Connect-Tek USB-2 Manuel d'utilisateur télécharger pdf (Page 8)

Application Note

www.tektronix.com/usb8

Impedance Measurement Test

Due to the high signal rates of USB 2.0 High-Speed mode,

trace and packaging impedance have become critical

parameters. The USB 2.0 High-Speed speciﬁcation now

requires differential impedance measurements of cables,

silicon and devices.

The USB 2.0 speciﬁcation requires that the differential

TDR impedance step response be set to 400 ps. The USB

speciﬁcation deﬁnes the impedance limits referenced from the

DUT connector. In general, the impedance should be between

70 Ohm and 110 Ohm at a given distance from the connector.

Cables are also required to meet speciﬁc impedance limits.

These limits are 90 Ohm +/- 15%.

Test Equipment

The impedance measurement test requires a time domain

reﬂecto-meter, such as the DSA8200 digital serial analyzer

sampling oscilloscope with 80E04 TDR sampling module,

which offers unmatched TDR performance on up to eight

channels simultaneously. Add the IConnect product to convert

from TDR measurements to S-parameters.

Figure 10 shows a TDR measurement made with the

DSA8200 sampling oscilloscope. The Min and Max measure

within the tolerance speciﬁed by the USB differential

speciﬁcation of 90 Ohm +/- 15%.

Instrumentation Requirements for USB

2.0 Physical Layer Testing

USB 2.0 opens up a whole range of USB consumer

applications to make the PC a more user friendly and valuable

tool in the workplace and home. With any consumer product

opportunity, time to market is crucial. USB designers are

keenly aware that the correct tool aids in meeting schedule

objectives. Especially critical are the bandwidth, rise time and

sample rate of the oscilloscope, along with the test ﬁxture and

fully automatic test software.

USB 2.0 physical layer validation and electrical compliance

testing require a host of test equipment, as the chart above

illustrates.

Figure 10. TDR measurement made with a DSA8200 sampling oscilloscope coupled

with an 80E04 TDR sampling module.

Test

Equipment

Signal

Quality

Test

Inrush

Current

Check

Droop

Test

Receiver

Sensitivity

Test

CHIRP

Test

Impedance

Measurement

Test

Real-time

Oscilloscope

Y Y Y Y Y

Time Domain

Reﬂectometer

Data Generator Y

Test Fixture Y Y Y Y Y Y

Test Software Y Y Y Y Y

Differential

Probes

Y Y

Single-ended

Probes

Y Y Y

Current Probes Y

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 8

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