Form of backend

In principle we could scan the LO and use a fixed and very simple IF with just one fixed frequency. Given that we are struggling for sensitivity this seems unattractive. The stability would probably not be good either. We therefore need a multichannel backend. The obvious choices are a set of filters (as in the MRAO and Onsala systems) and an analogue correlator along the lines developed by Andy Harris. More modelling is needed to determine the number of filters required but it seems unlikely that a great deal of additional information will be obtained by using more than about 4. The bandwidth should increase with increasing offset from the line. It is of course possible to make a cross-correlation filter spectrometer to use with a correlation front-end although twice as many filters are needed. The analogue correlator form looks attractive as a compact device suitable for mass production. The existing design is limited to about 4 GHz by the analogue multipliers but faster devices are being worked on. An alternative approach using passive detectors is under development at MRAO for CMB work. Because the frequency spacing is fixed, one would need at least 16 lags to cover plus and minus 8 GHz of IF. We suggest that the analogue correlator be adopted for further investigation with filters as a safe fallback.