如果组件的最窄带宽明显大于脉冲带宽，信号将不得不与不必要的噪声功率竞争，从而降低灵敏度。
If the mostnarrowband component has a bandwidth appreciably wider than the pulsebandwidth, the signal will have to compete against more noise power thannecessary, again reducing sensitivity.
因此，为了计算噪声功率，接收机的频率响应可以近似为以发射频率为中心的带通滤波器，其带宽等于波形带宽。
Thus for the purposeof noise power calculation, the frequency response of the receiver can beapproximated as a bandpass filter centered at the transmit frequency with abandwidth equal to the waveform bandwidth.
实际使用的滤波器并不具有完全矩形的通带特性。
Real filters do nothave perfectly rectangular passbands.
为了分析噪声功率，使用传递函数H(F)描述的滤波器噪声等效带宽βn。
For analyzing noisepower the noise-equivalent bandwidth βn of a filter described by thetransfer function H(F) is used.
图2.24说明了这一概念。
Figure 2.24illustrates the concept.
Figure 2.24. 描述滤波器噪声等效带宽的概念Illustration of theconcept of noise equivalent bandwidth of a filter.
噪声等效带宽是一个理想矩形滤波器的宽度，其增益等于实际滤波器的峰值增益，理想矩形滤波器和实际滤波器在平方频率响应下的面积必须相等。
The noise equivalentbandwidth is the width an ideal rectangular filter with gain equal to the peakgain of the actual filter must have so that the area under the two squaredfrequency responses are equal.
这一条件保证了在白噪声输入下，两个滤波器的输出噪声功率相同。
This conditionguarantees that given a white noise input, both filters exhibit the same outputnoise power.
因此得到
Thus
其中接收机功率增益Gs定义为|H(F)|2的最大增益。
where the receiverpower gain Gs is defined as the maximum gain of |H(F)|2 .
则滤波器H(F)输出端的总噪声功率N为
The total noise powerN present at the output of the filter H(F) is then given by
通过滤波器H(F)的白噪声不再是白噪声，而是具有功率谱|H(F)|2。
White noise passedthrough a filter H(F) is no longer white, but instead has the power spectrum|H(F)|2 .
如果|H(F)|2近似为双边带宽βn Hz的矩形滤波器，则滤波器输出噪声的自相关函数近似为sinc函数，该函数在滞后1/βn秒处为第一零点。
If |H(F)|2is approximated as a rectangular filter of two-sided bandwidth βnHz, the autocorrelation function of the noise at the filter output isapproximately a sinc function with its first zero at lag 1/βnseconds.
但是，在第3章中可以看到，接收机输出通常以大约1/βn秒的间隔进行采样。
However, it will beseen in Chap. 3 that the receiver output is normally sampled at intervals ofapproximately 1/βn seconds.
因此，连续采样接收机输出的噪声分量之间仍然是不相关的。
Consequently, thenoise component of the successive receiver output samples are stilluncorrelated with one another.
根据式（2.77）的简单公式，任何源或电路输出白噪声的功率谱密度可以描述为Boltzmann常数与等效温度T′的乘积。
The power spectraldensity of white noise at the output of any source or circuit can be describedas the product of Boltzmann’s constant and some equivalent temperature T′, mimicking the simple formulationof Eq. (2.77).
——本文译自Mark A. Richards所著的《Fundamentals of Radar Signal Processing（Second edition）》
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