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The lower limits of receiver noise are explained in terms of the fundamental physical constants of vacuum tubes and microwave crystal rectifiers. With negative-grid vacuum tubes, the receiver noise, expressed in absolute temperature units, can be about 30Â°K at 10 mc but increases to about 1000Â°K at 1000 mc. For crystal superheterodyne at all frequencies below 10,000 mc, the receiver noise can be down to 600Â°K, limited only by the crystal mixer. Above 10,000 mc the crystal rectifier parasitic elements-spreading resistance and barrier capacity-cause the receiver noise to increase rapidly to values of approximately 7000Â°K at 70,000 mc. For the future, high mobility semiconductors and new rectifier geometry promise substantial reduction of the parasitics. Also, a cooling mechanism promises limitless reduction in noise; a receiver noise of 150Â°K for all frequencies below 100,000 mc seems feasible.