TeachSpin’s Pulsed NMR is THE instrument to help students understand how the data delivered by sophisticated research or commercial apparatus is produced.
TeachSpin's Pulsed NMR Spectrometer Includes:
* a permanent magnet
* a crossed-coil sample probe
* and three electronic modules:
- a pulse programmer
- an oscillator/power amplifier/mixer
- and a receiver
i. The pulse programmer creates the appropriate pulse stream.
ii. 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.
iii. The rf power amplifier creates large rotating B fields at the sample so that a 90° pulse requires a pulse of only 5µs duration.
iv. 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.
v. The free-precession decay and the spin-echo signals can be measured using either a digital or analog oscilloscope.
Special Features Designed for Teaching:
* 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.
*Although the original PS1 series PULSED NMR SPECTROMETER is no longer being built, we maintain these pages for the convenience of our users and continue to support the instruments. As far as we know, all of them are still in action, including the very first one, which was delivered to Carnegie-Mellon University in the spring of 1994.
TEACHSPIN'S PULSED NMR SPECTROMETER PS2-A
with Continuous Wave capability sets the standard in Pulsed NMR systems designed for teaching.
In response to requests received over the twelve year life of our original Pulsed NMR, PS1-A and B*, this design has a host of new and exciting features. First of all, one can study NMR in both hydrogen and fluorine nuclei. The new magnet has a higher field (.49 Tesla and 21 MHz for protons) and a set of electrically controlled gradient coils. The magnetic field is stabilized to at least ±002 gauss over twenty minutes. The electrical shim coil homogenizes the magnetic field so that T2* for a mineral oil sample is 10 milliseconds. The combination of a stabilized and homogonized magnetic field makes it possible to detect inequivalent fluorine nuclei in the FID signal.
This spectrometer has phase sensitive detection (with a built in phase shifter) as well as linear amplitude detection. It allows both in-phase and quadrature detection of both pulsed and CW signals. That's right, this unit has a built-in slow magnetic field sweep and lock-in detector for doing CW resonance. This allows the students to compare CW absorption and dispersion with the FFT of the free-induction decay signals. This can be done for both hydrogen and fluorine.
The same pulse sequences are available, that is CP and GM sequences, for measuring T1 and T2, in liquids, soft solids, and some hard solids. This unit has a recovery time five times shorter than PS1-A, B, namely ten microseconds. That means students can study many solids whose T2 is about five microseconds or longer.
For those of you who have used the original unit for many years, we believe you will be delighted with the new instrument capabilities. Your students will be even more excited. Check out the details on the Pulsed/CW NMR pages of this website.
Jonathan F. Reichert
President TeachSpin Inc.
* We discontinued our original Pulsed NMR Spectrometer PS1-B because some key electronic components are no longer being manufactured. (Have no fear we have plenty in stock for repair work.)