Magnets flux density vs temperature
Because thermal motion randomizes magnetic domain, the flux density of magnets decreases with increasing temperature. NdFeB magnets has a temp co of dB/B/dT = -1.1e-3 /C.
Magnetostriction
The inductor humming is caused by magnetostriction, magnetic material changing shape in response to a changing magnetic field.
RS232 receiver
It is customary to use a RS232 driver IC to convert a TTL signal to RS232 signal level. The RS232 driver IC includes a charge pump to generate the required negative voltage. But if we look at the RS232 receiver spec, we'll notice that the transition actually happens at positive voltage; for example, MAX232A RS232 input threshold low is typically 1.3V and threshold high is 1.8V. So it is possible to send digital output (0 to 3.3V or 5V) to a RS232 receiver IC and be decode correctly (even when it is outside of the RS232 spec).
Rise/Fall time measurement
The rise time of the first order low-pass step response is 0.35/BW. The measured rise time is the square root of the sum of the square of the signal rise time and the square of the scope rise time. This is only strictly true for Gaussian step and Gaussian impulse response, but it is a good approximation with the signal rise time is greater than the scope rise time. The scope frequency response can be well approximate by a second order low pass with a damping factor of 1/sqrt(2).
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