宽带与窄带的区分方法

lisim51

  刚才看到有人说通过比较峰峰值和平均值之间的差值（6dB）可以判断出是否是窄带或宽带？为什么？原因如下（刚注册第一次发帖，希望成功）
A second discrimination for the determination of signal
characteristics, is the amplitude comparison between a peak
and an average measurement. Both measurements are
preferably made with the same instrument settings, especially
with an identical resolution bandwidth setting. If no amplitude
changes are observed between the two measurements, a signal
is considered narrowband. A signal is considered broadband if
an amplitude change between the two measurements is
observed, with the average measurement yielding the lower
amplitude. In practice, EMI standards that call out this
discrimination method, like CISPR 25, specify an amplitude
difference of, for example, 6 dB which is used as a decision
criterion. Per CISPR 25, a signal is considered to be
narrowband if the amplitude difference between the peak and
average detected signal is less than 6 dB. If the amplitude
difference is greater than 6 dB, the signal is determined to be
broadband. This approach is meaningful since the relative
amplitude accuracy of the instrument is to be considered as
well as other uncertainty factors that are introduced by different
instrument settings between the two measurements (e.g.,
change of reference level setting).
Figure 5 demonstrates the principle of this method, by
depicting the functionality of the peak and average detector.
The peak detector will determine the envelope of the signal to
be measured, which results in a low frequency signal at the
detector output or a DC signal in case the signal to be measured
is a CW signal. Since the peak detector determines the
amplitude envelope, it will provide the maximum signal
amplitudes. The average detector is often implemented as a low
pass filter that is placed after the peak detector in the signal
processing chain. This low pass filter, often referred to as video
filter, will be used as an integrator by setting the bandwidth
value to either a predefined value, called out in a standard (e.g.,CISPR 16-1-1, which specifies an integration time) or to a
value that is smaller than the lowest spectral component of the
signal to be measured. For example, a video bandwidth setting
of less than 100 Hz will result in the display of the average
value of the signal depicted in Figure 4. It should be noted that
the instrument is to be used in linear display mode in order to
obtain the average value of the signal under investigation. The
proper video bandwidth setting can be easily determined
empirically by reducing the video bandwidth step-by-step and
observing the resultant amplitude change. If further reductions
in video bandwidth do not cause further reductions in measured
amplitude, the proper video bandwidth for making an average
measurement has been found.
The comparison of peak and average detected signal
amplitudes, allows the conclusive determination of signal
characteristics without changing the resolution bandwidth. This
method can also be automated easily and thus allow further
automation of the overall compliance measurement process.