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The USB performs differential transmission of signals. To measure the
signal waveform, a differential probe is used. When a differential
probe is unavailable, the signal waveform between the signal and GND is
observed using an ordinary probe. The signal waveform between the
signal and GND is shown below. |
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* Click on any graph on the right to display a larger
version.
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For 1.5Mbps signal lines, even when the BLM21BD102SN1 (Chip Ferrite
Bead) with a large impedance (of 1 k-ohms at 100 MHz) is used, noise
can be suppressed with little waveform distortion. When noise of 100
MHz or less also causes a problem, the DLP31SN221SL2 (Chip Common Mode
Choke Coil) is used, having a relatively large impedance at these high
frequencies. |
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