Pass it on!
Welcome to Tech Talk, Pico Macom's series of technical notes, featuring useful information and equipment tips. This special edition installment of Tech Talk examines distribution design applications.
Distribution Design
The goal of distribution design is to provide consistent signal levels to every TV set. To accomplish this, we must cable, split, tap and amplify the signal. If performed correctly, every TV set will receive the same level of signal across all channels in a quality that represents the headend launch signal. If performed incorrectly, signal levels will be inconsistent, causing pictures to be grainy, have distortions or both. By following some simple practices, you can provide good picture quality every time.
Losses and Amplification
There are some simple concepts that must be understood first:
Frequencies: Low frequencies and higher frequencies have different loss characteristics as they passes through cable and equipment.
Cable: The size and type of cable play a significant role in how much signal will be lost at the low and high frequency ranges. Always check with the manufacturer for loss specifications of the cable you are using for your design.
Splitters: The splitter is a power divider, usually dividing the signal power evenly between the output ports. In some cases there is an odd number of output ports. For these applications there may be different loss values marked for each output; there may be a slightly higher loss on the higher frequencies. A 3dB reduction of signal occurs every time the signal is split. As an example, a 2-way splitter will have a 3.5dB loss on each output port. A 4-way splitter will have a 7dB loss and an 8-way splitter will have a 10.5dB loss on each output port.
Taps: Taps provide an input, output (through) and tapped output. The tap is a sampling of the through signal. Typically, the input to output signal reduction is around 1.5dB, while the input to tap port loss is the value on the label. The loss will be slightly higher at higher frequencies.
Attenuators: Attenuators (pads) reduce the signal level evenly across all channels. The value printed on the label is the amount of loss.
Amplifiers: Amplifiers will increase the signal level. They are usually designed to provide more gain on the higher frequencies to overcome cable and passive equipment losses. The operation of amplifiers is best when they are provided input signal levels that are even across all channel frequencies. In the concept called “unity gain,” every amplifier in line (cascade) is provided the same level on the input and amplifies equally on the output. Each amplifier is placed at the same signal loss distance as the prior amplifier.
Example:
Now that we have looked at the basics, let’s run through a distribution example. This is a three-leg design, although with simple modification it could adapt to almost any application. We will assume the following equipment performance for this design:
- Headend launch level of 38dBmV on CH 2 and 44dBmV on CH 54
- 6-series cable loss 0.75dB at CH 2 and 2.0dB on CH 54 per 50´ (all lengths are 50´ except 200´ for leg 3)
- Tap through loss even loss 1.5dB, Tap loss = value (30,27,24,20,16,12,9)
- Splitters even loss 2-way 3dB, 4-way 7dB
- Attenuators = value
Legs one and two are designed to be directly fed out of the headend, while leg three provides a 200´ cable run before a 12dB pad and then a tapped run. When designing the distribution, select the shortest cabling path that will allow you to pass by all TV locations. Ideally the tap can be placed so that two or three TV sets can be fed from the same tap. Calculate the passive and cable loss to the tap location, then select the tap value that will provide the signal levels desired at the TV locations.
Repeat this process until you have run out of signal or you’ve reached a point to amplify and restart the process. Amplifiers require a signal range based on input level based on channel range. If not sure, check with Pico Macom for the correct amplifier for your application.
Now that you can see how simple it can be, you are ready to start on your project. Use Pico Macom specifications for cable, splitters, taps, and amplifiers and browse the catalog for connectors, attachment supplies and test equipment.
Good luck and great pictures!