How long line is the line?
It has to do and the signal propagation velocity, article on FR4 fiberglass PCB board copper wire in the signal rate of 6 in/ns.
In short, as long as the signal is going online round-trip time is greater than the rise time of a signal, PCB circuit board
s go line should be done to deal with a transmission line.
We see a signal in a long line transmission what happens.
Suppose there is a 60 inches long PCB line, as shown in figure 1, return path is a layer of PCB ground plane near the signal lines, signal lines and ground plane between the open circuit at the far end.
Figure signal spread online forward in this article, take 10 ns at the end of the transmission line to go back to the source side and take 10 ns, is in a round-trip time is 20 ns.
If the above signal path as ordinary current loop back and forth, there should be no current return path, because is open at the far end.
But the reality is not the case, return path on the signal after the initial period of time is current.
This section in the circuit board factory in dongguan PCB drawing go online with a rise time of 1 ns signal, in the first 1 ns time, signal lines also travelled only 6 inches, don't know the remote is open circuit or short circuit, then feel how PCB impedance signal, how to decide?
If the signal path back and forth as ordinary current loop will generate contradiction, therefore, should be dealt with according to the transmission line.
In fact, the stray capacitance between signal line and back ground plane, as shown in figure 2.
When the signal propagation process, the forward voltage at A point does not change constantly, for parasitic capacitance, change means an electrical current, voltage direction as shown in figure the dotted line.
Therefore the impedance of the signal to feel is the impedance of the capacitor present, parasitic capacitance constitutes the current return path.
Signals travel in forward passes through each point will feel an impedance, the impedance is changing the voltage applied to the parasitic capacitance, commonly known as transient impedance of transmission line.
Figure 2 when reach the remote signal, remote voltage to signal after the final voltage, voltage changes no longer.
Parasitic capacitance still exist, but not the change of the voltage, capacitance is equal to open circuit, dc is the corresponding situation.
As a result, the performance of this PCB signal path of short-term and long-term performance is different, the starting within a short period of time, is a transmission line.
Even if the transmission line remote open circuit, the signal during the jump, the performance of the transmission line in limited will also be like a resistances.
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