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LNB ; Low Noise Blockdown Converter

LNB는 위성용 수신기에서 위성주파수를 셋톱박스 주파수로 낮추어주는 주파수 변환 장치를 의미한다.

주로 위성 수신용 파라볼릭 안테나의 수신부, 즉 파라볼릭 판에서 반사되어 온 전자파가 한점에 모이는 곳에 달린 feed-horn에 부가적으로 붙는 장치이다.
 보통 위성이 보내는 주파수인 C band (4~8GHz)와 Ku band(12~18GHz) 용 주파수를 받아서 셋톱박스가 처리할 수 있을 만한 수백MHz에서 2Ghz 까지의 신호로 변환해준다.

그 특성에서 알 수 있듯이, LNB는 Mixer와 LNA의 조합으로 만들어진다. 위성에서 수신된 높은 주파수는 LNA(저잡음증폭기)를 통해 우선 잡음을 최소화하면서 미약한 신호레벨을 높이고, Mixer를 거쳐 주파수가 하향변환하게 된다.

그래서 일반적인 형태로는
LNA - Down Mixer (LO포함) - BPF - IF Amp
와 같은 구조로 되어 있는 경우가 많다.

LNB는 그 용도상 위성수신 안테나와 함께 외부에 노출되어 있기 때문에, 무게가 가볍고 온도변화에 둔감해야 하는 등 기후에 강해야 한다.

 

Low Noise Block Downconverters

    The incoming satellite signal passes through the feedhorn and exits into the receiving system's first stage of electronic amplification called the low noise block downconverter or LNB. A certain amount of noise is generated within any electronic circuit. Any noise created by the LNB circuitry itself will be amplified and passed on to succeeding stages. For best overall system performance, this noise must be kept to a minimum.

    Noise temperature to noise figure conversion chartThe LNB sets the noise floor for your entire satellite receiving system. Less noise here means that more signal will actually arrive at the indoor receiver. Today's high performance LNBs use Gallium Arsenide (GaAs) semiconductor and High Electron Mobility Transistor (HEMT) technologies to minimize the noise level of the LNB.

    The noise performance of C band LNBs is quantified as a noise temperature measured in degrees Kelvin (K), while Ku band LNB noise performance is expressed as a noise figure measured in dB. Today's C band LNBs commonly achieve a noise temperature of 40 K or less, while Ku band noise figures of less than 1 dB are commonly available. In either case, the lower the LNB's noise performance rating, the less noise introduced into the LNB by its own circuitry. The conversion chart presented below shows the relationship between these two commonly used LNB noise measurement systems

 

 

 
Ku band LNBs

     Universal Ku-band LNBThree distinct frequency sub bands or spectrums are available from various Ku band satellites: the 10.75 to 11.7 GHz, the 11.7 to 12.5 GHz, and the 12.5 to 12.75 GHz frequency spectrums. Care should be taken to ensure that your system uses the LNB which matches the Ku band frequency spectrum or spectrums used by the satellites which you desire to view.

    Wideband or universal Ku band LNBs are now available which can switch electronically between any of the above frequency spectrums to provide complete coverage of the entire Ku band frequency range. The receiver or IRD sends a switching voltage (13 or 17 volts d.c.) to the LNB which automatically changes the LNB input frequency range to the desired frequency spectrum (10.70 to 11.75 GHz or 11.7 to 12.75 GHz). Keep in mind that the universal LNB has an IF output frequency range of 950 to 2150 MHz and can only be used effectively with a receiver or IRD which also has a comparable IF input frequency range.

 

 

 
 
LNB/Dish Trade Offs

     A satellite transponder's effective isotropic radiated power (EIRP) is expressed in dBW (for decibels referenced to 1 Watt of power). The various footprint maps for satellites serving the Middle East (such as those shown in The World of Satellite TV) allow system designers to determine the transponder EIRP likely to arrive at the site location. The southern beam transponders on AsiaSat 1, for example, deliver an EIRP of 33 dBW to Kuwait, while AsiaSat 2 transponders deliver an EIRP of 39 dBW. This information can be used to determine the appropriate combination of antenna size and LNB noise temperature to receive a signal that will exceed the minimum threshold requirements of the indoor satellite TV receiver.

    The tables below can be used to compute the combination of antenna size (given an antenna efficiency of 65 percent) and LNB noise temperature needed to produce a signal that exceeds an analogue receiver threshold of 7 dB (diagonal blue line). If the intersection of the EIRP line (from left to right) and the antenna line is above the blue diagonal line for the LNB in use, then the resulting signal level would be above receiver threshold. keep in mind that sparkle free reception of analogue (non-digital) TV signals requires at least a margin of 2 dB above receiver threshold (7 to 10 dB without threshold extension, depending on manufacturer).

    Table 2 below allows the reader to determine the appropriate combination of antenna aperture and LNB noise temperature for dishes ranging from 3.9 to 7.6 meters in diameter.

    Table 3 (below) illustrates the equivalent effect of combining a 2.5 dB or 1.2 dB noise figure Ku band LNB with antenna apertures ranging from 60cm to 2.4m in diameter. Any improvement due to a drop in Ku band LNB noise figure, however, is only achievable under clear sky conditions. Unlike C band signals, which are unaffected by changing weather conditions, Ku band signals are adversely affected by the presence of moisture in the Earth's atmosphere. The presence of rain, or even rain clouds, will dramatically raise the noise temperature of the sky and therefore raise the noise temperature of the receiving system as well.

    Another LNB specification commonly encountered is the amount of amplification or gain provided by each unit. This is also measured in dB. Generally, consumer LNBs produce 50 to 60 dB of gain, multiplying the received signal by as much as 1,000,000 times. This provides the receiver with the necessary amount of amplification for efficient operation. In most cases, the gain figure itself matters less than the LNB's noise contribution.

 

 

 
LNB Install Tips

     Potential probem sources at the feedhorn & LNBA rectangular flange on the back of the feedhorn mates with a similar flange located at the front of the LNB. A neoprene gasket is inserted between these to flanges to prevent any moisture from entering this junction. During assembly, be sure that this gasket is properly seated as any moisture entering through this critical junction will degrade signal reception and possibly damage either or both of these components. Many manufacturers now make a combination of a feedhorn and LNB called an LNF. This combination product eliminates the junction between feed and LNB as a source of potential moisture problems.

    The IF output connector on the back of every LNB is another potential source for the ingress of moisture. After attaching the coaxial cable to this connector, the junction should be sealed from the weather, either by using a special waterproofing compound such as coax seal which wraps tightly around the outside of the connection, or by flooding the inside of the coax's F connector with a waterproofing silicon sealer. If you elect to flood the F connector, be sure to first unplug the receiver and wait for the compound to dry before plugging the receiver back into an a.c. power source.

    Some digital satellite TV systems, primarily those which transmit on a relatively narrow carrier, require an ultra-stable LNB that uses a phased lock loop circuit to keep its local oscillator frequency from drifting too far away from its nominal value. Other digital satellite TV systems require an LNB with low phase noise performance. Be sure that you check with the digital satellite service operator to determine the specific LNB requirements for receiving their satellite services.

 

 

 
Linear Polarization

원형편파 및 선형편파
편파란 위성에서 송출하는 방식을 말하는 것으로서 국내에서 수신가능한 대부분의 해외위성방송은 선형편파 ( 수평/수직 - horizontal/vertical)이다.

원형편파로는 국내의 무궁화위성 3호의 KBS는 좌편파(LHCP)를 사용하며 일본의 NHK는 우편파(RHCP)를 사용한다. 또한 러시아의 이동위성(STATSIONAR - Gorizont)은 위의 두 원형편파를 모두 사용한다. 이원형편파는 전용 피드혼이 필요하다. 즉 KBS는 방송의 세기가 워낙 강해서 보통의 LNB로 클립을 사용하지 않고도 시청이 가능하나, 아날로그 NHK의 경우는 편파기를 넣어 주어야만 깨끗한 방송을 시청할 수가 있다. 하지만 러시아의 이동위성에서 방송하는 오스탄키노(Ostankino)나 레떼르(RTR)같은 방송은 위성자체도 이동위성이며 편파 또한 원형 편파를 사용해서 LNB를 선형으로 사용할 경우 화면의 질이 좋지 않다.

국내에서 대부분 시청하는 위성( Palapa, AsiaSat, Apstar, PanAmSat)은 대부분 선형을 사용한다

물론 운영자의 해석이 틀린 것일 수도 있다.. 이때는 주저말고 운영자에게 메일을...

    우편파 ( Right hand circularly polarized - RHCP)
    좌편파 ( Left hand circularly polarized - LHCP)
    수평파 ( Horizontal polarized - HP)
    수직파 ( Vertical polarized - VP)

 

 

 
Circular Polarization

     C band satellites such as the INTELSAT (C band only), Arabsat 1 C, Gorizont and Express spacecraft use an alternate polarization format known as circular polarization. For the best possible reception of circularly polarized satellite transmissions, you will need to use a feedhorn that has been constructed to receive these signals.

    Instead of beaming the microwave energy along a linear plane, whether vertical or horizontal, circular polarization is transmitted in a helical rotating pattern, with right hand circular rotating in a clockwise direction as seen from the satellite, and left hand circular signals rotating in a counterclockwise direction. Although standard linear feedhorns can still pick up any circular polarized signal, half of the available signal power will be lost.

    There are several manufacturers that offer special feedhorns that can receive both the linear and circular polarization formats. Many linearly polarized feedhorns also can be modified to receive circularly polarized signals with the addition of a rectangular insert made from a dielectric material such as Teflon.

 

 

 
Polaris's

     Most communications satellites maximize their use of the limited frequency spectrums assigned for satellite communications by overlapping the transponders, with their polarization switching from one sense of polarization to the opposite sense every other transponder. This allows twice as many channels in the same amount of space. In order to select the correct polarization, most feedhorns incorporate a small probe that is rotated until best reception is obtained.

    The probe is rotated by means of a small servo-motor which is powered by the indoor receiver or IRD. By sensing the strength of the incoming signal, some receivers can select the correct polarization setting automatically. However, most receivers are programmed during the installation process to recall the correct polarization format for each individual satellite stored in memory. A few manufacturers use a ferromagnetic device which electronically adjusts feedhorn polarization, instantaneously and silently. This introduces a small amount of signal loss, typically 0.1 to 0.2 dB, which for most applications is negligible. Ferromagnetic Polaris's have no moving parts that can cause maintenance problems in the future.

 

 

 
 
Hybrid Feedhorns

    Dual band hybrid feedhorns place both the C and Ku band waveguide openings directly over the focal cloud of the antenna. This type off feedhorn will give the satellite receiver direct access to all of the TV services carried on dual band satellites such as PAS 4 or INTELSAT 704. The placement of both the C and Ku band feed openings in such close proximity to each other, however, will reduce the level of C band satellite TV signals over what a good C band only feed can achieve. This may be an important consideration for system designers who wish to use the smallest dish possible to receive C band satellite TV services.

    An alternative design approach to receiving dual band satellite signals is to attach an optional Ku band feedhorn to one side of an existing C band feed which illuminates an antenna with an f/D greater than .35. Several manufacturers make add on Ku band feeds for this purpose which have a bracket that mates with existing mounting holes on their C band feedhorns. The add on Ku band feed is positioned so that its waveguide opening is on a plane that is 90 degrees from the plane of the polar axis of the dish.

    So called shallow dishes with an f/D of .35 to .45 can generate multiple focal points spaced at intervals from the main focal point of the antenna. The add on Ku band feedhorn is mounted so that it can pick up one of these secondary focal points.

    If used on a large C band antenna, the add on Ku band feed will capture enough signal to exceed the threshold rating of the receiver even though the secondary locations immediately adjacent to the main focal point are of lesser intensity. To receive C and Ku band signals from the same satellite, the operator will have to change the antenna's pointing direction along the Clarke Orbit to compensate for the switch to the secondary focal point.

 

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