The boundary between the optical medium and the coaxial cable is the optical receiver. In this article, we will consider their design and settings.
Contents of the article series
- Part 7: Optical Receivers
- Part 8: Optical backbone network
- Part 9: Headend
- Part 10: Troubleshooting on the cable TV network
The task of an optical receiver is to transfer a signal from an optical medium to an electrical one. In its simplest form, this can be done using a passive device that captivates with its unpretentiousness:
However, this engineering miracle provides very mediocre signal parameters: with an optical signal level of -1 - -2 dBm, the output parameters barely fit into GOST, and signal overestimation leads to a significant increase in noise.
To be sure of the quality of the delivered signal, the FTTB architecture requires the use of more complex devices:
Receivers found in our network: Vector Lambda, Telmor MOB and domestic Planar.
All of them differ from their passive younger counterpart in more complex circuitry, which includes filters and amplifiers, so you can be calm about the signal reaching the subscriber. Let's take a closer look at them:
The Telmor optical receiver has a block diagram inside. Such a scheme is typical for the OP.
The required optical signal level is usually from -10 to +3 dBm, during design and commissioning, the optimal value is -1 dBm: this is a decent margin in case of transmission line degradation and at the same time, a low level creates less noise when equipment circuits pass.
The AGC circuit (AGC) built into the optical receiver is precisely engaged in the fact that, by regulating the level of the incoming signal, it keeps the output in the specified parameters. This means that if for some reason the optical signal suddenly changes significantly, but remains in the AGC operating range (approximately from 0 to -7 dBm), then the receiver will properly send a signal to the coaxial network with the level that was set during setup . For particularly important cases, there are devices with two optical inputs, each of which is monitored and can be activated either manually or automatically.
All active OPs contain an amplifying stage, which also provides the ability to control the slope and output signal level.
Control of optical receivers
To set up the signal parameters, as well as change and control the built-in service functions inside the receivers themselves, there are usually simple controls. For the MOB shown in the photo above, this is a separate board, which is optionally installed in the case. Also, as an alternative, it is proposed to use a quick-detachable board, which is installed only for the time of configuration in the ports on the main board. In practice, this is not very convenient, of course.
The control panel allows you to set the input attenuator (as it increases, the output signal is reduced according to the gain), enable or disable (as well as set fixed values) the AGC, set the slope parameters and configure the ethernet interface.
The Chelyabinsk OP Planar has a clear indicator of the optical signal level, and the settings are carried out in a simple way: by turning and changing inserts that change the characteristics of the amplifier stage. The power supply is located in the hinged cover.
And made in the design of "technoporno" OP Vector Lambda has a two-digit screen and only three buttons.
To distinguish positive from negative values, this OP displays negative values ββin all segments, and positive zero and +1 displays half the screen height. For values ββgreater than +1,9, simply writes "HI".
Such controls are convenient for on-site setup, but for remote monitoring and control, almost all receivers have an ethernet port. The web interface allows you to control and change parameters, and SNMP polling is supported for integration with monitoring systems.
Here we see the same typical block diagram of the OP, on which it is possible to change the parameters of the AGC and attenuator. But the slope of this OP is set only by jumpers on the board and has three fixed positions.
Next to the diagram, parameters important for control are displayed: the levels of the input and output signals, as well as the voltage values ββreceived from the built-in power supply. 99% of the failures of such OP occur after the deterioration of these voltages, so they should be monitored to prevent accidents.
The word Transponder here means the IP interface and this tab contains the settings for the address, mask and gateway - nothing interesting.
Bonus: on-air TV reception
This is not related to the theme of the series, but I will only briefly talk about terrestrial TV reception. Why now? Yes, just if we consider the network of an apartment building, then it depends on the signal source in the coaxial distribution network whether the network will be cable or terrestrial.
In the absence of an optical fiber with a CATV signal, an on-air broadcasting receiver, for example, Terra MA201, can be installed instead of the OP:
Several antennas (usually three) are connected to the input ports of the receiver, each of which provides reception of its own frequency range.
Π‘ΠΎΠ±ΡΡΠ²Π΅Π½Π½ΠΎ, Ρ ΠΏΠ΅ΡΠ΅Ρ
ΠΎΠ΄ΠΎΠΌ Π½Π° ΡΠΈΡΡΠΎΠ²ΠΎΠ΅ ΡΠ΅Π»Π΅Π²Π΅ΡΠ°Π½ΠΈΠ΅ Π² ΡΡΠΎΠΌ ΠΎΡΠΏΠ°Π΄Π°Π΅Ρ Π½Π΅ΠΎΠ±Ρ
ΠΎΠ΄ΠΈΠΌΠΎΡΡΡ, ΡΠ°ΠΊ ΠΊΠ°ΠΊ ΡΠΈΡΡΠΎΠ²ΡΠ΅ ΠΌΡΠ»ΡΡΠΈΠΏΠ»Π΅ΠΊΡΡ Π²Π΅ΡΠ°ΡΡΡΡ Π² ΠΎΠ΄Π½ΠΎΠΌ Π΄ΠΈΠ°ΠΏΠ°Π·ΠΎΠ½Π΅.
For each of the antennas, you can adjust the sensitivity to reduce noise, as well as, if necessary, supply remote power to the amplifier built into the antenna. The signal then passes through an amplifier stage and is summed up. The ability to adjust the output level comes down to turning off the stages of the cascade, and the tilt adjustment is not provided at all: you can get the desired spectrum shape by adjusting the sensitivity of each antenna separately. And if there are kilometers of coaxial cable behind such a receiver, then the attenuation in it is already being fought by installing and configuring amplifiers, the same as on the cable network.
If desired, you can combine signal sources: collect both cable and terrestrial, and at the same time satellite signal into one network. This is done using multiswitches - devices that allow you to sum and distribute signals from different sources.
Source: habr.com