CANOGA PERKINS 2270 Fiber Optic Modem - Manual (Page 22)

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Canoga Perkins
To accomplish this, the SCT POL jumper needs to be changed. The position set at the factory is
NORM, which assumes a negligible delay in the cable and DTE user device. The INV position can
improve performance if that delay is an odd-multiple of one-half a clock period. Using an oscillo-
scope, the SCT and TXD A-lead waveforms can be examined to confirm that the falling edge of the
clock does not occur close to the changes in TXD. If this edge is too close to the data transitions,
the INV position must be used.
NOTE: This will not change the SCT to TXD phase relationship viewed on the interface,
but will compensate for the phasing problem.
2.7.7.2 SCT Returned On External Clock Leads
If the DTE is capable of returning the transmit clock (supplied by the modem) back to the modem
on the SCTE leads as an external transmit clock, better clock data phasing can be achieved at the
transmitter.
This can result in a significant improvement at higher data rates. Close switch position 4 on the
front panel for the modem to use the incoming clock for the transmitter and check the cables to be
sure they support SCTE.