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Do we need to consider a PCB trace as a transmission line when the Input is a DC signal.

As per my knowledge we used to consider PCB trace as a transmission line when length of the trace is more than Wavelength/20.

Wavelength is derived from the signal frequency ,for DC signal frequency is zero so it has infinite wavelength associated with the signal. So no need to consider the PCB trace as transmission line at DC. May I know I am correct or not.

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  • \$\begingroup\$ Zero frequency means infinite wavelength. \$\endgroup\$
    – stretch
    Sep 26, 2021 at 1:55
  • \$\begingroup\$ correct it was my mistake I will correct it .Thank you \$\endgroup\$
    – Hari
    Sep 26, 2021 at 8:43
  • \$\begingroup\$ Remember that a varying load is also an output, though. \$\endgroup\$ Sep 26, 2021 at 13:37

2 Answers 2

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Do we need to consider a PCB trace as a transmission line when the Input is a DC signal.

No. What you need to take into account is the DC resistance of the trace, and thus the voltage drop across it, if the flowing current will be relatively high.

However, if EMI is a concern, then you may need to be careful because if the supposedly DC signal has high-frequency spikes on it (e.g. coming from a poorly filtered switching regulator) then the PCB trace may act as an antenna for these AC signals. This may result in poor EMI performance (i.e. radiated emission). Even if the DC signal is clean enough, what you need to take into account is the incoming high-frequency signals (i.e. radiated immunity) because, again, the trace may act as an antenna for incoming high-frequency signals as well, if EMI is a concern.

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  • \$\begingroup\$ Thank you. It is a very informative answer. \$\endgroup\$
    – Hari
    Sep 25, 2021 at 17:00
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Yes, it 1st still physically a transmission line, but the behavior becomes simpler to analyze because there are no capacitive or inductive effects.

The DC series resistance and possibly shunt resistance effects are still present. In most applications these effects will be negligible.

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