Contemporary Research QMOD-SDI Integration Guide - Manual (Page 15)

Contemporary Research 15 QMOD-HD Integration
Distribution and Attenuation
If you’re putting together a design for a new RF system, there are several factors to keep in mind.
The key difference between RF and typical baseband systems is that you aren’t factoring cable
length in terms of cumulative resistance and capacitance over all the cable in the system – it’s an
antenna system, not a baseband system. The math is different. Also, you’ll see power speced in
terms of dB or dBmV in literature. All RF is measured in dBmV, but we often use the shorter dB
term because, well, we all know what it stands for in RF.
Splitters and Combiners
2-way Splitter, –3 dBmV 4-way Splitter, -7 dBmV loss
Splitters are the essential building block of an RF system. Every splitter is made up of one or more
2-way splitters, each level having a -3 dBmV loss. A 4-way splitter is made of 2 2-way splitters
feeding one 2-way splitter, with a combined loss of -7 dBmV, and 8-way has 3 levels, with a 10.5
dBmV loss, and so on. Used one way, they are splitters, sending signals to multiple cables. Flipped
around, they are combiners, bringing together multiple paths into one.
The internal transformers keep the signals isolated, so when use a splitter to feed TVs, you can
consider each TV as a separate signal path you can trace from the TVs, through each tap and
splitter, back to the source. This is a key design concept we’ll cover in a bit.
Active Combiners and Amps
In general, active combiners, such as our QCA9-33, have an internal splitter array, coupled with an
amplifier that compensates for combining losses. Most products in this genre provide essentially
unity gain – you get out what you put in, with a little extra. While there isn’t an industry standard
terminology, we call this type of combiner an Active Combiner, and one with a much larger
amplifier an Amplifier Combiner.
This concept, when using a very low-noise amplifier, simplifies design because you don’t have to
factor in combining losses. Also, when using QMODs and the QCA9-33, the 33 dBmV output is more
than enough to feed a small to medium sized TV network.
Larger systems need more power. The best approach is to use one or more active combiners, then
feeding one central 45-55 dBmV amp, usually called a Launch Amp. Bigger amps add more noise,
so the best approach is to use a bigger amp at the end so you’re not re-amping noise.