Application Note: High-Speed Testing

By IMRA America, Inc.

As circuits and electronic components become increasingly faster, their speed is beginning to outstrip the capabilities of conventional test and measurement equipment. For example, telecom vendors are developing 40 Gbit receivers and chip sets. Typical sampling oscilloscopes have 50 GHz bandwidth, giving system-limited rise/fall times of ~ 17 psec. While this is fast enough to give an eye-diagram for 40 Gbit logic, it is inadequate for detailed measurement of state transitions, whose fine details can affect circuit performance and bit-error-rates. Measurement systems with at least 5-times higher bandwidth would greatly benefit better circuit designs, fault detection, and diagnostic capabilities. The question arises: What kind of test equipment will be used to test the newer and faster circuits?

Photoconductive (PC) sampling and electro-optic (EO)sampling are proven technologies for measuring extremely high speed electrical waveforms, giving effective bandwidths up to the terahertz (1000 GHz) regime. What is more, EO and PC sampling probes have been demonstrated to give very little loading of the circuit being measured, making these methods superior in terms of being non-invasive.