User guide

8
BER Tester with Agilent N4982A Clock Recovery Unit
The N4962A serial BERT 12.5 Gb/s can also be used with a broadband clock
recovery unit, like the low-cost Agilent N4982A clock recovery unit. This
“AnyRate” CRU will recover a clock from a PRBS bit stream from 622 Mb/s to
13.5 Gb/s—perfect for use with the wide operating range of the Agilent BERT.
By recovering the clock from the DUT output, the BERT can accommodate
tests where polarization mode dispersion (or other physical effects) can
change the period of the data. This is illustrated in Figure 9.
N4982A CRU
LF TrigO
HF TrigO
JitterI
EXT CKI
TX CKO
IN
~IN
~OUT
OUT
Splitter
Out
Out
In
DUT with PMD
RX CKI
RX CKO
TX CKI
N4962A serial BERT
12.5 Gb/s
CLK output
~CLK output
Data input
E/O
O/E
Figure 9. Block diagram – BER measurement with Agilent clock recovery unit
A passive power splitter is required to duplicate the output signal of the DUT to
feed the BERT and CRU input. If the output from the DUT is severely degraded
or very low in amplitude, a buffer or retiming stage may be required to boost the
amplitude to meet the sensitivity of the BERT and CRU.
If the DUT has a differential output, one side should be delivered to the BERT
IN, and the other side should be connected to the CRU input. This will eliminate
the power splitter from the block diagram, and will allow operation at lower DUT
output levels. This is shown in Figure 10.