Using Modulated Carriers
C–2
Copyright © 2006 Harmonic Inc.
The transmitter has a higher chance of being clipped since modulated carriers
reach peak value occasionally, and there is a possibility that the modulated
carriers will line up in time and cause clipping. Clipping is not preferred for
transmitters with high digital loading.
Objective 2
When following Objective 2:
•
The goal is to achieve the same peak carrier RF value, so that the CNR
remains unchanged compared to unmodulated carriers.
•
Average CSO and CTB will be better than for unmodulated carriers,
because the average channel’s peak RF level is 3 dB lower for the
modulated carriers.
This objective can be achieved by setting up the transmitter with modulated
carriers in video mode when running Autosetup. See Figure C-1 on page C-3.
Remember that the modulated carriers are about 2 dB lower in average RF
power. This is what the transmitter’s built-in detector will determine. The
transmitter responds to this drop at the detector by setting the pad 2 dB lower
than the factory optimized setting. However, in this case, we know that the
peak value of the carrier will be 2 dB higher. To cope with this, Video Mode
adds 2 dB of pad and returns the peak carrier back to its original level. Thus,
the pad value returns to its original value, and the CNR returns to the same
value as in the unmodulated carrier case.
With a PWL 4100S transmitter set up this way with modulated carriers, the
average power is 2 dB below the setup for carriers without modulation. The
probability of clipping the laser is reduced. This provides better capability to
add digital loading, above the 550 MHz video band.
Conclusion
Figure C-1 on page C-3 is a diagram of the Autosetup results. Using
Autosetup to achieve one of the two objectives described above will satisfy
most applications.
However, Autosetup will not satisfy all applications. You also need to fine-
tune the transmitter to fit your specific requirements and preferences
precisely. You can accomplish this by adjusting the pad manually after
following the Autosetup procedure. See Chapter 4,
.
