Signal Sources and Operating Frequency Band

The need for transportability coupled with operator- and intrinsic safety considerations dictates the use of solid state sources operating at low voltage. Of these, Gunn oscillators are the least susceptible to noise and, if locked to a high stability source, are easy to stabilise and to scan in frequency. At lower frequencies they are both inexpensive and widely available as components of intruder alarm systems, but their cost rises rapidly with increasing frequency, and becomes uncomfortable beyond about 100 GHz. 

The best operating frequency will clearly be that at which the sample absorption is the greatest. Whilst the formula of Equation 1.48 suggests that this absorption increases indefinitely with frequency, no allowance has been made for the ever-decreasing thermal population of the states contributing to the transition. When this is factored in, one obtains the formula  

The peak absorption coefficient of OCS, 10 m , occurs at 462 GHz. This is by no means, however, the optimum working frequency due to the non-ideal behaviour of most MMW detectors. Commercial Schottky barrier mixer diode detectors show a quadratic roll-off in sensitivity at frequencies 100 GHz. If this is factored into Equation 6.1, the peak sample sensitivity occurs around 300 GHz, and the response is so flat that even at 100 GHz it has only fallen off by a factor of two. What is common to both curves is the dramatic fall-off in sample sensitivity at frequencies 100 GHz, reinforcing the point that the band 26-40 GHz is ill suited to high-sensitivity analytical spectroscopy. 

Our final selection of a source was based on the consideration that the two common MMW active atmospheric constituents, oxygen and water, have spectral lines of sufficient sensitivity only around 60 GHz, at 118 GHz and at 183 GHz. The capability to measure them was a must have feature of our spectrometer most other target species had spectral lines spread over a broader spectrum. With these considerations in mind a Gunn source was purchased to cover the 58-63 GHz band, together with a doubler and a tripler. In order to fill in the frequency gap between the harmonics a further source, comprising a 70-80 GHz Gunn plus a doubler was also used. Together these have provided access to the spectra of most species of interest 

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