Also called a low-noise block, 1 2 low-noise converter ( LNC ), or even low-noise downconverter ( LND ), 3 the device is sometimes inaccurately called a low-noise amplifier ( LNA ).It serves as the RF front end of the satellite receiver, receiving the microwave signal from the satellite collected by the dish, amplifying it, and downconverting the block of frequencies to a lower block of intermediate frequencies (IF).
This downconversion allows the signal to be carried to the indoor satellite TV receiver using relatively cheap coaxial cable; if the signal remained at its original microwave frequency it would require an expensive and impractical waveguide line. The microwave signal from the dish is picked up by a feedhorn on the LNB and is fed to a section of waveguide. One or more metal pins, or probes, protrude into the waveguide at right angles to the axis and act as antennas, feeding the signal to a printed circuit board inside the LNBs shielded box for processing. The lower frequency IF output signal emerges from a socket on the box to which the coaxial cable connects. The pins visible at the top and left side of the hole project into the waveguide and act as antennas to receive the signal, converting it to radio frequency alternating currents which are processed by the circuit board. The lower frequency output signal is taken from the coaxial cable jacks, bottom. This phantom power travels to the LNB; opposite to the signals from the LNB. The low noise amplifier section of the LNB amplifies this weak signal while adding the minimum possible amount of noise to the signal. This is the signal to noise ratio at the input divided by the signal to noise ratio at the output. 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 LNBs components. Active cooling to very low temperatures can help reduce noise too, and is often used in scientific research applications. The noise figure quoted in the specifications, important for determining the LNBs suitability, is usually representative of neither that particular LNB nor the performance across the whole frequency range, since the noise figure most often quoted is the typical figure averaged over the production batch. As microwave satellite signals do not easily pass through walls, roofs, or even glass windows, it is preferable for satellite antennas to be mounted outdoors. However, plastic glazing is transparent to microwaves and residential satellite dishes have successfully been hidden indoors looking through acrylic or polycarbonate windows to preserve the external aesthetics of the home. 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 sum signal is filtered out and the frequency difference signal (the IF) is amplified and sent down the cable to the receiver. For example, to downconvert the incoming signals from Astra 1KR, which transmits in a frequency block of 10.7011.70 GHz, to within a standard European receivers IF tuning range of 9502,150 MHz, a 9.75 GHz local oscillator frequency is used, producing a block of signals in the band 9501,950 MHz. Typically, a local oscillator frequency of 10.60 GHz is used to downconvert the block to 1,1002,150 MHz, which is still within the receivers 9502,150 MHz IF tuning range. 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.
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