Footnotes
Keywords: class-spectral-axis-v12

... frequency1
  In fact, there often are several local oscillators in a radio-observatory. At the Plateau de Bure Interferometer, two of them are used to stop both the signal and image tuned frequencies. This was useful at the time when the 1mm receivers were double-sideband and the sideband separation was obtained with Walsh phase modulation/demodulation.

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