Each pole costs 20 dB a decade and 90° of phase

Every additional real pole in a transfer function adds another −20 dB/decade of rolloff and, eventually, another −90° of phase lag.

Why it works

It’s why stacking poles (extra filtering stages, extra lag elements) steadily erodes phase margin — each one you add brings the phase closer to the −180° instability threshold.

Do it exactly

Estimate with the rule, then check it against the calculator that models it properly.

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