LeCroy SDA, Руководство оператора

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SDA Operator’s Manual
SDA-OM-E Rev H
383
Bit Error Rate
The SDA measures bit error rate directly on the captured bit stream by using the recovered clock
to sample the waveform, and a user selectable threshold. The data are assumed to be NRZ so a
high level is interpreted as a “1” and a low level is interpreted as a “0”. The bit stream that is
decoded in this process is compared, bit-by-bit with a user-defined known pattern. Since the
instrument does not have any information as to which bit in the pattern it has received, a
searching algorithm is used to shift the known pattern along the received data until a match is
found. A match is determined when more than half of the bits are correct for a given shift of the
known pattern. No match can be found if the bit error rate is over 50%, or if the wrong pattern is
selected. In this case, the bit error rate will indicate 0.5 meaning that exactly ½ of the bits are in
error which, of course, is the worst case.
Bit Error Map
A further level of debugging is available through the bit error map. This display is a view of the bit
errors in the data stream relative to any framing that may be present in the signal. There are
several options for framing that may be set. The general form of the data signal is shown here.
The header portion is a fixed pattern that can be set to any pattern. The header must be one or
more bytes if it is present. The software searches for the header and treats the bits between
headers as a frame. Each frame is displayed as a line of pixels in an x-y map and each
successive frame is displayed below the previous one in a raster fashion. Bit errors are computed
only on the payload sections of the data stream.
Framing can also be defined by a specific number of bits without a header. An example of this is
a pseudorandom bit sequence (PRBS) of a specific length, 127 bits for example. In this case,
setting the frame size to 127 will display one repetition of this sequence per line of the error map.
Bit errors are displayed in a lighter color, whereas non-errors are shown in dark blue. By
displaying bit errors on a frame-by-frame basis, pattern dependent errors can be clearly seen as
lightly colored vertical lines in the error map.