marginal oscillator for NMR
Date: mid 20th-late 20th Century
Inventory Number: 2004-1-0408a
Classification: Nuclear Magnetic Resonance
Subject:
Maker: Department of Physics, Harvard University (founded 1884)
Inventor: Robert V. Pound (1919 - 2010)
Cultural Region:
Place of Origin:
City of Use:
Dimensions:
not available
Accessories: variety of spare parts cataloged as 2004-1-0408c-g
DescriptionHybrid (vacuum tube / transistor) marginal oscillator mounted together with an electromechanical drive assembly on a yellow wood board, with four decaying pieces of rubber tubing nailed to the bottom of the board as feet to insulate the oscillator from ambient vibration.
The circuitry of the oscillator is shielded from external radiation by a cast aluminum case screwed shut, which when opened reveals a lab-built lattice of wire, transistors, resistors, and a capacitor composed of several parallel plates attached to a rotating axis and which during part of its rotation passes through another array of parallel plates interlocking with it without touching. The oscillator also uses a single vacuum tube, mounted outside the case (presumably for heat dissipation). Also on the outside of the case are three manual controls: an attenuator switch, another unlabeled switch, and a feedback knob adjustable by discrete intervals from 0 to 10. The box also has electrical inputs or outputs labeled "low audio," "high audio," "coil" (where the Rf coil connects), and "rf out." Through at least one of these, the signal would be output to an oscilloscope (for monitoring).
Mounted beside the oscillator is an electromechanical drive assembly made up of an electric motor (with a standard electrical cord and a simple on/off/reverse controller) and two gearboxes. The motor's rotational motion was routed through the gearboxes to change the angular speed of an output shaft. The output shaft of the gearbox can be coupled and uncoupled from the capacitor inside the marginal oscillator by means of small screws. A variety of metal, wood, and plastic shims have been placed beneath the motor and gearboxes in order to more precisely align the shafts.
The circuitry of the oscillator is shielded from external radiation by a cast aluminum case screwed shut, which when opened reveals a lab-built lattice of wire, transistors, resistors, and a capacitor composed of several parallel plates attached to a rotating axis and which during part of its rotation passes through another array of parallel plates interlocking with it without touching. The oscillator also uses a single vacuum tube, mounted outside the case (presumably for heat dissipation). Also on the outside of the case are three manual controls: an attenuator switch, another unlabeled switch, and a feedback knob adjustable by discrete intervals from 0 to 10. The box also has electrical inputs or outputs labeled "low audio," "high audio," "coil" (where the Rf coil connects), and "rf out." Through at least one of these, the signal would be output to an oscilloscope (for monitoring).
Mounted beside the oscillator is an electromechanical drive assembly made up of an electric motor (with a standard electrical cord and a simple on/off/reverse controller) and two gearboxes. The motor's rotational motion was routed through the gearboxes to change the angular speed of an output shaft. The output shaft of the gearbox can be coupled and uncoupled from the capacitor inside the marginal oscillator by means of small screws. A variety of metal, wood, and plastic shims have been placed beneath the motor and gearboxes in order to more precisely align the shafts.
Signedunsigned
FunctionA marginal oscillator is any oscillator that does not use clipping to control the oscillating level; it is composed of a resonator (LC circuit), a linear amplifier to replace the energy lost in the resonator, and a low-distortion automatic gain control to regulate the level of oscillation.
The marginal oscillator allowed an NMR experimenter to locate the precise frequency at which the solid or liquid sample placed inside the Rf coil (between the poles of the magnet) would resonate. The oscillator is an LC circuit, where the capacitor is the array of parallel plates on the rotating shaft, and the inductor is the coil in the probe around the sample. The rotation of the shaft varies the capacitance of the capacitor, which changes the oscillation frequency of the system. The coil emits radiation at that frequency, which is not absorbed except at the resonance frequency of the material in the probe, which is related to the strength of the permanent magnet and the atoms present in the substance. Then the radiation is absorbed, the voltage in the circuit falls. This can be detected for instance by connecting the voltage signal to an oscilloscope.
This instrument was meant to sit on the red platform of the large permanent magnet (2004-1-0407) and drive the Rf inductance coil (2004-1-0408b) which rested between the poles of the magnet. Technically, the marginal oscillator is made up of both 2004-1-0408a and 2004-1-0408b operating in tandem.
R.V. Pound's original marginal oscillator, built around 1950, used only vacuum tubes. This hybrid tube/transistor model was built later for use in Physics 191r. Items 2004-1-0408c, d, and e are spare parts used to maintain the oscillator.
in order to sweep the frequency of the oscillator by tiny increments by very gradually turning the axis of the capacitor mounted inside the aluminum case. By properly arranging the gearing, the system could be set to turn the capacitor as slowly as once in 40 days. (Items 2004-1-0408f and g are spare motors and gearboxes.)
The marginal oscillator allowed an NMR experimenter to locate the precise frequency at which the solid or liquid sample placed inside the Rf coil (between the poles of the magnet) would resonate. The oscillator is an LC circuit, where the capacitor is the array of parallel plates on the rotating shaft, and the inductor is the coil in the probe around the sample. The rotation of the shaft varies the capacitance of the capacitor, which changes the oscillation frequency of the system. The coil emits radiation at that frequency, which is not absorbed except at the resonance frequency of the material in the probe, which is related to the strength of the permanent magnet and the atoms present in the substance. Then the radiation is absorbed, the voltage in the circuit falls. This can be detected for instance by connecting the voltage signal to an oscilloscope.
This instrument was meant to sit on the red platform of the large permanent magnet (2004-1-0407) and drive the Rf inductance coil (2004-1-0408b) which rested between the poles of the magnet. Technically, the marginal oscillator is made up of both 2004-1-0408a and 2004-1-0408b operating in tandem.
R.V. Pound's original marginal oscillator, built around 1950, used only vacuum tubes. This hybrid tube/transistor model was built later for use in Physics 191r. Items 2004-1-0408c, d, and e are spare parts used to maintain the oscillator.
in order to sweep the frequency of the oscillator by tiny increments by very gradually turning the axis of the capacitor mounted inside the aluminum case. By properly arranging the gearing, the system could be set to turn the capacitor as slowly as once in 40 days. (Items 2004-1-0408f and g are spare motors and gearboxes.)
Curatorial RemarksLab-made. Called a "Pound Box"?
ProvenanceUsed in Harvard's advanced undergraduate physics lab (Physics 191r) before coming to the collection.
