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For eye mask testing industry defined masks are compared to the

transmitter eye-diagram. Pass/Fail is quickly determined. Mask margins

can be automatically determined. Eye mask test to industry defined hit

ratios (a relatively new concept defined as the allowed number of hits

compared to the total number of waveform samples) is also automatically

performed. Eye mask tests are almost always performed using a reference

receiver. A reference receiver defines the entire measurement system to

have a specific low pass frequency response, the most common being a

fourth-order Bessel low-pass response with the –3 dB frequency at 75%

of the data rate.

For example, a 10 Gb/s reference receiver would have a 7.5 GHz band-

width. A reference receiver allows the waveform to be viewed closer to

what a receiver in an actual communications system would see.

Application overview

For any high-speed communications signal, the channel and basic signal

characteristics must be assessed for compliance with standards and

interoperability with other devices in the system path. Digital Communica-

tions Analyzers (DCA’s) based on wide bandwidth sampling oscilloscopes

are recognized as the industry standards for accurate analysis of optical

waveforms in R&D, device validation and volume transceiver manufactur-

ing. In addition to basic eye-diagram and pulse waveform characteriza-

tion, DCA’s perform advanced jitter analysis and channel impedance

characterization.

Transmitter compliance testing and eye-diagram analysis

Viewing the eye-diagram is the most common method to characterize the

quality of a high-speed digital transmitter signal. Industry standards such

as SONET, SDH, Fibre Channel and Ethernet rely on eye-diagram analysis

to confirm transmitter specifications. The eye is examined for mask

margin, amplitude, extinction ratio and overall quality. Tests are commonly

performed using a well defined reference receiver to provide consistent

results both in manufacturing test, incoming inspection, and system level

applications. Standards based reference receivers and test procedures are

built into the DCA’s to provide compliance test capability.

In these standard tests automatic histogram analysis determines signal

levels to derive key waveform parameters including but not limited to:

• Extinction ratio: How efficiently laser power is converted to

information power

• Optical modulation amplitude (OMA): A measure of modulation

power

•

Eye height and width: An indication of how open the eye is

• One and zero levels: The logic levels of the eye

• Signal to noise ratio: Signal strength compared to noise

• Duty cycle distortion and crossing percentage: A measure of eye

symmetry

• Basic peak-to-peak and RMS jitter: A measure of the timing stability

of the signal

Applications:

Communications Waveform Measurements

Eye mask tests are performed with a reference receiver based test system

Parameters are automatically derived from the eye-diagram

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Agilent Technologies N4970A Manuale Utente Pagina 10

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