Olson Technology OTOT-1000C-9-FQ - Manual (Page 7)

that indicates proper modulation. For the factory test condition, this is +10dBmV at
547.25MHz. For other channel loadings, consult the next section. Do not forget to
account for the loss in the connecting cable.
INPUT DRIVE vs. CHANNEL LOADING
REDUCING LINK GAIN
The laser RF drive level is the primary determining factor of link distortion and CNR
performance. The basic limitation on input drive is total input power. Figure 2 shows the
approximate input levels versus channel loading. The left axis shows the nominal RF
input level. The unit will work with levels within ±2dB of this value. The right axis shows
the test point reading for optimum modulation. Most systems run their digital channels at
6dB below the analog channels. This is a very convenient level for calculating loading.
At 6dB down, simply divide the number of digital channels by 4 and add to the number of
analog channels to get the total equivalent analog channel loading. The factory test
input is 77 analog channels with 42 digital channels at 6dB down. The number of
equivalent analog channels is 77 + 42/4, which equals 87.5. This is the 87.5 ch /
+18.6dBmV input point on the graph. Some systems use an OMI meter to set laser
modulation. The OTOT-1000C-FQ’s have been individually adjusted for optimum
performance. Setting all units for the same OMI, instead of using the test point, will result
in reduced transmitter performance. If the channel loading is less than 40, you may
decide not to increase the levels by the maximum possible amount. This will provide
improved distortion at the cost of CNR. The best rule of thumb is to use the maximum
possible levels for long haul links, and lower drive levels as the links get shorter.
Short links can have reduced performance due to receiver overloading. Reducing the
laser RF drive will cause reduced CNR performance. Attempting to lower the laser
output power will have the even worse result of degrading both CNR and IMD. The
correct solution is to lower the receiver input level with an optical attenuator such as the
OTOA-1000. It is recommended that this be done at the receive location.