Electronic – Why do fundamental circuit laws break down at high frequency AC

achigh frequency

We're just starting the whole RF scene having dealt with DC and low frequency AC for all our previous courses.

I understand that at high frequency AC, fundamental circuit laws don't apply anymore and the classic passive component models need to be changed. The justification for this was that at high frequency AC transmission, the wavelength becomes much smaller and can sometimes be smaller than the wiring on PCBs etc.

I understand that this is an issue when transmitting through free space with electromagnetic waves but why is this an issue with actual physical wires and PCBs being driven by an AC source? I mean it's a direct connection, we aren't using electromagnetic waves to propogate through free space and so wavelength and stuff shouldn't matter right?

Best Answer

Actually, it is all about the waves. Even when dealing with DC, it is all managed by the electrical and magnetic fields and waves.

The "fundamental laws" aren't breaking down. The rules you have learned are simplifications that deliver accurate answers under certain conditions - you haven't yet learned the fundamental laws. You are about to learn the fundamental laws after having used simplifcations.

Part of the assumed conditions for the simplified rules is that the circuit is much smaller than the wave length of signal(s) involved. In those conditions, you can assume that a signal is in the same state across the circuit. That leads to a lot of simplifications in the equations describing the circuit.

As the frequencies get higher (or the circuits larger) so that the circuit is an appreciable fraction of the wavelength, that assumption is no longer valid.

The effects of wavelength on the operation of electrical circuits first became obvious at low frequencies but with very large circuits - telegraph lines.

When you start working with RF, you reach wavelengths such that the size of a circuit that sits on your desk is an appreciable fraction of the wavelength of the signals used.

So, you start having to pay attention to things you could conveniently ignore before.

The rules and equations you are now learning also apply to simpler, lower frequency circuits. You can use the new things to solve the simpler circuits- you just have to have more information and solve more complicated equations.

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