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).
The threshold voltage can be derived from the input signal via a low-
pass filter. This will work fine for most applications. But applications
that do not provide a continuous data stream at the input (for example,
any application using bursts) cannot use this low-pass filter, because
the threshold voltage will drift from the correct level when there is no
input. In such cases, the threshold can be specified manually. It is then
no longer derived from the input signal (see the following figure). The
manually set threshold voltage must of course be within the input range.
The difference between the data path and the CDR path is that the
comparator of the CDR is always single-ended. Thus, this comparator
always needs a threshold voltage that lies between the high and low
levels of the incoming signal.
The differential threshold of the data path comparator has no relation
to the single-ended threshold of the CDR path comparator. This means
that in differential mode, the two thresholds will be different and in
single-ended mode (either normal and complement) they will/can be
equal (except during measurements).
The following figure shows a simplified block diagram. It does not reflect
the different input modes (especially the differential case), but it
matches both single-ended cases.
Low Pass
Filter
Threshold
Voltage
Threshold
Voltage
DC tracking on/off
switches
Input
Connector
Comparator
Comparator
CDR
Data
Processing
recovered
Clock
Bit Rate Range
The Serial BERT provides bit rates from 150Mbit/s up to 12.5 Gbit/s,
depending on the instrument's options.
However, several specific properties and limitations need to be taken
into account when working at low bit rates. The limitations apply to the
instrument according to the following hysteresis curve:
Setting up the Error Detector
4
Agilent J-BERT N4903 High-Performance Serial BERT
119
Summary of Contents for J-BERT N4903
Page 1: ...S Agilent J BERT N4903 High Performance Serial BERT User Guide s Agilent Technologies...
Page 68: ...2 Setting up Patterns 68 Agilent J BERT N4903 High Performance Serial BERT...
Page 158: ...4 Setting up the Error Detector 158 Agilent J BERT N4903 High Performance Serial BERT...
Page 314: ...6 Evaluating Results 314 Agilent J BERT N4903 High Performance Serial BERT...
Page 374: ...7 Jitter Tolerance Tests 374 Agilent J BERT N4903 High Performance Serial BERT...
Page 394: ...8 Solving Problems 394 Agilent J BERT N4903 High Performance Serial BERT...
Page 434: ...Index 434 Agilent J BERT N4903 High Performance Serial BERT...
