Zdirection

1
Pulse widths could be 10-2 µs
100 MHz Electro-magnet/Permanent magnet Systems
Z-direction
Supercon magnet systems above 100MHz up to 900MHz
as known currently
Z-direction
Magnet Current source
Superconducting current carrying coils
2
CW RF Oscillator mw power
RF Crystal Detector
CW Mode of detection
Pulsed Mode of Detection
Display/Record
Pulsed RF Transmitter
Signal receiving and detection
High gain RF receiver/detector
Probe with sample coil and sample in Magnetic
Field
Time domain signal to computer for FFT
Signal generation
Display monitor/Plotter
3
Flowing Current and Induced Magnetic Fields in a
Solenoid
Electrons (Blue Circles) Move and the
Conventional Current Flows in the Opposite
direction.

The MOTION of the (red) isolated (?) northpoles
indicate the induced field distributions and in
reality there are no lines existing for the LINES
of FORCES (as drawn in the previous slide. It is
a virtual line and LOCUS of the point North pole.)
4
PULSE WIDTH
PEAK to PEAK PULSE Amplitude
5
Free Induction Digitized
Analog to Digital Converter A D C
Received Analog Signal
Digitized Signal
FID analog signal
FID digitized points
6
All the signal shapes have been calculated in MS
EXCEL
In practice FFT program calculates Frequency
domain spectra from the time domain signal
Integration
FID
FT Imag.
Time Domain Signal
FT Real
Pulsed detection mode
Frequency Domain Spectra after FT
Similar to the CW mode Spectra
7
HR NMR in Liquids 100W NMR of Solids 3KW PP
Matched 50 O
Sample coil in the Probe with sample
Power Amplifier
High Power RF Pulses to Probe
Spectrum to display monitor/Plotter
Rectangular RF Pulse
FID
CW RF Source
Gate
Low noise RF Preamplifier
DC Pulse
computer FFT
High Gain Signal Amplifier
Pulse Programmer
Time Domain signal
Phase Sensitive Detector
ADC
Reference Signal
Transmitter
Receiver
8
Basic Probe unit is a Resonance Circuit with
tunable split capacitors configuration for
matching.
Sample tube with sample
Low noise pre-amplifier /high gain
receiver/PSD/Digital computer/Plotter
High-Power pulse transmitter
RF Bridge (Hybrid Junction)
RF Source (sweep generator)
Scope
RF Signal receiver- detector
Receiver off time
Pulsed RF mode
CW Mode
CW Mode
50O
9
2
3
4
1
5
Probe sample
Crossed Diodes
Transmitter ON time
Receiver Silent or dead time
Receiver
Receiver OFF time
DATA acquisition starts at this time
After the RF pulse, the FID is the impulse
response from the sample spin system. The pulsing
and FID can be repeated and added to acquire the
averaged signal for better signal to noise ratio
10
Signal level
Noise Level
Achieving a Sharp signal depends on the
homogeneity of the magnetic filed Shimmimng the
magnetic field using gradient correction coils
and sample spinning are the provisions in the
spectrometer system for improving the homogeneity
Long T2
Short T2
11
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12

OH-CH2-CH3
Acidic medium spin coupling for OH protons do
not show up
CH3
3.61 ppm
1.13 ppm
5.24 ppm
CH2
0
OH
TMS d 0 ppm
Moderate Resolution
d 1.13 ppm
HR PMR Spectrum
This calculated and simulated 60 MHz spectrum
has the chemical shift and frequency values as
obtained from a real NMR spectrum of alcohol. The
above figure plotted using MS Excel application
and line drawing from MS WORD drawing tools. High
Resolution spectrum as shown above would be
possible with good homogeneity of the magnetic
field.
13
In the Pulsed Field Gradient PFG and Magnetic
Resonance Imaging MRI techniques calculated field
gradient are externally superposed.
The inherent unwanted and incidental
inhomogeneities are reduced by Shimming and
sample spinning
NMR frequency will vary linearly along the length.
3 lines inside
6 lines inside
Along the length same nmr frequency for the sample
Linear Field Gradient along z-axis.
Equal number of lines pass through fixed area of
cross section along the length