... plane:¹

In mechanics, $\zeta\mathrel{\stackrel{\scriptscriptstyle\mathrm{\Delta}}{\scriptstyle=}}\alpha/\omega_c$ is called the damping ratio of a mass-spring-dashpot resonator.

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... loop.²

See, for example, http://www.analog.com/static/imported-files/tutorials/MT-223.pdf.

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