Satellites use comparatively high
radio frequencies (
microwaves) to transmit their TV
signals. 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. The purpose of the LNB is to use
heterodyning 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. 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: ;
C-band: f_\text{IF} = f_\text{LO} - f_\text{recv} ;
Ku-band: f_\text{IF} = f_\text{recv} - f_\text{LO} where \scriptstyle f is a 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 950–2,150 MHz, a 9.75 GHz local oscillator frequency is used, producing a block of signals in the band 950–1,950 MHz. For the block of higher transmission frequencies used by
Astra 2A and
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 1,100–2,150 MHz, which is still within the receiver's 950–2,150 MHz IF tuning range. In a C-band antenna setup, the transmission frequencies are typically 3.7–4.2 GHz. By using a local oscillator frequency of 5.150 GHz the IF will be 950–1,450 MHz which is, again, in the receiver's IF tuning range. For the reception of
wideband satellite television
carriers, 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. ==Low-noise block feedhorns (LNBFs) ==