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144 Bu il d Y o ur O w n Q u a d c o p t e r

Figure 6.18 Three-motor-lead waveform at 100% power level.

I decided that it would be useful to capture a waveform showing the simultaneous
power pulses for all three motor leads. This waveform is shown in Figure 6.18 with the
motor operating at a 100% setting.
My careful inspection revealed that all three power pulses repeat every 750 µsec or
approximately 1.333 kHz. I was a bit curious to know if I could relate that frequency to the
audible noise emanating from the motor. I downloaded a free app to my smartphone that
provided a time versus frequency spectral analysis. Figure 6.19 shows the phone screen with
the spectral plot recording the motor noise when the motor is operating at 100% power.
Many spectral lines are present in the plot, including one at about 1.4 kHz. The strongest
one is around 2.8 kHz, which is the second harmonic of the 1.4-kHz frequency. From my

Figure 6.19 Smartphone audio spectral plot.