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Application Note - Auto Search Synchronization
GB1400 User Manual 2-45
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