QAM Marker
Signal Source
rev1.0.2 June 20, 2014
Broadcast Insertion Considerations
For maintaining maximum plant-wide shielding effectiveness, the QAM Marker signal must be
available throughout the coaxial distribution network. For that reason, installation strategy is
important.
1. A single QAM Marker may serve, if the plant is configured to optically transport the broadcast
spectrum from a single location (the headend) with no further electrical conversion before
each fiber node.
2. Multiple QAM Marker units may be needed, if the headend optical output serves multiple hub
locations with recombined signaling or remapped channel plans. The QAM Marker output
signal must be combined with all other broadcast signals at the input to the final laser.
Directions in the document are based on the assumption that channels adjacent to the QAM
Marker (16 and 17 for 138 MHz operation and 88 and 89 for 612 MHz operation) are QAM
channels. If either of the adjacent channels is an analog carrier the QAM Marker must be
operated in manual mode. Manual mode configuration is covered later in this document.
In the automatic mode, the QAM Marker adjusts the marker signal level relative to the RMS
power level of an adjacent QAM channel. When the difference in power levels between the
adjacent QAM channels is less than 3 dB, the QAM Marker will use the channel with the lower
RMS power level as a reference. When the difference in power is greater than 3 dB, the QAM
Marker will use the channel with the higher power level as the reference.
The QAM Marker automatic leveling feature references on its Combined Input port the
simultaneous availability of the QAM Marker output signal, and either one or both QAM signals
adjacent to the selected output frequency. IMPORTANT: The Marker OUT port of the QAM
Marker is limited to ONLY the Marker test signal. The QAM Marker only references the
adjacent QAM signals and does NOT forward the payload of the adjacent QAM signals.
Where this combination (Marker output signal and either or both adjacent QAM signal) is
available at an open combiner port feeding the broadcast laser, or where that combination is
available on the laser output test point, the QAM Marker can be operated in the automatic
leveling mode. Note that the Combined Input port QAM signal level must be between -20 and
+30 dBmV. If the input levels are too low, an amplifier with an appropriate gain may be used and
if the input levels are too high, an attenuator may be used.
If the headend configuration cannot supply the required combination, it is quite likely that the
automatic leveling feature cannot be used and the QAM Marker output level will fall within the
unused laser output spectrum above the broadcast channels. The QAM Marker level must be set
manually based upon the nearest adjacent QAM signal amplitude.
The manual output range is -10 dBmV to +35 dBmV. After installation of the QAM Marker the
output level should be checked relative to the adjacent QAM channel levels at a convenient field
test point, such as a node electrical output. If the QAM Marker is not 30 dB (+0.0, -2.0 dB) below
the adjacent QAM signals the output should be adjusted. Care taken at the headend during initial
set-up will minimize any further adjustment.
