Present and Future Capabilities of Microwave Crystal Receivers

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 30°K at 10 mc but increases to about 1000°K at 1000 mc. For crystal superheterodyne at all frequencies below 10,000 mc, the receiver noise can be down to 600°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 7000°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 150°K for all frequencies below 100,000 mc seems feasible.