tolerable level of an interfering signal at a parametric amplifier varies _ from approximately 10 W for frequencies offset by less than 10% from the desired signal to approximately 10'7 W for offsets greater than 50%. These data are combined with those from Table 1 to deduce the specification of a filter to give the necessary protection to the parametric amplifier. The results are shown in Table 2. Another relevant parameter of a filter is the insertion loss at the desired frequency and the effect of this on signal to noise ratios. The best parametric amplifiers currently achieve system noise temperature of about 40K; a filter with an insertion loss of 0.2 dB at room temperature would degrade signal to noise ratios with this system by 40% and, furthermore, would limit the amount of possible improvement by future developments in parametric amplifiers. It is difficult to combine high rejection and low insertion loss in a filter, especially if the fractional separation of the pass and reject bands is small. Table 2 Filter attenuations required to protect the parametric amplifier from the power levels of Table 1. The maximum and minimum values correspond to the most pessimistic and optimistic estimates respectively of the sidelobe levels from the SPS.
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