Local Oscillator

In order to use DSB systems (or a SSB one with modest rejection) we need to put the LO at the line frequency, 183.31 GHz. First harmonic mixers would require 91.155 GHz, which is easy with a fixed-tuned Gunn. Alternatively it may be more economical to adapt components from the standard ALMA LO system even though the tuning flexibility and phase stability are not required. Fundamental mode mixers are better because there would be fewer LO harmonics and somewhat lower noise. These could be driven with a Gunn plus a doubler, but would need quite a lot of power, especially for several mixers. Using biased mixers rather than self-biasing ones would reduce this problem. No tuning is needed, except possibly a step of a few MHz to move it out of the way of a particular line. Although with an SSB system one can in principle fit for the frequency, phase locking the LO to the system clock is clearly advisable, so that all the interference spikes are at accurately known frequencies (and with zero fringe rate).