The previous article mentioned that dV/dt is not a frequency, and dV/dt is a description of the time domain. The time domain and the frequency domain can be converted by Fourier transform.
Folding frequency Fknee
Therefore, here is to introduce an important concept that relates rise time to frequency: the corner frequency, the calculation formula of the corner frequency is:
Among them, Fknee represents the corner frequency, and Tr represents the rise time. This value represents the cut-off point of the spectrum containing 95% of the signal energy.
According to the Fourier transform, the square wave can be decomposed into the harmonic superposition form of the infinite sine wave. The edge of the square wave, that is, the steeper the rising edge or the falling edge, the more harmonics it contains, and the turning frequency represents the harmonic energy. Accumulate the harmonic order value of 95% of the total energy. It can be seen that the higher the Fknee, the higher the speed, and the higher the speed, the higher the corner frequency, thus linking the speed to the frequency.
Through these theoretical studies, we can more fully understand why the shorter the rise time of the CLK signal, the larger the higher harmonic components, the greater the impact on EMC, so we are in the circuit design process to meet the system timing. The rise time of these high frequency signals should be minimized.
2. Another important parameter - bandwidth
Often there is another parameter that comes into contact with bandwidth.
This concept is mainly due to the need to convert the frequency response into rise time after switching from analog to digital.
Specifically, when the signal passes through the transmission line, it can be seen that each spectral component of the signal propagates simultaneously in the transmission line, and some of the spectral components are attenuated due to loss, and some are enhanced by distortion. After arriving at the receiving end, it is necessary to regenerate the signal by the spectral component response, and convert the frequency response into the rising time. This parameter is involved at this time. Therefore, device manufacturers usually give this value to tell the user the accuracy of the device.
For example, an oscilloscope manufacturer typically gives each vertical amplifier a maximum operating bandwidth, and the probe marks the corresponding maximum bandwidth, and spectral components that exceed this bandwidth are discarded. Based on experience, a 10% to 90% rise time can be obtained from the time domain graph to obtain an approximate relationship with the bandwidth: BW=0.35/Tr. It can be seen that BW and FKnee are quite similar.
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