Spectrometer monochromatic microwaves

Fig. 3.1. Schematic diagram for a standard C.W. X-band spectrometer. The sample is placed vertically into the centre of the cavity in the region of maximum microwave magnetic field. Various forms of stationary waves are set up in the cavity, depending on its shape. The basis of the instrument is the Klystron as a source of monochromatic microwaves, the permanent electromagnet, giving a homogeneous field through the microwave cavity (or resonator). The field is swept to generate the spectrum, and the modulation coils provide a rapid sampling (often at ca. 100 kHz) to give phase-sensitive...

Superherodyne spectrometers are now not common in laboratory microwave experiments, but superheterodyne detection plays a major role in radio astronomy, as we shall see later. The reasons are obvious one cannot modulate the energy levels of extraterrestrial molecules, and a radio telescope collects radiant energy at all frequencies simultaneously. One does not have a primary monochromatic source of radiation, as in laboratory experiments. 

A typical microwave spectrometer is illustrated in Fig. 2. The essential elements of a microwave spectrometer are a microwave source, absorption cell, detection system, and a system for measuring the source frequency. Microwave sources—the klystron and, more recently, the backward-wave oscillator (BWO)—generate a very narrow band of frequencies so that the source is essentially monochromatic. Furthermore, the source frequency can be conveniently varied and is often phase stabilized to give good frequency stability. 

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