The main factor in introducing noise in PCB design
1. Due to the existence of loop impedance, a voltage drop is inevitably generated in the loop, resulting in a ground potential difference between the logic devices. The signal sent by the gate circuit is a fixed potential at the local ground potential. If there is an offset between the reference potential of the transmitting end and the receiving end, then another potential will be received.
2. The threshold level of some logic series products is a function of temperature. The signal transmission from a lower temperature gate to a higher temperature gate may have a reduced tolerance or a negative tolerance.
3. The rapidly changing return signal current flows through the ground path inductance, causing a change in the ground voltage between the logic devices. These ground voltage differences have the same effect on the received signal potential as the DC ground potential difference described above. This is a form of inductive crosstalk.
4. Signals on adjacent lines may be coupled to each other through their mutual capacitance or mutual inductance, causing crosstalk to a specified line. Crosstalk is superimposed on the expected received signal, possibly skewing a good signal to the adjacent switching threshold.
5. Ringing, reflection, and long lines distort the shape of the binary signal. The signal at the receiving end is smaller (or larger) than the transmitting end. The tolerance is that the signal distortion has some allowable limits.
The first two cases exist in all electronic systems, regardless of their speed. The latter three are unique to high speed systems. These three high-speed effects all vary with the size of the transmitted signal: the greater the return current of the signal, the higher the ground potential difference caused. The larger the signal voltage (or current), the more crosstalk is generated, and the larger the transmitted signal, the more severe the ringing and reflection. Therefore, no matter whether it is a low-speed or high-speed system, noise is inevitably introduced, and the voltage tolerance gives the system room to adjust.
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