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.

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 LNB's shielded box for processing.

The lower frequency IF output signal emerges from a socket on the box to which the coaxial cable connects.

Active cooling to very low temperatures can help reduce noise too, and is often used in scientific research applications.

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.

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 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.

With the launch of the first DTH broadcast satellite in Europe (Astra 1A) by SES in 1988, antenna design was simplified for the anticipated mass market.

In the UK, the "minidish" sold for use with Sky Digital and Freesat uses an LNBF with an integrated clip-in mount.

The simplification of antenna design that accompanied the first Astra DTH broadcast satellites in Europe to produce the LNBF extended to a simpler approach to the selection between vertical and horizontal polarized signals too.

Here is an example of a North American C-band LNB: Here is an example of a standard linear LNB: In Europe, as SES launched more Astra satellites to the 19.2°E orbital position in the 1990s, the range of downlink frequencies used in the FSS band (10.70–11.70 GHz) grew beyond that catered for by the standard LNBs and receivers of the time.

Reception of signals from Astra 1D required an extension of receivers' IF tuning range from 950–1,950 MHz to 950–2,150 MHz and a change of LNBs' local oscillator frequency from the usual 10 GHz to 9.75 GHz (so-called enhanced LNBs).

The local oscillator frequency is switched in response to a 22 kHz signal superimposed on the supply voltage from the connected receiver.

S-band was chosen for these satellites because its frequencies efficiently penetrate the atmosphere and provide high-quality transmissions to small-diameter 80 cm antennas in regions that experience heavy rainfall such as Indonesia.

A similar Ku- or C-band reception performance requires greater transmission power or much larger dish to penetrate the moist atmosphere.

A quattro LNB has a single feedhorn and four outputs, which each supply just one of the Ku sub-bands (low band/horizontal polarization, high band/vertical polarization, low/vertical and high/horizontal) to a multiswitch or an array of multiswitches, which then delivers to each connected tuner whichever sub-band is required by that tuner.

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.

Multiple tuners may also be fed from a satellite channel router (SCR) or unicable LNB in a single cable distribution system.

A Unicable LNB has one output connector but operates in a different way to standard LNBs so it can feed multiple tuners daisy-chained along a single coax cable.

Up to 32 tuners can be allocated a different frequency in the IF range and for each, the SCR LNB downconverts the corresponding individually requested transponder.

In cases where only a single cable is possible, such as apartment blocks, Sky Q compatible multiswitches can be used, which instead use BSkyB SCR.

So monoblock LNBs are usually a compromise solution designed to operate with standard dishes in a particular region.

Most receivers sold nowadays are compatible with at least DiSeqC 1.0 which allows to switch automatically between 4 satellites (all of contemporary Monoblock LNBs), as user changes channel on remote control.

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.