GE L60 Instruction Manual page 616

OVERVIEW CHAPTER 9: THEORY OF OPERATION
The third type of distortion is pulse asymmetry. Modern carrier sets claim to be free of this problem, but historically it has
been observed that either the mark or the space signals were extended at the receiving end compared with the originally
sent signal. Distinction between the delay and asymmetry is relatively straightforward: if the rising edges and the falling
edges of the transmit and receive signals are spaced by the same period of time, one deals with a straight delay. If the
spacing is different between the rising and falling edges is different, pulse asymmetry takes place on top of the delay. In
this case, one of the numbers is labeled as delay, and the difference with respect to the other number is labeled a pulse
asymmetry. Both need to be entered as settings in order to deal with this distortion.
The following figure presents two cases of this channel distortion. For the extended mark, the falling edge must be shifted
forward in time (accelerated). For extended spaces, the rising edge must be advanced. If not corrected, the pulse
asymmetry renders the system unusable for distortions longer than one quarter of a power cycle. This problem shows the
advantage of modern DSP technology. Assuming that the signal can be impaired by short lasting noise, it is very difficult to
perform this correction accurately in the analog world.
Figure 9-25: Pulse asymmetry and correction (channel delay omitted for simplicity)
The following figure illustrates the alignment algorithm. It shows local current, received RX voltage, and the remote pulse
aligned with the local pulses accounting for the channel delay setting.
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9-34 L60 LINE PHASE COMPARISON SYSTEM – INSTRUCTION MANUAL