A solid state broadband microwave noise source

Recent studies of avalanche breakdown have led to a thorough understanding of the noise properties of a stable-burning avalanche discharge in a reverse biased silicon p-n junction. Avalanche diodes specially designed for uniform breakdown are nearly ideal noise sources because of their high noise output, high efficiency, broad bandwidth, low temperature dependence and high reliability. In order to reduce circuit parasitics, volume, and weight we have integrated an optimized avalanche diode with its current regulating bias circuit into a microstrip-line circuit on a ceramic substrate. Such a microwave noise source has a potential frequency range from dc to 18 GHz thus replacing one temperature limit diode and six gas discharge tubes required to cover the same range. The noise source can be biased from an unregulated voltage supply with 20 to 35 v and approximately 30 mA. The following preliminary measurements were obtained: The spectral noise power density was found to be > 30 db above kT from 1 to 11 GHz with a spectral density variation within ± 0.7 db. Tuning out reflections from connectors and adaptors reduced the spectral density variation to less than ± 0.2 db from 2.0 to 5.5 GHz and to less than ± 0.3 db from 8.0 to 11.2 GHz. The stability was found to be better than ± 0.01 db over a 12-hr period and better than ± 0.1 db over a 500-hr period. The temperature dependence, of the spectral power density measured at 2.5 GHz was found to be better than --0.002 db/°C. The dependence of spectral power density on bias voltage was better than -0.005 db/v. To study the reproducibility of diode fabrication we mounted 20 diodes in microwave pill packages. At a bias current of 20 mA the spectral power density at 10 GHz was 33.0 db with a spread of ± 0.15 db for 18 of the 20 diodes.