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收检厘米波和波长更短的信号的几种新技术能使我们得到较以往好得多的噪声性能。在过去十年中,噪声温度已从大约3000°K降低到5°K以下。目前时兴的或在未来具有发展前途的低噪声器件有:行波管、参量放大器、隧道二极管、脉泽,光子计数器以及光敏检波器。本文讨论获得低噪声的各种技术,并摘要地叙述每种技术取得的成就。行波管内有效噪声的降低,是通过使电子注流穿过一个被延伸的低速度区域来完成的。S波段的行波管已测到过低达250°K的噪声温度。在运用电子注和半导体二极管的参量放大器件中都获得了低噪声。在任何未致冷的微波放大器中,电子注参量放大器的测得的噪声温度最低。致冷的半导体二极管参量放大器已给出甚至更低的噪声温度。隧道二极管是最新的负阻微波器件。就极低的噪声性能来讲,似乎还存在着某些限制,尽管它在许多低噪声应用中表现了很大的长处。到目前为止,脉泽是微波低噪声放大器方面最好的器件。几度的固有脉泽噪声温度是易于获得的。光子计数器和光敏检波器比较稳定,并且可以指望它们有很大的前途。
Several new techniques for collecting centimeter-wave and shorter wavelength signals have enabled us to achieve much better noise performance than ever before. Over the past decade, the noise temperature has dropped from about 3000 ° K to below 5 ° K. The current low-noise devices that are fashionable or promising in the future include traveling-wave tubes, parametric amplifiers, tunnel diodes, masers, photon counters and photodetectors. This article discusses various techniques for obtaining low noise and summarizes the achievements of each technique. The reduction of effective noise in the traveling wave tube is accomplished by passing the electron beam through an extended low velocity region. Traveling wave tubes in the S-band have measured noise temperatures as low as 250 ° K. Low noise is achieved in parametric amplification devices that use electronic diodes and semiconductor diodes. In any uncooled microwave amplifier, the measured noise temperature of the electronic parametric amplifier is the lowest. Refrigerated semiconductor diode parametric amplifiers have given even lower noise temperatures. Tunnel diode is the latest negative resistance microwave device. There seems to be some limitations with respect to very low noise performance, although it shows great benefits in many low noise applications. By far, maser is the best device for microwave LNAs. Several degrees of inherent maser noise temperature is readily available. Photon counters and photodetectors are relatively stable and can be expected to have a great promise.