Comark EXACT-V2, Руководство пользователя, Страница: 27 / 94

USER MANUAL
MPD-1611291-A
EXACT-V2 High End Rack Modulator / Exciter
3/10/2017
MPD-1611291-A
Page 27/94
The “RF Maintain” feature consists in keeping an RF signal presence out of the
modulator in order to allow a constant power at the modulator RF output during an
input stream error (the function “RF Maintain on Input Stream Error” shall then be
enabled).
The overall objective is to avoid any RF signal interruption at the output of the
modulator for protecting the amplifier.
When the “RF maintain” feature is selected, specific data is not broadcasted during the
switching time from one input stream to another. A PRBS-like signal is used to maintain
the original RF signal and RF power level. Thus the broadcasted signal cannot be
decoded by any receiver.
2.7.2 Crest Factor Reduction Management
PAPR (Peak to Average Power Ratio) issues are associated with high power peaks
present in the signal. By correctly managing PAPR reduction (Crest Factor reduction),
the user will be able to avoid high power peaks from the modulator and so increase the
output power level from the amplifier. It will then allow the best coverage for a given
transmission site. However, this PAPR reduction feature will decrease MER performance
as well as shoulder levels. It must be used carefully after studying the following
explanations.
The PAPR solution uses two clipping modules for the best efficiency. Figure shows the
location of the two modules as well as the user parameters.
Figure 11: PAPR block diagram
2.7.2.1 Crest Factor Parameter
In order to reduce these peaks and avoid damage to the amplifier, the exciter uses a
“crest factor” parameter. It is used to set the clipping level of the signal. The range is