Cable TV networks for the smallest. Part 7: Optical Receivers
The boundary between the optical medium and the coaxial cable is an optical receiver. In this article, we will consider their design and settings.
Content of a series of articles
The objective of the optical receiver is to transfer the signal from the optical medium to the electric one. In its simplest form, this can be done using a passive device, which captivates with its unpretentiousness:
However, this engineering miracle provides very mediocre signal parameters: at an optical signal level of -1 - -2 dBm, the output parameters hardly fit into GOST, and an overestimation of the signal leads to a significant increase in noise.
To be sure of the quality of the delivered signal with the FTTB architecture, it is necessary to use more complex devices:
Receivers found in our network: Vector Lambda, Telmor MOB and domestic Planar.
From their passive younger brother they all differ 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 : The Telmor optical receiver has a panel inside with a picture of a structural diagram. Such a scheme is typical for OP.
The required level of the optical signal is usually from -10 to +3 dBm, when designing and commissioning, the optimal value is -1 dBm: this is a decent margin in case of degradation of the transmission line and at the same time, a low level creates less noise when passing equipment circuits. AGC
circuit integrated in the optical receiver ( AGC) is just engaged in the fact that by adjusting the level of the incoming signal, it keeps the output in the given parameters. This means that if for some reason the optical signal suddenly changes significantly, but remains in the operating range of the AGC (from about 0 to -7 dBm), the receiver will regularly transmit to the coaxial network the signal with the level that was set during setup . For especially 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 amplification stage, which also provides the ability to control the slope and level of the output signal.
Optical Receiver Management
To configure the signal parameters, as well as change and control the built-in service functions, simple controls are usually present inside the receivers themselves. The MOB shown in the photo above has a separate board, which is optionally installed in the case. Also, as an alternative, it is proposed to use a quick-detachable board that is installed only at 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 values of the input attenuator (with an increase in which the output signal decreases correspondingly to the gain), turn on or off (as well as set fixed values) the AGC, set the tilt parameters and configure the ethernet interface.
The Chelyabinsk OP Planar has a clear indicator of the level of the optical signal, and the settings are carried out in a simple way: a twist and a change of inserts that change the characteristics of the amplifier stage. The power supply is located in the hinged lid.
And made in the design of "technoporn" OP Vector Lambda has a two-digit screen and only three buttons.
To distinguish positive values from negative values, this OP displays negative values by all segments, and a positive zero and +1 indicates half the height of the screen. With values over +1.9, it simply writes “HI”.
Such controls are convenient for on-line configuration at the facility, but for the possibility of remote monitoring and control, almost all receivers have an ethernet port. The web interface allows controlling and changing parameters, and SNMP polling is supported for integration with monitoring systems.
Here we see the same typical structural diagram of the OP, on which there is the possibility of changing 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.
The parameters important for control are displayed next to the circuit: the levels of the input and output signals, as well as the voltage values received from the built-in power supply. 99% of failures of such OPs 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: live broadcast
This does not apply to the theme of the series, but I will only briefly talk about the broadcast reception of TV. 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 KTV signal, an on-air broadcasting receiver can be installed instead of an OP, for example, Terra MA201:
Several antennas (usually three) are connected to the input ports of the receiver, each of which provides its own frequency range.
Собственно, с переходом на цифровое телевещание в этом отпадает необходимость, так как цифровые мультиплексы вещаются в одном диапазоне.
For each of the antennas, you can adjust the sensitivity to reduce noise, and also, if necessary, apply remote power to the amplifier built into the antenna. Further, the signal passes the amplification stage and is summed. The ability to adjust the output level boils down to cascade stages, and tilt adjustment is not provided at all: you can get the desired shape of the spectrum by adjusting the sensitivity of each antenna individually. And if there are kilometers of coaxial cable behind such a receiver, then they are already struggling with the attenuation by installing and tuning amplifiers, the same as on the cable network.
If desired, you can combine the signal sources: to collect in one network and cable, and terrestrial, and at the same time also a satellite signal. This is done using multiswitches - devices that allow you to summarize and distribute signals from different sources.