黄土高原环境异质性与植被的恢复与重建

黄土高原是一个独特的地理区域,由于对其原生植被的不同认识,自然区划历来富有争议。为因地制宜地进行植被建设,在辨析植被属性有关论点(黄土无林、草原次生等)的基础上,主要由现代植被证据进一步讨论黄土高原的自然地带。分析了生物气候条件在不同地域之间的分异性,阐述了植被地带特征。为充分说明植被地带性,还从历史的角度探讨了植被建设的效果。表明黄土高原环境的非均质性可表征为森林、草原等地带,不能认为黄土高原不具有森林发育的地带性环境。相对于森林地带北界森林线,森林草原地带北界应为树木线。植被建设不应局限于一种土地利用模式,不能无视疏林及稀疏灌丛在森林草原地带的客观存在。

A fast-settling frequency-presetting PLL frequency synthesizer with process variation compensation and spur reduction

This paper proposes a fast-settling frequency-presetting PLL frequency synthesizer. A mixed-signal VCO and a digital processor are developed to accurately preset the frequency of VCO and greatly reduce the settling time. An auxiliary tuning loop is introduced in order to reduce reference spur caused by leakage current. The digital processor can automatically compensate presetting frequency variation with process and temperature, and control the operation of the auxiliary tuning loop. A 1.2 GHz integer-N synthesizer with 1 MHz reference input Was implemented in a 0.18μm process. The measured results demonstrate that the typical settling time of the synthesizer is less than 3μs,and the phase noise is -108 dBc/Hz@1MHz.The reference spur is -52 dBc.