Port vs Terminal

Today, I would like to discuss about some confusing terms - port and terminal.

In a sense, there is not much difficulty in understanding both terms. Many people know that port or terminal is a gate that signals go into/out of a circuit. Yes, they mean that!

Then, a question might arise if there is any subtle difference in them because typically rf or high frequency circuits' in and out part we call a port, otherwise, typically in low frequency or typical analog circuit designers use a terminal for the same meaning. For example, if you refer to the famous CMOS IC design (Razavi's) book, in chapter 8 - feedback, the author mention about impedance transformation due to feedback effect calls terminal modification, not port modification.

Why designer in high frequency circuits call a port circuit's in and out and why low frequency circuit designers call it a terminal?

Of course, there is a subtle difference between two terms.

Let's find out what is the difference.

In RF signal measurements, we cannot simply put a probe to a circuit to measure voltage or current. This is because the wavelength of the signal to be measured is small so that its phase/amplitude changes depending on the length of the probe and its wire. That's why it is difficult to measure rf signal with a conventional oscilloscope.

The reason that a port is used in high frequencies is that we need to define signal and ground condition accurately. Due to phase shift sensitivity of a short wavelength, we need to define signal and ground condition all way such a way that they need to be defined exactly. In typical low frequency analog circuit, wavelengths are long engough it does not matter you use relatively long probe wire or not. For example, if a wavelength of 300m and your probe is 30Cm long, the phase change due to probe length etc is not an issue. However, if a wavelength is 13mm, and your probe has 30Cm long, depending on its actual length, ie. 29.3Cm or 30.21Cm, will affect the oscilloscope measure amplitude (due to phase shift). Importantly, at high frequencies, due to short wavelengths, it is critical to define signal and ground condition accurately.

From the arguements above, I can now say that a port is one that has well-defined signal and ground condition and a terminal is relatively not well defined on them. For example, a coaxial cable is two conductive material running through across the cable and one is defined for a signal and the other is for ground. Both signal and ground lines go together always so that they are well defined anywhere we use them. However, a wire can be used for a terminal connection. If we just use a single wire to communicate a signal between external and internal part of a circuit, we are automatically assumed that the ground potential is well-defined so that we can just pass a signal without much attention to where the ground potential is defined. Therefore, just a wire to transfer a signal is enough to deliver energy to a circuit.

It maybe a subtle and quite often they are being used interchangeably. But, it is good to know!