Signal Modulation and Detection Processes

Although the only direct modulation process to be discussed in detail in this monograph is frequency modulation it is by no means the only detection method possible. The most commonly employed method in anal3ftical microwave spectroscopy has been Stark modulation, but as has been discussed above, it is not well suited to cavity MMW spectrometry. 

Alternatively one may modulate the sample concentration by periodically removing it from and replacing it within the absorption cell as done, albeit rather slowly, in pulsed supersonic jet absorption spectroscopy. The sample absorption coefficient can be modulated by double resonance using a second microwave source to pump a transition that has one energy level in common with the 

The two constraints that make direct observation of weak absorption signals impracticable are the presence of pink noise, which contains a preponderance of low-frequency power compared with white noise, and the enhancement of this and other sources of noise by the rectification process through which MMW signals are detected. Both predicate the use of some kind of modulation at a frequency greater than the noise comer frequency and the use of a phase-coherent detector to convert the resulting modulated signal to a DC level suitable for display or for post-detection computer processing. 

A widely used practice is to apply a sinusoidal frequency modulation (FM) to the MMW source. This too produces an amplitude modulation of the transmitted signal. Any fi equency dependent component in the MMW transmission line, whether it be a sample, a cavity or a reflecting component, will show similar behaviour. It is important to minimise those spurii by careful construction and attention to mechanical matching of components such as coaxial connectors and waveguide flanges. 

A simplistic description of this process for sinusoidal FM of a source output HWHM Av at angular frequency w, is obtained by expanding the frequency varying spectral signal seen at the detector F(y + Avsino) ) in a Taylor series  

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