Radio‐frequency oscillators used as detectors of nuclear paramagnetic resonance absorption employ an auxiliary source at audio frequency to modulate the static magnetic field. A small component of voltage at this frequency is usually induced into the tank coil of the oscillator. If this voltage is permitted to appear at the grid of the oscillator, mixing with the rf voltage will occur. This produces a false signal indistinguishable from the resonance signal. A simple LC filter put between tank circuit and oscillator tube is found to be extremely effective in removing false resonances. The response of the oscillator to signals generated by a Watkins calibrator is measured in the range from 5 to 25 Mc. Results of these and ``Q'' measurements show the oscillator sensitivity to be unimpaired by the addition of the LC network.

1.
R. V.
Pound
 and
W.
Knight
,
Rev. Sci. Instr.
21
,
219
(
1950
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2.
G. Watkins, thesis, Harvard University, 1952 (unpublished).
3.
N. A. Schuster, thesis, Washington University, 1951 (unpublished).
4.
The time constant of the integrating network following the lock‐in detector was 5 sec.
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© 1958 The American Institute of Physics.
1958
The American Institute of Physics
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