AROUND THE BLOCK....
strange-looking little box on the front of your satellite dish? What does it
do, and how does it do it? All will be revealed in our easy to follow guide to
that remarkable piece of space-age technology, the low noise block converter...
Without doubt the single most important component in any home satellite
TV system is the odd-shaped widget attached to the end of the arm, in front of
the dish. It's called the LNB, or low-noise block-converter, and it's job is to
collect and detect the incoming signals, amplify them, and then convert them to
a more manageable form before sending them down a cable to the set-top
receiver. It all sounds fairly straightforward....
Nowadays we take LNBs completely for granted, they're just another
anonymous little box full of electronics bits and pieces; they're cheap,
efficient and thoroughly unremarkable, until you realise precisely what they
have to do. By the time the microwave signals beamed from TV satellites have
passed through 36,000 km of space and the Earth's atmosphere and made their way
to your dish they're unimaginably weak, just a few hundredths of a billionth of
a watt in fact, and that is against a background roar of artificial and
naturally occurring noise, coming from
the Earth and space. Any additional noise, generated by the electronic
components inside the LNB, could easily wipe out such puny signals.
In the early days of radio
astronomy and satellite communications microwave amplifiers -- or parametric
amplifiers as they were known -- had to be immersed in tanks of liquid helium
or nitrogen, this had the effect of reducing noise levels by slowing down the
molecular motion of the materials that made up the electronic components in the
circuits. Clearly that would be impractical, not to say rather too dangerous
for a domestic receiving system, fortunately for us and the rest of the
satellite television industry a solution was found.
Satellite television as we know it now simply would not have been
possible without the development of a
device known as the gallium-arsenide field effect transistor or GasFet. This
component, used in the amplifier circuits inside an LNB, has extraordinarily
good low-noise characteristics, but unlike its predecessors, operates quite
happily at normal atmospheric temperature. The first GasFets were developed in
Japan in 1974, but it wasn't until the early 1980s that the first commercial
quantities were produced, leading to the development of the LNA or low-noise
amplifier, the forerunner of the LNB.
The LNA made home satellite receiving systems practical and affordable,
though ten years ago there were no direct-to-home (DTH) satellite channels, and
in order to receive signals from the first generation of low-power geosynchronous satellites, dishes
had to be several metres in diameter. However, in spite of that monster home
satellite dishes became a fairly common sight in American back yards, and at
the time there were plenty of TV channels to watch, distributed by satellite to
cable stations across the country, though scrambling soon put paid to that!
The LNA made way for the LNB following the gradual implementation of an
international treaty (World Administrative Radio Conference or WARC) which set out the frequency allocations for
direct broadcast (DBS) television; this resulted in a shift in satellite
transmission frequencies during the latter part of the 1980's, from the C-Band
(3.5 to 4.5 gigahertz -- a gigahertz or GHz for short, is a thousand million
cycles per second), to the KU-Band (10 to 17GHz). The C-Band was first used by
Telstar, the daddy of all satellites, and it is still in widespread use by many
of today's telecommunications satellites, but the C-Band is unsuitable for DTH
transmissions due to restrictions on power levels and its potential to
interfere with terrestrial microwave equipment.. The KU-Band, on the other hand
is little used outside of STV but the higher frequencies involved are extremely
difficult to handle once they've been received, so the block-converter part of
the LNB has the task of reducing the KU-Band signals to a lower frequency band,
so they can be processed more easily.
So what comes out of the socket or 'F-connector' on the back of an LNB?
In fact this is a two-way link with the outside world, and the cable that takes
the signal from the LNB to the set-top receiver also carries the LNB's DC power
supply from the receiver, and more often than not, polarity switching signals
as well. (satellite signals are 'polarised', to make more efficient use of the
frequency spectrum -- that's another subject for another day...). On a typical
Astra system the intermediate frequency or IF signal coming out of the LNB
contains all of the various TV channels, spread across a frequency band
extending from around 950Mhz to 1,500MHz
(1.5GHz). It's similar in effect to the broad range of signals coming
from a conventional roof-top TV aerial, in fact the span of frequencies used by
an LNB's IF output is not too far removed from the UHF band occupied by
terrestrial TV. This also means that the signal can be sent to the set-top
receiver along coaxial cable, similar to the type used for normal TV aerials.
The final question has to be are there any differences between the
various makes and types of LNB on the market?
The simple answer is most certainly yes, you only have to glance through
the advertisements in this magazine to see some huge variations in prices.
Price normally equates directly to performance, and that means noise, or rather
lack of it. This can be expressed in two ways either as noise temperature,
(measured in degrees Kelvin), or the more commonly quoted noise figure, stated
in decibels; this denotes the difference between the signal-to-noise (S/N)
ratios of the signal before and after it has passed through the LNB. Over the
past few years the noise figures of LNBs have been steadily falling. Five years
ago 2 to 2.5dB was considered good, today the norm is between 1.2 and 1.8dB.
High-performance LNBs generally have noise figures below 1dB, these would be
used on smaller dishes, in difficult signal areas, or on multi-satellite
systems. The second important criteria is frequency coverage. Most LNBs are
designed to operate over the band of frequencies used by TV satellites like
Astra. Special application or dual-band LNBs that operate over a alternative
bands (C-Band, X-Band, Ka-Band etc.), or a wider band of frequencies are
produced in far smaller numbers and consequently cost a lot more.
LNBs are extraordinary items of technology, and we've only just
scratched the surface. We could fill this magazine several times over with the
full technical inside story, and we haven't even begun to consider all the
gubbins on the front end of an LNB, including things like feedhorns, polarisers
and even the dish itself, but perhaps
now you'll see those little metal boxes
in a rather different light because without them there would be no satellite
television, no Bart Simpson, no Start Trek TNG, no WWF, mind you, there
wouldn't be any endless aussie soaps or ropey quiz shows either. Hmmm......
R.Maybury 1992 0508