How do I incorporate output and input impedances connected to my circuit?
First of all you should think inn terms of equivalent circuits of the sources, i.e. Thevenin circuit. So, your TTL source has in principle a 0-5 V signal excursion (although there are internal voltage drops and limited capabilities to go rail to rail), but combined with a series impedance. The circuit is indeed non-linear and we need to simplify a bit: a voltage source with ideally a square wave and a series resistance is suitable. Which value for the resistance: it depends on the TTL family, and considers also that the values are in reality different for 0-&gt;5V ad 5-&gt;0V transitions. See past post o this site: Input and Output Impedance of a TTL NAND Gate
A good guess may be at about 100ohm.

The you draw the circuit as a single-line combination of components, all in series. Take into consideration that there are series resistance, but also shunt resistance to ground.

The load is 75ohm, and if you connect the source and the load with a cable, or a log transmission line, this must have 75 ohm characteristic impedance to avoid reflections.

The network you put in your box should be better detailed. It will have an input impedance (resistance), that &quot;sees&quot; the source and its more-or-less 100 ohm, and an output impedance that &quot;sees&quot; the 75 ohm load. Somewhere there is the cable or transmission line, possibly between the network-in-the-box ad the load. So that network should also be approximately matched to the 75 ohm at its output. For the matching at the input, if you do not have a feeding line but the source is directly connected, you might have some voltage drop but no serious reflections.