Connect-Tek USB-2 Manuel d'utilisateur télécharger pdf (Page 9)

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Understanding and Performing USB 2.0 Physical Layer Testing and Debugging

Selecting Tools for USB 2.0 Physical

Layer Testing

Real-Time Oscilloscope

A real-time oscilloscope is the most crucial test instrument

for USB 2.0 measurements. When selecting an oscilloscope

for these measurements, it is important to consider the

oscilloscopes rise time, bandwidth and sample rate. The

following section deals with the required performance

characteristics of the real-time oscilloscope.

Effect of Oscilloscope Bandwidth/Rise Time on

Measurement Accuracy

Rise time needs of the oscilloscope depend closely on the rise

times or slew rate of the signals to be measured. The following

empirical formula gives the relation between measured rise

time [RT(measured)], oscilloscope rise time [RT(oscilloscope)]

and signal rise time [RT(signal)];

RT(measured) = [ RT(signal)

+ RT(oscilloscope)

]

The following table illustrates the variation of percentage error

versus the ratio of oscilloscope rise time to signal rise time,

based on this relationship.

Rise/Fall Time vs Oscilloscope Bandwidth and

Rise Time

* Based on a signal with a 500 ps rise time.

When the oscilloscope rise time speciﬁcation is ﬁve times

that of the signal, the error decreases to 1.8%. However,

lower oscilloscope rise times would signify higher error in

measurements with respect to signals. Therefore, in order to

measure a signal with a rise time of 500 ps, the oscilloscope

used should ideally have a rise time of 100-180 ps, like a

DPO7254.

Figure 11. DPO7254 digital phosphor oscilloscope.

Bandwidth (GHz) Rise Time (ps) Measured Rise Time* % Error

4 100 509 1.80%

3 120 514 2.80%

2 180 531 6.20%

1 340 604 21%

1 400 640 28%

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

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Connect-Tek USB-2 Manuel d'utilisateur Page 9