(c) The characteristics of typical radio frequency amplifiers in a disturbing field With such a great disparity between wanted and unwanted power levels the feasibility of operating a radio telescope even in the far-out field of the SPS depends on the level of rejection of the SPS signal which can be obtained by filtering at the 2.45 GHz frequency. It is possible to provide adequate protection from out-of-band interfering signals for all the stages of a radio receiver except the first. The first stage is typically a parametric amplifier that has been optimised for minimum noise figure so that maximum sensitivity is achieved. Protection of a parametric amplifier by means of a filter results in a degradation in noise performance due to the insertion loss of the filter and the noise that it contributes. The amount of degradation increases with the amount of protection that is required and also as the unwanted frequency approaches the desired frequency. Measurements have been made of the effect of out-of-band interference on several of the relatively narrow bandwidth (£ 1%) parametric amplifiers in regular use at Jodrell Bank. Three main effects have been observed:- (i) gain compression for the case where the interfering frequency is within a few percent of the desired frequency. (ii) detuning and gain peaking where the frequency separation is greater than in (i). (iii) cross modulation i.e. the generation of an inband signal in the parametric amplifier from two out-of-band signals. Many astronomical observations would be prejudiced by gain changes -4 of 0.01% and/or spurious in-band components of 10 times the receiver -9 noise. Interference levels of about 10 W at the parametric amplifier are sufficient to cause these changes if the frequencies are within a few percent of one another. Stronger interference can be tolerated if the frequency offset is greater. 5. Filtering requirements The discussion in the preceding section indicates that the maximum
RkJQdWJsaXNoZXIy MTU5NjU0Mg==