A low-noise block downconverter is the receiving device mounted on satellite
dishes used for satellite TV reception, which collects the radio waves from the
dish. Also called a low-noise block, low-noise converter, or even low-noise
downconverter, the device is sometimes wrongly called an low-noise amplifier.
The LNB is a combination of low-noise amplifier, frequency mixer, local
oscillator and intermediate frequency amplifier. It receives the microwave
signal from the satellite collected by the dish, amplifies it, and downconverts
the block of frequencies to a lower block of intermediate frequencies. 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 LNB is usually a small box suspended
on one or more short booms, or feed arms, in front of the dish reflector, at
its focus. 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 PCB 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.
The LNB gets its power from the receiver or set-top box, using the same coaxial
cable that carries signals from the LNB to the receiver. This phantom power
travels to the LNB; opposite to the signals from the LNB.
A corresponding component, called a block upconverter, is used at the
satellite earth station dish to convert the band of television channels to the
microwave uplink frequency. Amplification and noise
The signal received by the LNB is extremely weak and it has to be
amplified before downconversion. The low noise amplifier section of the LNB
amplifies this weak signal while adding the minimum possible amount of noise to
the signal. The low-noise quality of an LNB is
expressed as the noise figure. 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 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. Every LNB off the production line has a
different noise figure because of manufacturing tolerances. The noise
figure quoted in the specifications, important for determining the LNB's
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.
Block downconversion Satellites use comparatively high radio
frequencies 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. Plastic glazing however 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 the superheterodyne principle to take a
block of relatively high frequencies and convert them to similar signals carried
at a much lower frequency. 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 is amplified and sent
down the cable to the receiver: C-band
Ku-band where 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, 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 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
oscillator. LNBFs
With the launch of the first DTH broadcast satellite in Europe by SES in
1988, antenna design was simplified for the anticipated mass-market. In
particular, the feedhorn and the polarizer were combined with the LNB
itself into a single unit, called an LNB-feed or LNB-feedhorn, or even an
"Astra type" LNB. The prevalence of these combined units has meant that
today the term LNB is commonly used to refer to all antenna units that provide
the block-downconversion function, with or without a feedhorn.
The Astra type LNBF that includes a feedhorn and polarizer is the most
common variety, and this is fitted to a dish using a bracket that clamps a
collar around the waveguide neck of the LNB between the feedhorn and the
electronics package. The diameter of the LNB neck and collar is usually 40mm
although other sizes are also produced. In the UK, the "minidish" sold for use
with Sky Digital and Freesat uses an LNBF with an integrated clip-in mount.
LNBs without a feedhorn built-in are usually provided with a flange around
the input waveguide mouth which is bolted to a matching flange around the
output of the feedhorn or polarizer unit.
Polarization It's common to polarize satellite TV
signals because it provides a way of transmitting more TV channels using a
given block of frequencies. This approach requires the use of receiving
equipment that can filter incoming signals based on their polarisation. Two
satellite TV signals can then be transmitted on the same frequency and
provided that they are polarized differently, the receiving equipment can
still separate them and display whichever one is currently required.
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. The probe inside the LNB waveguide
collects signals that are polarized in the same plane as the probe. To maximise
the strength of the wanted signals, the probe is aligned with the polarization
of the incoming signals. This is most simply achieved by adjusting the LNB's
skew; its rotation about the waveguide axis. To remotely select between the two
polarizations, and to compensate for inaccuracies of the skew angle, it used
to be common to fit a polarizer in front of the LNB's waveguide mouth. This
either rotates the incoming signal with an electromagnet around the waveguide or
rotates an intermediate probe within the waveguide using a servo motor but such
adjustable skew polarizers are rarely used today.
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. Astra type LNBFs incorporate two probes in the waveguide,
at right angles to one another so that, once the LNB has been skewed in its
mount to match the local polarization angle, one probe collects horizontal
signals and the other vertical, and an electronic switch determines which
polarization is passed on through the LNB for amplification and
block-downconversion. Such LNBs can receive all the
transmissions from a satellite with no moving parts and with just one cable
connected to the receiver, and have since become the most common type of LNB
produced. Common LNBs
= C-band LNB= Here is an example of a North American
C-band LNB: Local oscillator: 5.15 GHz
Frequency: 3.40–4.20 GHz Noise figure: ranges from 25 to 100
kelvins. Polarization: Linear
= Ku-band LNB= Standard North America Ku band LNB
Here is an example of a standard linear LNB:
Local oscillator: 10.75 GHz Frequency: 11.70–12.20 GHz
Noise figure: 1 dB typical Polarization: Linear
Universal 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 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. The launch of Astra 1E and subsequent
satellites saw the first use by Astra of the BSS band of frequencies for new
digital services and required the introduction of an LNB that would
receive the whole frequency range 10.70–12.75 GHz, the "Universal" LNB.
A Universal LNB has a switchable local oscillator frequency of 9.75/10.60 GHz
to provide two modes of operation: low band reception and high band reception.
The local oscillator frequency is switched in response to a 22 kHz signal
superimposed on the supply voltage from the connected receiver. Along with the
supply voltage level used to switch between polarizations, this enables a
Universal LNB to receive both polarizations and the full range of
frequencies in the satellite Ku band under the control of the receiver, in
four sub-bands: Here is an example of a Universal LNB
used in Europe: Noise figure: 0.2 dB typical
Polarization: Linear North America DBS LNB
Here is an example of an LNB used for DBS:
Local oscillator: 11.25 GHz Frequency: 12.20–12.70 GHz
Noise figure: 0.7 dB Polarization: Circular
= Ka-band LNB= Multi-output LNBs
= Dual, twin, quad, and octo LNBs= An LNB with a single feedhorn but
multiple outputs for connection to multiple tuners. Typically, two, four or
eight outputs are provided. Each output responds to the tuner’s band and
polarization selection signals independently of the other outputs and
"appears" to the tuner to be a separate LNB. Such an LNB usually may derive its
power from a receiver connected to any of the outputs. Unused outputs may be
left unconnected. Note: In the US, an LNB with two outputs
is termed a "dual LNB" but in the UK, the term "dual LNB" historically
described an LNB with two outputs, each producing one polarisation only, for
connection to a multiswitch and today "dual LNB" describes antennas for
reception from two satellite positions, using either two separate LNBs or a
single Monoblock LNB with two feedhorns. In the UK, the term "twin-output LNB",
or simply "twin LNB", is usually used for an LNB with a single feedhorn but
two independent outputs. = Quattro LNBs=
A special type of LNB intended for use in a shared dish installation to deliver
signals to any number of tuners. A quattro LNB has a single feedhorn and
four outputs, which each supply just one of the Ku sub-bands to a multiswitch or
an array of multiswitches, which then delivers to each connected tuner
whichever sub-band is required by that tuner.
Although a quattro LNB typically looks similar to a quad LNB, it cannot 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.
= SCR/Unicable LNBs= Multiple tuners may also be fed from an
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.
Instead of block-downconverting the whole received spectrum, an SCR LNB
downconverts a small section of the received signal selected according to a
DiSEqC-compliant command from the receiver, to output at a fixed frequency
in the IF. Up to 16 tuners can be allocated a different frequency in the
IF range and for each, the SCR LNB downconverts the corresponding
individually requested transponder. Most SCR LNBs also include either a
legacy mode of operation or a separate legacy output which provides the
received spectrum block-downconverted to the whole IF range in the conventional
way. Optical-fibre LNBs
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. The four sub-bands’ IFs are stacked to
create one IF with a range of 0.95–5.45 GHz, which is modulated on an optical
signal using a semiconductor laser, to send down the fibre cable.
At the receiver, the optical signal is converted back to the traditional
electrical signal to "appear" to the receiver as a conventional LNB.
Monoblock LNBs A monoblock LNB is a unit consisting of
two LNBs and is designed to receive satellites spaced close together,
generally 6°. For example in parts of Europe, monoblocks designed to receive
the Hot Bird and Astra 19.2°E satellites are popular because they enable
reception of both satellites on a single dish without requiring an expensive,
slow and noisy motorised dish. A similar advantage is provided by the duo LNB for
simultaneous reception of signals from both the Astra 23.5°E and Astra 19.2°E
positions. Cold temperatures
It is possible for any moisture in an LNB to physically freeze due to ice
build-up in very low temperatures. This is only likely to occur when the LNB is
not receiving power from the satellite receiver. 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 Dish Network receivers remains powered as well as those
receivers that 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. See also
Bias tee Block upconverter
Orthomode transducer Signal-to-noise ratio
Transmit and receive integrated assembly Duo LNB
Single Cable Distribution Fibre satellite distribution
References External links
LNB mysteries explained. Explanation and block diagram of LNB
Noise Temperature and Noise Figure OnAstra - Official Astra
consumers/viewers' site SES - Official SES trade/industry site
Astra Recommendations VSAT Installation Manual with
explanation of the Low-noise block converter