How do you use Smith charts and S-parameters to design matching networks for your PCB?

The Smith Chart takes the infinite X Y plane of real and imaginary impedances and maps them into a finite circle. This allows us to view all the impedances possible in a finite circle, rather than an infinite plane. After all it is hard to find infinite plotting paper. The center point of this circle is the normalized real impedance, usually 50 ohms. Zero impedance, a short, is mapped to the center left and infinite impedance, an open, is mapped to the center right.

The left side of the chart represents ZL = 0, or a short circuit. The article mentioned it as the bottom side which I think is not correct. there are several online free tools that depict smith chart and can help during impedance matching

S12 and S21 are described backwards. S21 is the transmission from input to output, vice versa for S12. S-parameters and all other network parameters can use the ABCD parameters (transmission parameters) as their basis. This is most commonly done with transmission lines. These parameters are typically used as the intermediaries for other network parameters and the transfer function for a multiport network. One good reason for doing this is that they allow direct cascading and they are generalizable to any reference impedance without a renormalization procedure, which is not the case for S-parameters. I seem to recall Agilent doing this in their software (before it was Keysight).

To get the maximum energy transfer from one network into another network we need to match the output impedance of the first network to the input impedance of the second network. At DC it is easy, but when we add the complication of non-zero frequency we have reactive impedances to deal with. As it turns out we want the two complex reactive impedances to cancel, +jX - jX = 0. This is a neat trick while using complex numbers as both the real and imaginary values are changing at the same time. The Smith Chart comes to our rescue as we can do the necessary calculations graphically.

I have never heard of matching networks composed of "transistors and active amplifiers". Active components are usually the thing being matched, not the matching components themselves. Frankly, this AI generated article is a word salad of RF jargon. The Microwaves 101 article on Smith charts is vastly better written, as it provides intuition, history, context, and applications of the smith chart. Plus, nowhere does this article discuss the PCB. I would have expected at least an example of how to match a network on a real PCB, or how the substrate Dk affects trace impedance. This is just another example of a subpar AI generated article that adds no value, just noise.

Nonlinear/active components used for impedance matching? Never heard of this, usually you are matching an active component to hit a target impedance. Also for nonlinear components (amplifiers/transistors running near saturation), you want a bit of intentional mismatch in order to maximize power transfer.

To those saying active components can't be used for impedance matching, you absolutely can in RFIC design. Common-gate inputs for LNAs can inherently match the source, since the input impedance is 1/gm. RFIC designers have control of the transistor size and can accordingly adjust its parameters.

It is true that S-parameters can be used to design matching networks in a circuit design sense. Unfortunately, circuit design starts to break down in the GHz range and you will find that the circuit you design with an optimization algorithm does not function as you expected. This is because the small traces between the components, their mounting pads, and nearby ground will change the impedance of the network. This is because the parasitic capacitance and inductance of traces/pads will become similar to the component values in the matching network. So, the only way to continue improving the network is to tune the component values while monitoring with a spectrum analyzer or by monitoring S11 (if using a test board to tune the design).

I feel like LinkedIn is just using actual human experts in an effort to correct all this AI-generated gobbledygook, but they don't really value our hard work. Oh well... I see that this topic is categorized under PCB Design, but nothing was mentioned about the PCB specifically. The biggest deviations in impedance occur from parasitic capacitance/inductance in the PCB, especially at the higher frequencies where impedance matching becomes most important. Of course an AI generated article would miss this, but this is where your perfectly designed and simulated impedance matching network will fail on a real PCB, and if you don't know how to account for those parameters in a circuit model then you are just guessing in your design.

Typically, RF impedance matching involves adding lumped passive components such as LC matching networks or a pi pad. Upper part of Smith chart is inductive and lower part is capacitive. By combining shunt and series L and C elements, any value of the load impedance on Smith chart can be matched. Performance of impedance matching network should always be verified in practice with a Vector Network Analyser (VNA) because simulated values will likely require adjustments due to physical differences between ideal models and physical parts.

Where are the sources cited for the referenced online tools? This is the sort of information that AI generated content needs to improve upon.