I have a PCB board with the stackup shown below. I want to calculate what would be the impedance (and capacitance too?) of a 5" PCB trace on the L1 layer.
How to calculate it?
For example, I am trying to use this calculator (https://www.eeweb.com/toolbox/microstrip-impedance/), what should be the value of the fields?
trying to figure out how I can replace a 4" trace with an equivalent RLC Circuit.
First choice: Don't. Use the 'tline' element in LTSpice instead.
Second choice: You can model a transmission line with a sequence of pi or T sections. If you use a different transmission line calculator, for example the Saturn PCB one, or this online one, they will output capacitance and inductance per unit length in addition to the characteristic impedance.
Say your calculator says the capacitance is 1.7 pF per inch and the inductance is 12.2 nH per inch. Then you can break up your 4" trace into as many sections as you want. Each section of length L (for example L=0.4" if you break your 4" trace into 10 sections) will have 1.7 \$\times\$ L pF and 12.2 \$\times\$ L nH, arranged in either a pi or T circuit.
Keep solving your circuit with this set-up, increasing the number of sections each time, until your results stop changing (this step is why it's easier to just use a built-in model than to try to make an LC model yourself). The higher the frequencies present in your circuit are, the more sections you'll need to achieve an accurate model.
ltline, even if it tends to be sluggish given certain conditions. The tline is a rather simple approach, and it overshoots easily, even in ideal conditions: try, for example, a current source with pwl 0 1 0 1m, two or more tlines with Td=1 Z0=1 and a terminating 1 Ohm resistor. Plot all the intermediary voltages and you'll see what I mean. Alternatively, it might be even better to model your own RLC, even at the cost of extra elements/nodes, because then you can also model coupled inductors. Of course, there will be a penalty... \$\endgroup\$