Superheterodyne detection

An unusual example of a microwave spectrometer which uses superheterodyne detection and molecular modulation is a tunable-cavity spectrometer designed and built by Radford [13]. Microwave cavities were described in chapter 9, where they form the heart of a microwave magnetic resonance spectrometer. Compared with 

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. 

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Figure 10.24. The principal elements of a radio telescope employing superheterodyne detection for recording frequency-discrete line emissions or absorptions [34].

Some of the earliest spin-lattice relaxation time measurements were carried out using superheterodyne detection by partial saturation of the magnetic resonance by a microwave pulse, and subsequent monitoring of the transient recovery on an oscilloscope [147], which is essentially a type of saturation recovery measurement... 

Weak spectral absorption is, however, characterised by small changes in a large background power and is therefore ill suited to superheterodyne detection. That background is also detected and overloads the sensitive intermediate frequency amplifier tuned to the beat frequency that is derived from the mixing process. Instead it is customary to modulate the absorption in some way and to observe the modulation on the detected signal rather than the absorption itself... 

The optimum sensitivity of the Fourier transform spectrometer obtained by calculating the signal-to-noise ratio of a pulse Fourier transform spectrometer relative to a conventional absorption spectrometer has also been given. Suppose that a spectral range F has to be investigated in a total time, T. Both experiments will use a superheterodyne detection system with a balanced mixer. The noise is assumed to be white with a power density Pg per spectral unit. The S/N ratio is defined as the ratio of the peak signal amplitude to the rms noise amplitude. 

Bartolini, P., Eramo, R., Taschin, A. and Torre, R. (2007). A superheterodyne-detected optical-Kerr-effect study of supercooled water dynamics. In preparation. 

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