LNB: diferenças entre revisões
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Linha 15:
É um LNB com o "Feed Horn" (Alimentador ou Iluminador) integrado. Muitas vezes o "F" é omitido, como o LNB Ku, apesar de o alimentador estar integrado, não é comum denominar-se LNBF Ku.
==Block-downconversion==
Satellites use comparatively high [[radio frequencies]] ([[microwave]]s) to transmit their TV [[Signalling (telecommunication)|signal]]s. As microwave satellite signals do not easily pass through [[wall]]s, [[roof]]s, or even [[glass]] [[window]]s, satellite [[antenna (radio)|antenna]]s are required to be outdoors, and the signal needs to be passed indoors via cables. When radio signals are sent through [[coaxial cable]]s, the higher the frequency, the more losses occur in the cable per unit of length. The signals used for satellite are of such high frequency (in the multiple gigahertz range) that special (costly) cable types or [[waveguide]]s would be required and any significant length of cable leaves very little signal at the receiving end.
The purpose of the LNB is to use the [[Superheterodyne receiver|superheterodyne]] principle to take a block (or [[band (radio)|band]]) of relatively high frequencies and convert them to similar signals carried at a much lower frequency (called the [[intermediate frequency]] or IF). These lower frequencies travel through cables with much less [[attenuation]], so there is much more signal left at the satellite receiver end of the cable. It is also much easier and cheaper to design electronic circuits to operate at these lower frequencies, rather than the very high frequencies of satellite transmission.
The frequency conversion is performed by mixing a fixed frequency produced by a [[local oscillator]] inside the LNB with the incoming signal, to generate two signals equal to the sum of their frequencies and the difference. The frequency sum signal is filtered out and the frequency difference signal (the IF) is amplified and sent down the cable to the receiver:
IF frequency = received frequency - local oscillator frequency
The local oscillator frequency determines what block of incoming frequencies is downconverted to the frequencies expected by the receiver. For example, to downconvert the incoming signals from [[Astra 1KR]], which transmits in a frequency block of 10.70-11.70 GHz, to within a standard European receiver’s IF tuning range of 950-2150 MHz, a 9.75 GHz local oscillator frequency is used, producing a block of signals in the band 950-1950 MHz.
For the block of higher transmission frequencies used by [[Astra 2A]] and [[Astra 2B|2B]] (11.70-12.75 GHz), a different local oscillator frequency converts the block of incoming frequencies. Typically, a local oscillator frequency of 10.60 GHz is used to downconvert the block to 1100-2150 MHz, which is, again, within the receiver’s 950-2150 MHz IF tuning range.<ref>Bains, Geoff. "How It Works - Local Oscillators And Block-downconversion" ''What Satellite & Digital TV'' (January, 2011) pp82</ref>
For the reception of [[wideband]] satellite television [[carrier wave|carrier]]s, typically 27 MHz wide, the [[accuracy]] of the frequency of the LNB local oscillator need only be in the order of ±500 kHz, so low cost dielectric oscillators (DRO) may be used. For the reception of narrow bandwidth carriers or ones using advanced [[modulation]] techniques, such as [[16-QAM]], highly stable and low phase noise LNB local oscillators are required. These use an internal [[crystal oscillator]] or an external 10 MHz reference from the indoor unit and a [[phase-locked loop]] (PLL) [[oscillator]].
==Amplification and Noise==
To compensate for the inevitable attenuation of the extremely weak signal received, by its transmission along the cable to the receiver, and to keep introduced noise (unwanted signals) at a low level relative to the signal, the LNB provides considerable [[Amplifier|amplification]], both of the signal from the waveguide probe and of the converted IF signal. Special [[electronic engineering]] techniques are used to ensure the signal has less noise on the output than would be possible with less stringent engineering. If low noise engineering techniques were not used, the sound and picture of satellite TV would be of very low quality, if it could be received at all without a much larger dish reflector.
The low noise quality of an LNB is expressed as the [[noise figure]] (or sometimes [[noise temperature]]). This is the ratio of the amount of noise in the output to the amount in the input, in [[decibels]] (dB). The ideal LNB would have a noise figure of 0dB. In fact, every LNB introduces some noise, although clever design, expensive components and even individual tweaking of the LNB after manufacture, can reduce noise levels to very low levels.
Every LNB off the production line has a different noise figure because of manufacturing tolerances. When you choose an LNB, the noise figure quoted in the specifications is important to determine its suitability but the figure quoted is usually representative of neither that particular LNB nor the performance across the whole frequency range.
[[Image:Universal-euro-sat-lnb.jpg|thumb|right|120px|K<sub>u</sub>-band linear-polarized LNBF]]
The noise figure quoted for most LNBs is the ''typical'' figure and should be an average of the production batch. Even if it's an accurate average (only a few samples from the batch are actually measured), the sample range is not given and so you cannot tell how far from the quoted average your LNB may lie. In addition, the figures measured are often at just one frequency so the quoted figure tells you nothing about the amount of noise introduced at other frequencies, which may be considerably higher.<ref name="WS article"/>
==LNBFs==
With the launch of the first DTH broadcast satellite in Europe ([[Astra 1A]]) by [[SES Astra]] in 1988, antenna design was simplified for the anticipated mass-market. In particular, the [[feedhorn]] (which gathers the signal and directs it to the LNB) and the [[polarizer]] (which selects between differently polarized signals) were combined with the LNB itself into a single unit, called an LNB-feed or LNB-feedhorn (LNBF), or even an "Astra type" LNB. The prevalence of these combined units has meant that today the term LNB is commonly used to refer to all antenna units that provide the block-downconversion function, with or without a feedhorn.
[[Image:sky lnb.jpg|thumb|right|120px|LNBF for Sky Digital and Freesat in the UK]]
The Astra type LNBF that includes a feedhorn and polarizer is the most common variety, and this is fitted to a dish using a bracket that clamps a collar around the waveguide neck of the LNB between the feedhorn and the electronics package. The diameter of the LNB neck and collar is usually 40mm although other sizes are also produced. In the UK, the "minidish" sold for use with [[Sky Digital]] and [[Freesat]] uses an LNBF with an integrated clip-in mount.
LNBs without a feedhorn built-in are usually provided with a (C120) flange around the input waveguide mouth which is bolted to a matching flange around the output of the feedhorn or polarizer unit.
==Polarization==
Satellite TV signals are usually broadcast using alternating [[Polarization (waves)|polarization]] to maximise the number of transmissions carried in a given frequency range as reception equipment can distinguish between transmissions using the same frequency but different polarization. Throughout the World, most satellite TV transmissions use vertical and horizontal [[linear polarization]] but in North America, [[Direct broadcast satellite|DBS]] transmissions use left and right hand [[circular polarization]]. Within the waveguide of a North American DBS LNB a slab of [[dielectric]] material is used to convert left and right circular polarized signals to vertical and horizontal linear polarized signals so the converted signals can be treated the same.
[[Image:old flange lnb.jpg|thumb|right|120px|A 1980s LNB (2.18dB noise figure) without built-in polarization selection and with a C120 fitting for separate feedhorn and polarizer]]
The probe inside the LNB waveguide collects signals that are polarized in the same plane as the probe. To maximise the strength of the wanted signals (and to minimise reception of unwanted signals of the opposite polarization), the probe is aligned with the polarization of the incoming signals. This is most simply achieved by adjusting the LNB's ''skew'' - its rotation about the waveguide axis. To remotely select between the two polarizations, and to compensate for inaccuracies of the skew angle, it used to be common to fit a ''polarizer'' in front of the LNB's waveguide mouth. This either rotated the incoming signal with an electromagnet around the waveguide (a magnetic polarizer) or rotated an intermediate probe within the waveguide using a servo motor (a mechanical polarizer) but such adjustable skew polarizers are rarely used today.
The simplification of antenna design that accompanied the first Astra DTH broadcast satellites in Europe to produce the LNBF extended to a simpler approach to the selection between vertical and horizontal polarized signals too. Astra type LNBFs incorporate two probes in the waveguide, at right angles to one another so that, once the LNB has been skewed in its mount to match the local polarization angle, one probe collects horizontal signals and the other vertical, and an electronic switch (controlled by the voltage of the LNB's power supply from the receiver : 13 volts for vertical and 18 volts for horizontal) determines which polarization is passed on through the LNB for amplification and block-downconversion.
Such LNBs can receive all the transmissions from a satellite with no moving parts and with just one cable connected to the receiver, and have since become the most common type of LNB produced.
== Universal LNB==
[[Image:twin universal flange lnb.jpg|thumb|right|120px|A twin-output universal LNB with a C120 flange fitting for a separate feedhorn]]
In Europe, as [[SES Astra]] launched more satellites to the [[Astra 19.2°E]] orbital position in the 1990s, the range of [[downlink]] frequencies used in the [[Fixed satellite services|FSS]] band (10.70-11.70 GHz) grew beyond that catered for by the standard LNBs and receivers of the time. Reception of signals from [[Astra 1D]] required an extension of receivers' IF tuning range from 950-1950 MHz to 950-2150 MHz and a change of LNBs' local oscillator frequency from the usual 10 GHz to 9.75 GHz (so-called "Enhanced" LNBs).
The launch of [[Astra 1E]] and subsequent satellites saw the first use by Astra of the [[BSS band]] of frequencies (11.70-12.75 GHz) for new digital services and required the introduction of an LNB that would receive the whole frequency range 10.70-12.75 GHz - the "Universal" LNB.
A Universal LNB has a switchable local oscillator frequency of 9.75/10.60 GHz to provide two modes of operation – low band reception (10.70-11.70 GHz) and high band reception (11.70-12.75 GHz). The local oscillator frequency is switched in response to a 22 kHz signal superimposed on the supply voltage from the connected receiver. Along with the supply voltage level used to switch between polarizations, this enables a Universal LNB to receive both polarizations (Vertical and Horizontal) and the full range of frequencies in the satellite K<sub>u</sub> band under the control of the receiver, in four sub-bands:<ref>{{cite web|url=https://pro.ses-astra.com/resources/dealers/dish_inst_broch1.pdf|title=Professional Dish Installation|publisher=ASTRA (GB) Limited|format=pdf|date=March 2005|page=7}}</ref>
{| class="wikitable"
|-
! Supply Voltage
! Supply Tone
! LO Frequency
! Polarization Received
! Frequency Band Received
! IF Range Used
|-
| 13 V
| 0 kHz
| 9.75 GHz
| Vertical
| Low (10.70-11.70 GHz)
| 950-1950 MHz
|-
| 18 V
| 0 kHz
| 9.75 GHz
| Horizontal
| Low (10.70-11.70 GHz)
| 950-1950 MHz
|-
| 13 V
| 22 kHz
| 10.60 GHz
| Vertical
| High (11.70-12.75 GHz)
| 1100-2150 MHz
|-
| 18 V
| 22 kHz
| 10.60 GHz
| Horizontal
| High (11.70-12.75 GHz)
| 1100-2150 MHz
|}
== Example LNBs==
=== Standard North America K<sub>u</sub> band LNB ===
Here is an example of a standard linear LNB:
* Local oscillator: 10.75 GHz
* Frequency: 11.70-12.20 GHz
* Noise figure: 1 dB typical
* Polarization: Linear
[[Image:astra lnb.jpg|thumb|left|120px|[[SES Astra|Astra]] type LNBF]]
=== Standard European "Astra" LNB ===
Here is an example of a Universal LNB used in Europe:
* Local oscillator: 9.75/10.60 GHz switchable
* Frequency: 10.70-12.75 GHz
* Noise figure: 0.2 dB typical
* Polarization: Linear
===North America DBS LNB ===
Here is an example of an LNB used for [[Direct broadcast satellite|DBS]]:
* Local oscillator: 11.25 GHz
* Frequency: 12.20-12.70 GHz
* Noise figure: 0.7 dB
* Polarization: Circular
=== C-band LNB ===
Here is an example of a North American [[C-band]] LNB:
* Local oscillator: 5.15 GHz
* Frequency: 3.40-4.20 GHz
* Noise figure: ranges from 25 to 100 [[Kelvin]]s (uses Kelvin ratings as opposed to dB rating).
* Polarization: Linear
==Multi-Output LNBs==
[[Image:octo lnb.jpg|thumb|right|120px|An eight-output Octo LNBF]]
=== Dual/Twin/Quad/Octo LNBs ===
An LNB with a single feedhorn but multiple outputs for connection to multiple tuners (in separate receivers or within the same receiver in the case of a twin-tuner PVR receiver). Typically, two, four or eight outputs are provided. Each output responds to the tuner’s band and polarization selection signals independently of the other outputs and "appears" to the tuner to be a separate LNB. Such an LNB usually may derive its power from a receiver connected to any of the outputs. Unused outputs may be left unconnected (but waterproofed for the protection of the whole LNB).
Note: In the US, an LNB with two outputs is termed a "dual LNB" but in the UK, the term "dual LNB" describes antennas for reception from two satellite positions, using either two separate LNBs or a single Monoblock LNB with two feedhorns. In the UK, the term "twin-output LNB", or simply "twin LNB", is usually used for an LNB with a single feedhorn but two independent outputs.<ref name="WS article"/>
=== Quattro LNBs ===
A special type of LNB intended for use in a shared dish installation to deliver signals to any number of tuners. A quattro LNB has a single feedhorn and four outputs, which each supply just one of the K<sub>u</sub> sub-bands (low band/horizontal polarization, high band/vertical polarization, low/vertical and high/horizontal) to a [[multiswitch]] or an array of multiswitches, which then delivers to each connected tuner whichever sub-band is required by that tuner.<ref>{{cite web |url=http://www.onastra.com/288905/glossary?category=q|title=Astra Glossary - Quattro LNB |publisher=SES ASTRA| accessdate=December 30, 2010}}</ref>
Although a quattro LNB typically looks similar to a quad LNB, it cannot (sensibly) be connected to receivers directly. Note again the difference between a quad and a quattro LNB: A quad LNB can drive four tuners directly, with each output providing signals from the entire K<sub>u</sub> band. A quattro LNB is for connection to a multiswitch in a shared dish distribution system and each output provides only a quarter of the K<sub>u</sub> band signals.
[[Image: Unicable.jpg|120px|thumb|SCR LNB with three SCR taps for daisy-chaining multiple tuners]]
===SCR/Unicable LNBs===
Multiple tuners may also be fed from an [[Satellite channel router|SCR]] or [[Unicable]] LNB in a [[single cable distribution]] system. A Unicable LNB has one output connector but operates in a different way to standard LNBs so it can feed multiple tuners daisy-chained along a single coax cable.
Instead of block-downconverting the whole received spectrum, an SCR LNB downconverts a small section of the received signal (equivalent to the bandwidth of a single transponder on the satellite) selected according to a [[DiSEqC]]-compliant command from the receiver, to output at a fixed frequency in the IF. Up to 16 tuners can be allocated a different frequency in the IF range and for each, the SCR LNB downconverts the corresponding individually requested transponder.<ref>Bains, Geoff. "Inverto Unicable LNB" ''What Satellite & Digital TV'' (February, 2006) pp60-62"</ref>
Most SCR LNBs also include either a legacy mode of operation or a separate legacy output which provides the received spectrum block-downconverted to the whole IF range in the conventional way.
[[Image: FibreLNB.jpg|thumb|right|120px|A optical fibre LNB (with fibre connection and conventional F-connector for power input)]]
==Optical Fibre LNBs==
LNBs for [[fibre satellite distribution]] systems operate in a similar way to conventional electrical LNBs, except that all four of the sub-bands in the entire Ku Band spectrum of 11.70 GHz-12.75 GHz across two signal [[Polarization (waves)|polarisations]] are simultaneously block-downconverted (as in a Quattro LNB). The fours sub-bands’ IFs are stacked to create one IF with a range of 0.95 GHz-5.45 GHz (a bandwidth of 4500 MHz), which is modulated on an optical signal using a [[semiconductor laser]], to send down the fibre cable.
At the receiver, the optical signal is converted back to the traditional electrical signal to “appear” to the receiver as a conventional LNB.<ref>{{cite web |url=ttp://www.globalinvacom.com/products/fibreopticlnb2.php |title= FibreMDU Optical LNB |publisher= Global Invacom | accessdate=January 12, 2010}}</ref>
== Monoblock LNBs ==
[[Image:monoblock lnb.jpg|thumb|right|120px|A twin-output monoblock LNBF for [[Astra 19.2°E]] and Hot Bird with a collar size adapter fitted]]
A [[Monoblock LNB]] (also spelled "monobloc") is a unit consisting of two LNBs and is designed to receive satellites spaced close together, generally 6°. For example in parts of Europe, monoblocks designed to receive the [[Hot Bird]] and [[Astra 19.2°E]] satellites are popular because they enable reception of both satellites on a single dish without requiring an expensive, slow and noisy motorised dish. A similar advantage is provided by the [[Duo LNB]] for simultaneous reception of signals from both the [[Astra 23.5°E]] and [[Astra 19.2°E]] positions.
== Cold temperatures ==
It is possible for an LNB to physically freeze due to ice build-up in very low temperatures, obscuring the signal. This is only likely to occur when the LNB is not receiving power from the satellite receiver (i.e. no programmes are being watched). To combat this, many satellite receivers provide an option to keep the LNB powered while the receiver is on standby. In fact most LNBs are kept powered because this helps to stabilise the temperature and, thereby, the local oscillator frequency. In the case of UK [[BSkyB]] receivers, the LNB remains powered while in standby so that the receiver can receive [[firmware]] updates and [[Electronic Programme Guide]] updates. In the United States the LNB connected to [[Dish Network]] receivers remains powered as well as those receivers that receive software and firmware updates and guide information over the air at night.
==References==
{{Reflist}}
==See also==
{{commonscat|Low noise blocks (LNB)}}
*[[Block upconverter]] (BUC)
*[[Orthomode transducer]]
*[[Signal-to-noise ratio]]
*[[Bias tee]]
*[[Duo LNB]]
*[[Single Cable Distribution]]
*[[Fibre satellite distribution]]
*[[Low noise amplifier]] (LNA)
*[[Transmit and receive integrated assembly]] (TRIA)
== External links ==
* [http://www.satcure.co.uk/tech/lnb.htm LNB mysteries explained].
* [http://www.satsig.net/lnb/explanation-description-lnb.htm Explanation and block diagram of LNB]
* [http://www.satsig.net/noise.htm Noise Temperature and Noise Figure]
* [http://www.ses-astra.com SES Astra business website]
* [http://www.onastra.com SES Astra consumer website]
* [http://www.ses-astra.com/business/en/support/document-library/recommendations/index.php Astra Recommendations (for satellite reception equipment including LNB types)]
* [http://www.skyvision.net/sites/default/files/SkyVision%20VSAT%20Installation%20Manual%20(Version%201).pdf VSAT Installation Manual with explanation of the Low-noise block converter]
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