Digital RF Engineers To Master Knowledge Serial -2

Date:2015/10/13 9:36:51 Hits:

Through previous research we know the distribution of the spectrum of the digital signal is very wide, its highest frequency component range depends on the rise time of the signal rather than the data rate. When the first challenge such high-bandwidth digital signal transmission, the face is the impact of the transmission channel.

The real transmission channels, such as PCB, cables, backplanes, connectors, etc. bandwidth is limited, which would bring into the original signal in the frequency distribution is weak or completely filtered out, after the loss of high-frequency components on the waveform of performance is the edge of a signal slows, etc. appear on the signal is too rushed or shocks.

In addition, according to Faraday's law, varying signal transitions will produce eddy currents in the conductor to offset the change in current. The faster the rate of change of current (digital signal is equivalent to the rise or fall time of the signal shorter), more intense vortex conductor. When the data rate up to about 1Gb / s or more, the signal conductor current and induced current is essentially fully offset, the net current flow is restricted only on the surface of the conductor, which is the skin effect. It will increase skin effect losses and changing circuit impedance, change in impedance will change the phase relationship between the signals of each sub-harmonics, resulting in distortion of the signal.

In addition, most commonly used to manufacture circuit board FR-4 media is a glass fiber woven into its uniformity and symmetry than the poor, while the dielectric constant of FR-4 material further and signal frequency, so signals different frequency components of the transmission speed is not the same. Different transmission speed will further change the phase relationship between the signal of each harmonic component, so that the signal is worse.

Therefore, when the high-speed digital signal transmission in the PCB, high frequency components of the signal due to the loss of sales will be weak, the various frequency components can transmit at different speeds and at the receiving end and then added together, and also part of the energy impedance discontinuities such as vias, connectors, or line width variations Local produce multiple reflections, the combination of these effects will seriously change the shape of the waveform. To analyze such complex issues is a big challenge.

A point worth noting is that the amplitude of the signal attenuation, rise / fall time change on many factors, changes in transmission delay and frequency components are related to the different frequency components affected are not the same. The digital signal, its frequency components and signal transmission and related digital symbols (such as 0101 0011 frequency component stream and stream represents not the same), so different digital stream in the transmission by The impact is not the same, which is inter-symbol interference ISI (inter-symbol interference ISI).

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Digital RF Engineers To Master Knowledge Serial -2

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