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The stressed eye's crest factor

By Martin Rowe, Senior Technical Editor- June 1, 2010

 Applying a high-crest-factor disturbance to a serial data stream lets you verify if a receiver can meet a 10-12 BER requirement.Download the Crest Factor paper and slides used at DesignCon 2010.

When you think of crest factor, you probably think of AC mains voltage, not serial data streams. Crest factor, defined as VPEAK/VRMS, applies to any AC signal, including the modulating signal that causes sinusoidal jitter. "A New Method for Receiver Tolerance Testing Using Crest Factor Emulation (CFE)," a paper presented at DesignCon 2010 by Ransom Stephens of Ransom's Notes and John Calvin of Tektronix, explains how to apply high crest factor to a data stream. The paper explains how you can stress a receiver under worst-case conditions and measure BER (bit-error rate).

Using an arbitrary waveform generator, you can apply low-probability, high-amplitude jitter at known points in a serial data stream. The problem of accurately measuring BER at 10–12 or 10–15 is that the test time required to properly evaluate the full effects of random events can range from 30 min to many days. High-amplitude jitter can occur at any time, so if you can apply jitter at sensitive points, you can cut measurement time and increase repeatability in your measurements. The CFE measurement technique can reduce test time to less than 1 min and is 100% deterministic in its ability to be reproduced from run to run.

To perform the BER measurement, apply a high-amplitude jitter signal to a bit transition where both the median ISI (intersymbol interference) and half the amplitude of the sinusoidal jitter specified in a serial-data standard occur. If the receiver fails to correctly identify the bit (BER > 10–12), then you can repeat the test and verify if the error is indeed caused by the receiver. You'll know if the failure was caused by ISI, periodic jitter, or random jitter.

Download Ransom Stephens and John Calvin's paper and slides used at DesignCon 2010.