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The "noise diode" technique is now well-established for the absolute measurement of receiver noise figure at frequencies up to about 300 Mc. A method is described of extending this technique to much higher frequencies, using a frequency changer to produce the required rf test signal from an if noise source. An experimental procedure has been developed which enables the signal level to be accurately evaluated, subject to such limitations as those of Dicke's reciprocity theorem which are of little practical significance for the applications so far considered. As in the low-frequency case, measurements are in general independent of receiver bandwidth, there is no stray radiation problem, and the receiver output indicating device may follow any law within wide limits. The necessary components are easy to construct and can be calibrated without the use of additional apparatus. The system is particularly well suited to mixer crystal measurements since both conversion loss and over-all noise figure can be measured with equal facility, and the latter is readily analyzed in terms of conversion loss and noise ratio.