A marginal oscillator (1–50 MHz) operating at relatively high, stable oscillation levels, with very low noise, is described. Phase‐sensitive detection of the real and imaginary parts of the surface impedance of metals or the nuclear susceptibility can be carried out from 0.5 Hz as a result of the low 1/fnoise. The noise is dominated by this 1/fnoise below 200 Hz, while at higher frequencies it is white and the oscillator has a noise factor of 1.15 or 0.6 dB.