Sample Modulation and Signal Detection

Of the many possible forms of signal modulation, that of source FM has the advantage of being simple to apply and requiring no additional MMW components or structural modifications to the absorption cavity. The penalty is the considerable attention that must be paid to the interaction between source, cavity and sample to obtain reproducible results that are capable of analysis. For a start, the use of a cavity presupposes that the source frequency be kept in synchronism with the cavity resonance whenever data are taken, and in respect of achieving this source FM possesses a distinct advantage over other modulation methods. 

Our mode of operation was that originally demonstrated by Kolbe and Leskovar. In our cavity spectrometer the effect of frequency modulating the source is to transmit a MMW signal through the cavity to the detector whose signal output amplitude fluctuates at the same modulation frequency. This modulation signal disappears, or rather is converted into its second harmonic 

The servo feedback loop functions by reducing the amplitude of the FM superposed by the cavity response onto the MMW signal to as small a value as possible. This does not mean, however, that the MMW output becomes unmodulated. Rather, the original modulation is replaced by one at twice the modulation frequency, that goes undetected by the servo system s phase-coherent detector. It may, however, be observed and converted into DC by a second phase-coherent detector responding to twice the modulation frequency. In the system so far described, this DC voltage will correspond to the second derivative of the cavity frequency profile (Section 4.1) and will therefore take on a fixed value characteristic of the cavity-g and its coupling coefficient. 

The novel feature of the operation occurs if the source scan-range includes a fixed frequency spectral absorption. This absorption will be enhanced as the cavity resonance is tracked through it, by an increase in the effective path length. At this point the absorption is detected by the second phase-coherent detector that sees it as a variation of the second harmonic of the modulation frequency superposed on the fixed cavity signal. Thus the spectrometer will respond to all spectral features in the range through which the source is tracked rather than to a single one located at whatever frequency the cavity happens to be resonant. 

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