Connect-Tek USB-2 Manual de usuario descargar pdf (Pagina 7)

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Understanding and Performing USB 2.0 Physical Layer Testing and Debugging

Monotonicity Test

When performing a USB 2.0 high-speed compliance test,

a developer needs to verify that the signal under question

is monotonic. Monotonicity veriﬁes that a transmitted signal

should smoothly increase or decrease in amplitude without

deviation in the opposite direction. Non-monotonic signal

behavior is caused by metastability, high-frequency noise and

jitter problems in a circuit. Figure 7 compares a monotonic

signal with a non-monotonic signal using a USB 2.0 high-

speed signal with a rise time of 500 ps.

Test Equipment

To verify monotonic behavior of a signal, the oscilloscope

used should have a sample rate high enough to capture as

many sample points as possible on a rising or falling edge.

In addition, the oscilloscope should have enough bandwidth

to ensure that the high frequency non-monotonic transition

is not attenuated. Hence, an oscilloscope with a sample rate

of 10 GS/s and a bandwidth of 2 GHz or higher, such as the

MSO/DPO5204, DPO7254 or DPO7354, is the ideal tool for

monotonicity testing.

The monotonicity test for a USB 2.0 device is veriﬁed during

test packet examination. The option USB compliance test

package captures the test packet and examines each rising

and falling edge for monotonic behavior, as shown in Figure 8.

Set-up uses the high-speed signal quality test conﬁguration,

as illustrated in Figure 9. The TDSUSB2 compliance test

package, coupled with a high-performance oscilloscope,

automates this process and ensures repeatability of test

results.

Monotonic Signal Non Monotonic Signal

Equipment Set-up

with Application Running

Differential Probe

Test Fixture

Device Under Test

Win 2K PC

Port

Software

Test

DUT

Gnd

J32

Init

J33

D+ D-

Recep

Figure 7. Illustration of monotonic and non-monotonic USB 2.0 high-speed signals

with a rise time of 500 ps.

Figure 8. The option USB compliance test package captures the test packet and

examines each rising and falling edge for monotonic operation.

Figure 9. The monotonicity test set-up uses the high-speed signal quality test

conﬁguration.

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