Low-noise Block Downconverter - Block Downconversion

Block Downconversion

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, satellite antennas are required to be outdoors.

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.

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: IF frequency = local oscillator frequency - received frequency

K_u-Band: 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 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 still within the receiver’s 950-2150 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 950-1450 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.