025-000554 REV X7
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INPUT DRIVE vs. CHANNEL LOADING
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. The following charts show 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. Figure 2 below is for the OTOT-1000-3 and OTOT-1000-6 models.
Figure 3 on the next page is for all other optical power levels.
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 / +17.6dBmV input point on the graph.
Some systems use an OMI meter to set laser modulation. The OTOT-870’s & OTOT-1000’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.
REDUCING LINK GAIN
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.
FIGURE 2 - INPUT LEVEL vs. CHANNEL LOADING - LOW POWER UNITS
