This paper explores the feasibility of developing the "ultimate goal" of a flexible receiver (see figure1), comprised of an analogue to digital converter with an antenna as its input. This is done through some analysis which describes the use of sample and hold devices for frequency conversion within a radio. Through the use of a perfect multiplier a model is developed in order that the noise figure of a sample and hold device can be predicted. The results show that sample and hold devices have

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... higher noise figures than mixers and hence their use can give rise to reduced receiver sensitivity. The analysis also demonstrates that their noise figure performance is considerably improved through the use of RF gain and filtering. This demonstrates that the ability to sample at RF can only realistically be achieved with appropriate consideration of the necessary RF gain and selectivity, and thus flexible receivers need programmable filters. Interestingly the paper also concludes that an ideal sampler has an infinite noise figure and is therefore useless.