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diode laser spectroscopy

earth's field nmr

earth's field nmr gradient/field coil system

fabry-perot cavity

faraday rotation

hall effect

magnetic force

magnetic torque

magnetic torque's magnetic force balance

modern interferometry

muon physics

noise fundamentals

optical pumping

power/audio amplifier

pulsed/cw nmr

pulsed nmr

quantum analogs

signal processor
/lock-in amplifier

sonoluminescence

torsional oscillator

two slit interference, one photon at a time

two slit's cricket

individual parts

introduction | the instrument | experiments | specifications| prices

New Pulsed/CW NMR Designed for Teaching

• a permanent magnet
• a crossed-coil sample probe
• and three electronic modules:
  • a pulse programmer
  • an oscillator/power amplifier/mixer
  • and a receiver

1. The pulse programmer creates the appropriate pulse stream.
2. The synthesized continuous-wave (CW) 15MHz oscillator is modulated into microsecond pulse bursts by the pulse stream. The stable oscillator can be tuned to the resonant frequency of the sample protons in the magnetic field.
3. The rf power amplifier creates large rotating B fields at the sample so that a 90° pulse requires a pulse of only 5µs duration.
4. The receiver amplifies the small rf signal created by the precessing nuclear magnetization. The rf signal can then be demodulated by either the linear amplitude detector or by the mixer phase-sensitive detector. The mixer obtains its reference signal directly from the CW oscillator.
5. The free-precession decay and the spin-echo signals can be measured using either a digital or analog oscilloscope.

• A crossed-coil probe is used so that the students can see how the pulsed oscillator and the receiver function independently.
• The instrument is composed of three independent units, an oscillator-amplifier, a pulse programmer and a receiver.
• All the connections between the modules and the crossed-coil probe are made by the students on the front panel with coaxial cables. This allows students to understand what the hardware is doing. Errors in connecting cannot damage the instrument.
• External BNC connectors allow students to interrogate of each module independently.
• Students use the Oscillator/Amplifier to hand adjust the frequency of the rf pulse to match the free precession frequency of the nucleus they are investigating.
• Students directly connect the Pulse Programmer to the Oscillator and recognize that it controls the timing of the radio frequency pulses.
• With the Pulse Programmer, students select the duration, number, spacing and repetition rate of the rf pulses. The student, not a well programmed computer, makes the choices. Mistakes, which highlight misconceptions, are easily corrected.
• Watching the effect of varying the pulse width from 0 to more than 360 degrees makes some of the adjustments done automatically in research instruments more comprehensible.
• Reducing the repetition time until the signal shrinks illustrates the meaning of T₁.
• Students can monitor either the actual simple harmonic signal of the rf spins or the traditional Envelope of the Free Induction Decay. (In our demonstrations, we find the distinction is not always obvious in spite of textbook knowledge.)
• Apparent changes in T₂ due to self diffusion can be examined by monitoring the effect of changing delay time in a multi-pulse Carr-Purcell sequence.
• Students actually take raw data of voltage and time which they can plot in a variety of ways rather than just using the print out of an automated machine. This means they can better understand the calculations and graphs a research apparatus might supply.