Contemporary Research QMOD-SDI Integration Guide - Manual (Page 14)

Contemporary Research 14 QMOD-HD Integration
RF Output Levels and Combiners
Note, that when you combine RF channels, there is some loss, called insertion loss. A simple 2-
input combiner might drop the output a few dB. Larger 8- or more input combiners can drop much
more, in the range of -12 to 18 dB. There are also a few active, multi-port commercial combiners
that average less loss than similar passive combiners. Your RF supplier can supply insertion loss
information for their products.
So when we say the QMOD can output about 30 Db, that power can be cut in half or more in the
combining process. Also, when you’re combining the QMOD with cable, you’ll lower the power to
match or be bit less than the analog cable channels.
Launch Amp
In classic RF design, therefore, after all the filtering and combining is done at a site’s “head end” –
where the system originates the RF to the building. For typical commercial applications, a “launch
amp” is added after the processing gear to amplify the signal to all rooms. How much power you
need depends on the design of the entire RF system.
Combining with Existing Analog Systems
If you’re combining QMOD with an existing analog system, it’s possible that you’ll be mixing with
channels set to a much higher RF signal.
There are two options, depending on the architecture of the existing system.
 Mix after the existing combiner. The typical 8/16-channel passive combiner loses about
15 dBmV or more. Mix the combiner output and QMOD with a smaller low-loss combiner.
Note that the QMOD digital channel can be 6-10 dBmV less than the analog signal. If the
system feeds a launch amplifier, you can boost the output to compensate for the 4-7 dBmV
loss in this solution.
 Boost output of QMODs before combining. Feed the QMOD output to a booster amp
before combining with the analog channels. If you have several QMODs, feed them to a
separate combiner, then boost the total output. You could also use an active combiner that
will both combine and boost the signal level.