In this article, we will consider cable television high-frequency radio signal amplifiers on the coaxial part of the backbone.
Contents of the article series
- Part 6: RF Amplifiers
- Part 7: Optical Receivers
- Part 8: Optical backbone network
- Part 9: Headend
- Part 10: Troubleshooting on the cable TV network
If there is only one optical receiver in the house (or even in the whole block) and all the wiring to the risers is made with a coaxial cable, signal amplification is required at their beginning. Our network mainly uses Teleste devices, so I will tell using their example, but in principle the equipment of other manufacturers is no different and the set of functionality for configuration is usually similar.
The minimum number of settings has a model CXE180M:
As you probably remember from the previous parts, a signal has two important quantitative parameters: level and slope. It is them that can help correct the settings of the amplifier. Let's start in order: immediately after the input connector is attenuator. It allows you to reduce the input signal by up to 31dB (when you switch the blue jumper in accordance with the diagram, the range of the knob changes from 0-15 to 16-31dB). This may be necessary if the amplifier receives a signal of more than 70 dBuV. The fact is that the amplifying stage provides a signal level increase of 40 dB, and at the output we must remove no more than 110 dBΞΌV (at a higher level, the signal-to-noise ratio drops sharply and this figure is relevant for all broadband amplifiers and receivers with a built-in amplifier) . Thus, if 80 dBuV gets to the input of the amplifier, for example, then at the output it will give us 120 dBuV of noise and a crumbling figure. To avoid this, you need to set the input attenuator to the damping position of 10dB.
Behind the attenuator we see equalizer. It is necessary to eliminate the reverse slope, if any. This is achieved by reducing the signal level in the low-frequency zone by up to 20 dB. It is worth noting that we cannot eliminate the reverse slope by raising the level of the upper frequencies, we can only crush the lower ones.
These two tools are enough to correct minor signal deviations from the norm. If this is not the case, then you can use the following:
cable simulator, made in the form of an insert that can be placed horizontally or vertically, as the name implies, simulates the inclusion of a long cable section, on which the attenuation of mainly the upper frequencies of the range should occur. This allows you to reduce the forward slope, if necessary, by suppressing 8dB in the high frequency zone. This can be useful when installing amplifiers in a cascade over a short distance, for example.
After these manipulations, the signal passes the first stage of the amplifying stage, after which we see another insert, which allows us to further reduce the gain. The jumper following it will again help us to suppress low frequencies to obtain the necessary slope. These two settings are essentially analogous to the input attenuator and equalizer, but working with the second stage of the cascade.
At the output of the amplifier stage, we see test tap. This is a standard threaded connector to which you can connect a measuring device or a television receiver to monitor the quality of the output signal. Not all devices and almost no TVs are able to properly digest a signal with a level of one hundred or more dBuV, so test taps on any equipment are always made with an attenuation of 20-30 dB from the actual output value. This should always be kept in mind when taking measurements.
Before the exit, another insert is installed. The photo of the amplifier shows that the arrow depicted on it points only to the right output. And this means that there will be no signal in the left. Such inserts are in these amplifiers βout of the boxβ, and inside the box itself, the package contains another one:
It allows you to use the second output, but inevitably introduces a signal attenuation of 4 dB.
The amplifier model CXE180RF at first glance has twice as many settings:
In fact, everything is not so scary: with the exception of small differences, everything here is the same as in the above.
First, a test tap appeared at the input. It is needed to control the signal without disconnecting the cable from the amplifier input and, accordingly, without interrupting the broadcast.
Secondly, the diplex filters that have appeared, as well as the output attenuator and equalizer, are necessary for setting up the DOCSIS transmission channels, therefore, within the framework of this article, I will only say that the filters cut off the frequencies that are indicated on them and this can become a problem if the signal spectrum TV channels are broadcast on these frequencies. Fortunately, the manufacturer produces them with different values ββββand it is not difficult to replace them if necessary.
The twists (like a jumper that introduces attenuation of 10dB) affect only the return channel and are in no way able to change the television signal.
But the remaining three jumpers offer us to get acquainted with such technology as remote power.
When designing houses, amplifiers are often placed in places where there may be problems with the supply of electricity from switchboards. In addition, each plug-socket pair, which also implies a circuit breaker (which can be installed in the most unexpected place), represents a potential point of failure. In this regard, it is possible to power the equipment directly over the coaxial cable. Moreover, as can be seen from the marking on the overlay of the power supply, it can be both alternating and direct current with a very wide voltage range. So: these three jumpers enable the supply current to flow to the input, as well as to each of the two outputs separately, if we need to power the next amplifier in the cascade. When you turn on the riser with subscribers, of course, you cannot apply voltage to the output!
I already mentioned in parts that special trunk taps are used in such a system:
They use larger and more reliable elements, and a massive body provides heat dissipation and protection.
The power source in this case is a block with a built-in massive transformer:
It is worth saying that with the seeming optimality of the remote power supply scheme, amplifiers operating in this way are less likely to experience power failures at home without consequences, and when replacing them, the technical staff additionally have to look for and turn off the power of the unit itself so as not to work with live cables and, thus, while replacing one amplifier, the whole house remains without a signal. For the same reason, such amplifiers require a test tap at the input: otherwise, you would have to work with a live cable.
It would be interesting to learn from colleagues how widespread systems with remote power supply are, write in the comments if you use it, please.
If you need to connect a large number of TVs inside an apartment or office, you may encounter a lack of level after the divider chain. In this case, it is necessary to install an amplifier at the subscriber's territory, for which small devices with a minimum number of settings and a lower gain level are used.
For example, this:
Source: habr.com