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ONCE AROUND  THE BLOCK....

 

INTRO

What's that 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...

 

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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.

 

LIQUID NITROGEN

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.

 

MONSTER DISHES

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 PRICE OF NOISE

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......

 

---end---

R.Maybury 1992 0508

 


 

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