Tektronix 071-0590-00 Consideration In Determining The Eye Data Width, Delay Specifications

Application Note - Auto Search Synchronization

Consideration In Determining The Eye Data Width

In most circumstances a test signal data eye displayed on an oscilloscope will appear to be larger than that reported by the GB1400 Receiver. This is expected and is due to several factors. One is that low error rates are virtually impossible to see on an oscilloscope. Other factors such as set-up and hold time and signal characteristics will all have an effect on the measurement by decreasing the apparent eye size.

The value of an eye width measurement made on the GB1400 is when it is used in a relative manner. If a device is determined to be working correctly using a known good signal, its eye width can be measured on the GB1400 Receiver. That measurement can be used as stable reference standard against which all other devices can be measured. It is not possible for the GB1400 Receiver to duplicate all characteristics of the actual device that will be connected in its place, but it will usually suffice as a reasonable approximation.

Consideration In Determining The Data Eye Center

In theory, a plot of bit error versus delay setting will show a smooth curve, almost linear, which has no aberrations and transitions from horizontal to almost vertical at the data eye crossing. For a signal with this curve (a clean signal with no aberrations, jitter or wander), the measurement of the data eye center will be consistent using either of the methods.

In typical applications, the method which will assure a repeatable measurement of the data eye center is to measure each and every delay setting for a significant number of data bits. Because of probable aberrations in the data signals, measuring coarsely over the delay (using 70pS steps) MAY lead to inconsistent measurements due to the aberrations being seen during one search and not the next.

Delay Specifications

The GB1400 programmable delay has the following nominal specifications:

Range:	0 – 3.99 Ns
Resolution:	5 pS
Accuracy:	+/- 20 pS

GB1400 User Manual 2-45