Themes
Every model has an edge
The ideal amplifier, the small-signal transistor, the lumped element, the capacitor: each is excellent inside a range and wrong outside it, and the range is a number rather than a warning. No figure here is drawn without the frequency, amplitude or size at which the model in it stops being true.
Two routes to a number
A phase margin from the loop gain and an overshoot from the step response. A transfer function from the matrix and the same one from the poles it was factored into. A ladder solved by nodal analysis and by a chain-matrix product. Neither route in any of those pairs can confirm itself, and they share no arithmetic.
The schematic is a label
Two drawings of the same circuit with different placement are the same circuit, so the layout carries no information — which is exactly not true of a mechanism, where the geometry is the content. Schematics here are small, drawn in one hand, and put in a corner. The canvas belongs to the response.
Refused, not extrapolated
A network with no path to ground has no answer, and a solver that returns one is lying. A model asked to work outside its range declines and says why. Half the value of an assertion is what it rejects, so every refusal on this site was produced by running it rather than by describing it.
The parasitic is the component
A capacitor is a capacitance, a resistance and an inductance, and above a computable frequency the third one is the whole part. A source is an electromotive force and a resistance. The thing nobody draws is usually the thing that decides the answer, and it is always a number.
Measured, not tabulated
Butterworth is flat, Chebyshev is steep, Bessel has good delay — three adjectives and no quantities, in the most reproduced table in the subject. Every comparison on this site is taken off a solved network instead, which is how the third column, the one that decides whether a filter can pass an edge, turns out to have been missing.
The constants decide
A divider's ratio does not predict what happens when something is connected to it — its magnitude does. A filter's family does not say how much delay distortion it costs — a measurement does. The quantity that gets discarded to make a rule memorable is repeatedly the one that decides the outcome.