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Low-noise block downconverter


A low-noise block downconverter (LNB) is the receiving device mounted on satellite dishes used for satellite TV reception, which collects the radio waves from the dish and converts them to a signal which is sent through a cable to the receiver inside the building. Also called a low-noise block, low-noise converter (LNC), or even low-noise downconverter (LND), the device is sometimes inaccurately called a low-noise amplifier (LNA).

The low-noise quality of an LNB is expressed as the noise figure (or sometimes noise temperature). This is the signal to noise ratio at the input divided by the signal to noise ratio at the output. It is typically expressed as a decibels (dB) value. The ideal LNB, effectively a perfect amplifier, would have a noise figure of 0 dB and would not add any noise to the signal. Every LNB introduces some noise but clever design techniques, expensive high performance low-noise components such as HEMTs and even individual tweaking of the LNB after manufacture, can reduce some of the noise contributed by the LNB's components. Active cooling to very low temperatures can help reduce noise too, and is often used in scientific research applications.

The purpose of the LNB is to use the superheterodyne principle to take a block (or 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.

Throughout the world, most satellite TV transmissions use vertical and horizontal linear polarization but in North America, 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.

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 Ku 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 Ku band signals.

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 11.70–12.75 GHz across two signal polarisations are simultaneously block-downconverted (as in a quattro LNB). The four sub-bands’ IFs are stacked to create one IF with a range of 0.95–5.45 GHz (a bandwidth of 4.5 GHz), which is modulated on an optical signal using a semiconductor laser, to send down the fibre cable.

It is possible for moisture in an LNB to freeze, causing ice to build-up at very low temperatures. 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 by the dissipated heat from the circuitry of LNB. 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 a Dish Network receiver remains powered so the system can receive software and firmware updates and guide information over the air at night. In Turkey, another LNB type Digiturk MDUs are kept powered to receive VOD content, STB firmware, EPG data, and pay-TV keys in order to watch encrypted content.