulse has
disappeared, significant energy remains in the probe. The
higher the quality factor Q of the probe, the higher the dead
time is. This is a disadvantage for detection, because it will
be necessary to delay the measurement of this time: i) not
to destroy the low-noise preamplifiers and ii) not to su-
perimpose the low detection signal on the high amplitude
due to the probe signal.
In the model presented in this article, the dead time is
about 250
m
s before the two quadrature detection channels
return to zero (see Fig. 9). The detection of the useful signal
needs to be delayed by this delay and the detection circuit
is reinforced, by a network of diodes, to support the peak
amplitudes of several volts for a few hundred
microseconds.
8.6. The receiving chain: radio frequency preamplifier
The signal from the probe is too weak and cannot be
used directly, and so it must be amplified. The amplifi-
cation chain consists of two stages with radiofrequency
preamplifiers in series. The noise factor of a preamplifier
cascadeisalmostequaltothatofthefirst stage. A
"BA01500"preamplifier with the lowest factor is placed
first, followed by a RF2046 home-made broadband
preamp unit. The low-noise amplifier LN BA01500-35
has a frequency range varying from 0.1 to 500 MHz
with a gain of 35 dB. The intermodulation product
power at the output is þ12 dBm (output IP3), and the
undistorted input power is 35 dBm (1 dB compression
point). Thus, the maximum permissible input power is
therefore 35 dBm.
Fig. 9. Observation of the probe's dead time: about 250
m
s. At the top and in the middle are displayed the two quadrature detectionchannels (yellow and blue). At
the bottom is displayed the rectangular (purple) RF excitation pulse (50
m
s of length).
Fig. 10. Structure of a Quadrature Intermediate Frequency Mixer demodu-
lator (QIFM). Mixers: X2L-06e414 from Pulsar Microwave.
A. Louis-Joseph, P. Lesot / C. R. Chimie 22 (2019) 695e711 705
Our home-made preamplifier is made with three
RF2046 components in series: the overall gain of the
channel measured at the network analyzer (without
distortion)is35dBat6MHzand31dBat1MHzandthe
operating frequency range is from DC continuous to
1 GHz. The output intermodulation product (IP3) is þ23
dBm and the noise factor (NF)isequalto3.8dB.This
configuration of the elements as well as their perfor-
mances allows us to ensure the detection of signals on a
large dynamic range without distortion: typically from
some 100
m
Vto14mV.
8.7. Quadrature demodulation
Quadratic demodulation is based on the principle of
synchronous detection to obtain real and imaginary com-
ponents of the NMR signal. Its structure is based on a
Quadrature Intermediate Frequency Mixer (QIFM)
demodulator whose schema is given (see Fig. 10). It consists
of a 90
hybrid splitter, two mixers, and a 0
combiner
(summing). Each of these elements is listed in the table of
components (see Table A1).
At the input of the 90
hybrid divider, we have the
base frequency of the Local Oscillator (LO)inputspec-
trometer; this is actually the Larmor frequency of the
spectrometer. At the input of the 0
divider, we have the
FID: this is the high-frequency response RF. At the
output of the quadratic demodulator on channels I
(imaginary) and Q (quadratic), the signals are of the
form: i) cos [2
p
(
n
rf
þ
n
0
)t)] þcos [2
p
(
n
rf
n
0
)t)]; ii) sin
[2
p
(
n
rf
þ
n
0
)t)] sin [2
p
(
n
rf
n
0
)t). We therefore obtain
two quadrature components with all the "sum"and
"difference"frequencies of the demodulated signals:
2
p
(
n
rf
þ
n
0
)and2
p
(
n
rf
n
0
), plus harmonics. Only low-
frequency signals in the 10-kHz range are interesting,
and the signal must be filtered to retain only the lowest
frequencies using a low-pass filter.
8.8. Double active audio low-pass filter
An active double low-pass filter is required to keep only
the low frequencies of the demodulated signals (see
Fig. 11). Each filter is designed to charge the demodulators
on 50
U
and consists of two stages allowing an overall gain
of about 30 dB in voltage. As around the resonance, the
Fig. 11.
1
H signal (one component) of a water sample (5 mm) obtained (one SCAN: NS ¼1) after the demodulation stage and low pass filtering, observed on a
digital oscilloscope with FFT. Top: FID component. Bottom: spectrum: presence of the peak at 4.92 KHz. Note the linewidth of 423 Hz. The Larmor frequency was
n
0
¼6.4 MHz. The RF excitation pulse length is 50
m
s. See Appendix for the details of the experimental setup.
A. Louis-Joseph, P. Lesot / C. R. Chimie 22 (2019) 695e711706
measured frequency differences are about ten kilohertz,
the filter bandwidths are adjustable over a range from
100 Hz to 500 KHz.
The two filter channels must be adjusted to have the
same frequency and phase responses throughout the
useful bandwidth. This is to avoid unbalancing between
the real and imaginary components of the signal, which
would degrade the spectrum in frequency (quadrature
artifact, zero frequency…). Precise gain and phase ad-
justments in the bandwidth are made with a network
analyzer, the only true and effective way to visualize
the response of both channels as a function of
frequency.
9. Setting of spectrometer
9.1. Probe control
The tuning of the probe to the resonance frequency has
been described above.
Fig. 12. (a) No NMR tube: real (high, channel 1) and imaginary (low, channel 2) components at the output of the audio frequency amplifier after an RF excitation
pulse length of 50
m
s. (b) Example of one-scan quadrature detection of a low-magnitude
1
H signal (5-mm water tube). See Appendix for the experimental setup.
A. Louis-Joseph, P. Lesot / C. R. Chimie 22 (2019) 695e711 707
9.2. Cancellation of the DC offsets
Any DC voltage offset in the detection circuit generates a
zero frequency component that will affect the quality of the
spectra. To reduce this unwanted effect, it is important to
carefully cancel the offset voltages of the active double low-
pass audio filter to have a signal close to zero when the
input signal is nul.
9.3. Signals at the output of the audio amplifier without
sample
At first, we work without putting a sample in the
magnet to minimize the imperfections of the assembly:
adjustment of the LO level (10 dBm) and cancellation of
continuous offsets. In the absence of a sample, the voltage
offsets are less than 1 mV. In addition, note that the
deadtime of the probe after the RF pulse is 400
m
s (see
Fig. 12a).
9.4. Signals at the output of an audio amplifier with sample
The imperfections of the assembly having been mini-
mized, we introduce our sample (a 5-mm tube of water),
and we visualize the precession signals in quadrature
mode (see Fig. 12b). Note the very low magnitude of the
signals.
9.5. Optimization of the 90flip angle
The NMR signal obtained at this step, although
observable, is relatively small in amplitude. To maximize
the amplitude of the FID, it is now necessary to adjust the
strength excitation pulse for a tilting of M
0
equal to 90
.
For this reason, the duration of the pulse is varied until a
magnetization is obtained whose intensity in the xy plane
is maximum (see Fig. 13). The amplitude variation of the
magnetization vector is observed as a function of the
excitation pulse duration. In our case, the pulse duration is
between 13 and 14
m
s. Another approach is to determine
the pulse duration corresponding to an angle of 180
,for
which the macroscopic magnetization is then null (tilted
to the Ozaxis).
9.6. Increase in the S/N ratio
The signal-to-noise ratio of an NMR experiment in-
volves numerous factors such as: i) the magnetization M
0
,
ii) the filling factor and the quality factor of the probe, iii)
the noise factor of the preamplifier, and so on [22]. After
optimization of all the electronic hardware and parameters,
we can also increase the SNR during the acquisition. The
SNR can be improved by averaging the FID signals over a
large number of acquisitions (NS scans). Indeed, on an
average of NS acquisitions, the noise will be averaged to-
ward zero due to its random origin, whereas the useful
signal will increase on average. Thus, the SNR is improved
at the root of NS as displayed in Fig. 14.
10. Possible applications
Benchtop FT-NMR allows many applications in chem-
istry, such as the analysis or characterization of small
molecules, or the monitoring of chemical reactions. The
teaching of NMR methodologies (effects of excitation/
inversion pulses, pulse calibration, optimization of
acquisition parameters, as well as the description of some
experiments as the measurement of relaxation
Fig. 13. Adjustment of the RF excitation pulse duration to obtain a maximized signal corresponding to a 90pulse.
A. Louis-Joseph, P. Lesot / C. R. Chimie 22 (2019) 695e711708
parameters, spin echoes…) is also another aspect of their
use. The portable low-field NMR is particularly suitable for
nuclei with high natural sensitivity, such as
1
H,
19
F,
31
P,
and
29
Si. However, other «exotic »nuclei with high gy-
romagnetic ratios such as
7
Li,
11
B, and
23
Na can also be
detected [27,28].
A (home-built) mobile NMR spectrometer can also be
used to develop projects that aim to couple NMR and op-
tical microscopy in a semiconductor [23] to locally polarize
nuclear spins (dynamic polarization) and measure the
spatial profile of nuclear magnetization using an optically
detected NMR, with good spatial resolution. This
Fig. 14. Effect of averaging on the SNR. Comparison of the FIDs obtained by adding successively: (a) 2, (b) 32, and (c) 64 scans (5-mmwater tube). On (c), note the
vertical line setting at 3.45 kHz. See Appendix for the experimental set up.
A. Louis-Joseph, P. Lesot / C. R. Chimie 22 (2019) 695e711 709
fundamental study of nuclear spin diffusion does not
require a high magnetic field (less than 0.15 T). For this kind
of experiment, NMR functionalities are reduced to signal
excitation and detection for parameter optimization,
without the need for frequency resolution.
11. Conclusion
The low-field FT-NMR spectrometer presented in this
article is fully operational. This revisited instrument can be
exploited to carry out simple NMR measurements such as
the determination of chemical shifts (
d
), spin echoes, mea-
surement of T
2
(
1
H) or T
1
(
1
H) relaxation times, etc. On the
other hand, it provides a pedagogical approach to the
teaching of NMR to students at all levels, from the physical
concept to the electronic realization through the many
possible analysis applications [24, 25, 26].Itisalsoan
excellent demonstration element at scientific events for a
more general public. Finally, it can be inserted in real ex-
periments of physics requiring NMR to be coupled with
another measurement technique such as optics, for example.
Noteworthy technical developments of the instrument
are possible. Thus if the permanent magnet is equipped
with an additional pair of coils with currents in the range of
1e3 A, it can then be injected to create additional field
gradients to B
0
. The spectrometer is then able to perform
some simple 1D imaging experiments or to experimentally
measure molecular diffusion coefficients.
Although sensitivity is reduced at low-field strength,
quantification and measurement of relaxation and diffusion
parameters of neat samples or solutions are possible [27, 28].
This kind of portable NMR spectrometer could then appear
the ideal tool for outdoor NMR analysis of ecosystems.
Conflicts of interest
The authors declare no conflicts of interest.
Acknowledgements
A.L-J. thanks the physics department of the "
Ecole pol-
ytechnique"and its Polytechnician students, as well as the
LPMC (UMR CNRS 7643) for their contributions to the
development of mobile NMR. P.L. thanks CNRS for its sup-
port to fundamental research and the divulgation of
fundamental sciences.
Appendix
Definitions of acronyms and notations used
ARM: Advanced Reduced instruction set computing
Machines
ADC: Analog-to-Digital Converter
BNC: Bayonet NeilleConcelman (connector)
B
0
:Static magnetic field
BF: Basic Frequency
B
1
:RF coil.The term B
1
is used to characterize the field,
in the plane, orthogonal to the static B
0
field. In the
terminology frequently encountered in the fields of NMR,
the B
1
coil is a resonant LCR circuit tuned to the Larmor
frequency. This coil is used for the detection and the exci-
tation of the nuclei in FT-NMR experiments. Lis inductance,
Cis capacitance, and Ris resistance.
CW: Continuous Wave
dB: Decibel
dBm: Power in decibel: 0 dBm ¼1 mW/50
U
DDS: Direct Digital Synthesis
FFT: Fast Fourier Transformation
FID: Free Induction Decay
FT: Fourier Transformation
FPGA: Field Programmable Gate Area
HFFID: High-Frequency Free Induction Decay
LO: Local Oscillator
M
0
:Magnetization at equilibrium
NF: Noise Factor
NMR: Nuclear Magnetic Resonance
Q:Quality factor
QIFM: Quadrature Intermediate Frequency Mixer
RF: RadioFrequency
SMA: SubMiniature version A (connector)
SNR: Signal-to-Noise Ratio
TROSY: Transverse Relaxation Optimal SpectroscopY
TTL: Transistors Transistors Logics
T
2
:Spinespin relaxation time
T
1
:Spinelattice relaxation time
n
0
:Larmor frequency (MHz)
Table A1
Characteristics and nomenclature of electronic components.
Characteristics Designation Number
of devices
Frequency
range (MHz)
1500 G permanent magnet,
40-mm air gap, NMR
homogeneity on a useful
diameter of 20 mm,
divergent polar pieces on
a diameter of 200 mm,
height 400 mm
TE2M C4-1367
1500 G
1 6.2e6.8
Directional coupler 30 dB,
3W, SMA (Pulsar)
C5-08-411 2 5e1000
SMA connector (pulsar) 20 0.1e1000
Mixer level 7 dBm SMA
(Pulsar)
X2L-06-414 2 (LO/RF) 1e
1000: (IF)
DC-1000
Power divider/combiner, 2
way (0
), phase balance:
2
, amplitude balance:
0.3 dB
Insertion loss 0.8 dB, SMA
(Pulsar)
P2-09-411 3 5e1000
90
hybrid power divider
Phase balance 2
, amplitude
balance 0.4 dB, insertion
loss 0.5 dB, SMA (pulsar)
QE-01-412 1 2e10
Low-noise RF preamplifier
NF ¼2, gain: 35 dB,
RF output power: 5 dBm
Absolute maximum rating
input
RF power: 35 dBm
(Elhyte)
BA01500-35 1 0.1e500
A. Louis-Joseph, P. Lesot / C. R. Chimie 22 (2019) 695e711710
Experimental setup (except otherwise in the text)
All NMR and test experiments have been made with the
electronic setup outlined in Fig. 4. A permanent magnet
with a field of 1500 G (6.4 MHz) is used for the static
magnetic B
0
field (see Fig. 2). A home-made solenoid-type
probe device and a 5-mm water sample tube were used for
the tests. The frequency and the RF pulse duration have
been set with the frequency generator and the pulse
generator mentioned in the text. Data have been acquired
and processed using some digital oscilloscopes with sen-
sitivities of 1 or 2 mV/div.
Characteristics and nomenclature of the electronic
components
Table A1 provides the characteristics and nomenclature
of the electronic components used.
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Table A1 (continued )
Characteristics Designation Number
of devices
Frequency
range (MHz)
Low-noise RF preamplifier
NF ¼2.9, gain: 25 dB,
output RF power: 5 dBm
(mini circuit)
ZFL1000LNþ1 0.1e1000
Low-noise RF preamplifier
RF; NF ¼3.8, gain: 23 dB,
RF output power: 5 dBm
(home made)
RF2046 3 DC -1000
Power amplifier ZHL 5W11
Operational amplifier OP27 1 DC - 8
Operational amplifier OP37 1 DC - 60
Resistor 1 k
U
,10k
U
10
Variable capacitor 5 pFe500 pF 4
Probe (home made) 1 6.1e6.8
Sequencer board with ARM
þDDS (home made)
Or frequency generator and
laboratory pulse
generator (see text).
NXPLPC1767þ
DDS 9959
1
1
0.01e301
Analog or digital
oscilloscope with FFT
1
AC/DC adapter 24 V/6.4 A AHM150PS24 1
Total estimated cost ~10 kV
A. Louis-Joseph, P. Lesot / C. R. Chimie 22 (2019) 695e711 711
