MODULE SET-UP
ChromaFlex DMT24, DMT34 & DMT44 Multi-Wavelength DWDM Direct Modulated Transmitter Module – Hardware Interface Manual
9-1
MODULE SET-UP
9 . Module Set-up
9 .1 Setting RF Input Levels
The RF signals that are common to each laser transmitter are connected to the Broadcast (BC) input port as shown in the
rear panel connector table in section 6.1. The common RF loading into the BC RF input port should occupy the RF spectrum
to at least approximately 270 MHz to avoid SRS crosstalk degradation.
The Narrowcast (NC) signals for each of the individual laser transmitters is connected to the corresponding input port
as shown in the rear panel connector table in section 6.1. Note that the NC input should be all QAM signals and above
270 MHz. The RF combining is performed externally for the DMT44 module with four laser transmitters. Follow the same
guidelines for combining BC and NC RF signals as described above.
Determining the proper rear panel RF input level is important to ensure optimum transmitter performance. The following will
provide guidance in calculating the proper adjustment to the per channel RF level for your actual channel loading from the
reference channel loading and the total RF input power requirement.
Each DMTx4 transmitter is factory optimized for a rear panel nominaltotal input power of +31 dBmV for the combined BC
and NC signals. This level provides 3 to 4 dB of reserve gain for the AGC operation. The reserve gain is the amount of
headroom below the +31 dBmV reference level.
9 .1 .1 All QAM Channels
The all QAM reference channel loading and per channel input level is 155 ITU-T J.83 Annex B QAM 256 channels between
54-1002 MHz set to 9 dBmV. The RF per channel level of 9 dBmV is calculated from the total power as follows:
Total Power - [10*log
10
(channel load)]
31 dBmV - [10*log
10
(155)] = 31 dBmV – 22 = 9 dBmV
Using the same formula to adjust for a lower channel loading such as a 750 MHz system with 110 channels:
31 dBmV - [10*log
10
(110)] = 31 dBmV – 20.4 = 10.6 dBmV
Therefore with 110 of all QAM channel loading the correct rear panel RF input level is 10.6 dBmV at the input of both the BC
and NC ports.
9 .1 .2 Mix of Analog & QAM Channels
The mix of Analog and QAM reference channel loading and per channel input level is 30 NTSC analog (54-258 MHz) set to
13 dBmV and 124 ITU-T J.83 Annex B QAM 256 channels (258-1002 MHz) set to 11 dBmV. The RF level per channel for
analog and QAM is calculated from the total power as follows:
Since there is a 6 dB power level difference between the QAM and analog channels it is easier if we convert the QAM
channels to an equivalent analog power level in the formula below. The QAM RF level being 6 dB lower means the QAM
power level is 1/4
th
the analog level. As a point of reference, -3 dB is 1/2 power and -6 dB is 1/4
power.
Total Power - [10*log
10
(Analog channel load)+(QAM channel load/4)]
31 dBmV - [10*log
10
((30)+(124/4))] = 31 dBmV – 18 = 13 dBmV
Therefore the analog channels are set to 13 dBmV and the QAM channels are 6 dB lower or 11 dBmV.
Use the same formula to increase the number of analog channels to 60 and decrease the number of QAM channels to 94
and calculate the correct input level for each:
31 dBmV - [10*log
10
((60)+(94/4))] = 31 dBmV – 19 = 12 dBmV
Therefore the analog channels are set to 12 dBmV and the QAM channels 6 dB lower or 6 dBmV.
9 .2 Configuring The DMTx4 Transmitter Module
To set-up, configure and verify the RF drive level for the DMTx4 requires the operator to become familiar with the Hand-
held Display or GUI status and configuration menus. Refer to the ChromaFlex Chassis – Hardware Interface Manual as a
supplement to the following procedures which will reference the associated display parameters.
The following DMTx4 Hand-held Display menu tree will assist in navigating the Hand-held Display.
CHAPTER 9:
