Agilent Technologies J-BERT N4903B, Руководство пользователя, Страница: 175 / 572

N O T E
The “frequency” is the modulation frequency, and “amplitude” is the tracking of
the PLL.
Data rate and loop parameters
The data stream contains multiple frequencies, and the CDR needs to know the
expected data rate. The user entered information is necessary to lock the CDR, and
to detect a loss in lock condition. The entered frequency should be accurate, and
better than 100 ppm.
N4903B provides a CDR which has additional input parameters: loop bandwidth,
transition density, peaking, and SSC deviation.
Loop bandwidth is the input parameter to set the characteristics of the loop.
Additionally, the Peaking can be chosen from a drop down list.
Transition density affects the loop parameters, and it must be either entered, or
measured. Some standards define a loop bandwidth for a specific transition
density.
The SSC deviation parameter widens the window for loss in lock detection, and
detunes the center of detection, because SSC is generally asymmetric (down-
spread).
N O T E
CDR is not available together with Bit Recovery Mode (BRM) at data rates above
11.5GHz.
Threshold
The regular threshold voltage is not only used to determine the optimum sampling
for the data, but also to perform measurements such as eye diagram or output level
measurements, it is not possible to use it for the clock recovery.
For this reason, the clock recovery circuitry has it's own comparator for the
incoming data. This comparator also needs to know the threshold voltage (0/1
decision threshold).
Setting up the Error Detector
5
Agilent J-BERT N4903B High-Performance Serial BERT 175