User guide

transition density. In such a case enter the value given in the specification, so that
the CDR behaves according to the standard. If a standard from the preset list is
selected, this field is preset.
Loop Bandwidth
This is the range of the CDR loop bandwidth. In this field the user should enter the
loop bandwidth value; the range is within 100 KHz to 12MHz. If a standard from
the preset list is selected, this field is preset.
When the BERT error detector is used to characterize a data stream instead of a
receiver, the loop parameters should be set according to the used standard.
If the CDR is used to recover the bit stream from a receiver to be characterized due
to a lack of a clock output, choose the loop bandwidth significantly higher than the
receiver’s bandwidth. To characterize a DUT’s CDR it is the best practice to use
its recovered clock output instead of the BERT’s built in CDR. Choose a low loop
bandwidth to measure the jitter on an incoming data stream as the CDR will track
the incoming jitter up to the loop bandwidth and thus make it invisible to the error
detector.
Fine Adjust
Fine Adjust allows you to tweak the response of the CDR according to transition
density changes. This control can be used to reduce jitter in pattern with high
changes in transition density.
Peaking
The drop down list provides a maximum of three peaking values. If a standard from
the preset list is selected, this field is preset. A low value reduces the peaking near
the loop bandwidth while, a higher value improves the SSC response.
Auto Threshold
If the Auto Threshold option is selected, the Serial BERT continuously analyzes
the incoming data stream to detect the voltage level that is optimal for the
derivation of the clock signal.
This function is recommended for all applications where a valid data stream is
continuously arriving at the error detector.
For measurements with partly invalid data, such as burst tests, this function may
cause a clock loss. In such cases, deactivate Auto Threshold and
• enter a fixed voltage level manually, or
Setting up the Error Detector 5
Agilent J-BERT N4903B High-Performance Serial BERT 181