Anatomy of a Satcom Terminal

The circulator is used to make sure that the transmit signals go out through the dish and not back into the receive chain. It also makes sure that the receive signals come from the dish into the receive chain and not into the transmit chain. It works much like a roundabout in principle. This is often referred to as an Orthomode Transducer or OMT and is, these days, built into the feed assembly. It can also be a polarisation device but they all do the same job.

The Rx or receive filter is usually a waveguide filter which tightly controls the frequencies allowed into the receive chain. This has the effect of reducing the unwanted noise from space and prevents interference from outside of the receive band of frequencies. This is often built in to the feed assembly.

A Tx filter.

The Tx or transmit filter is usually a waveguide filter which tightly controls the frequencies allowed into antenna. This has the effect of reducing the unwanted signals from being accidentally transmitted onto the stallite and prevents interference to outside of the transmit band of frequencies. This is often built in to the feed assembly.

An LNA.

The LNA (Low Noise Amplifier), sometimes known as an LNB (Low Noise Block) on receive only terminals, is a very good amplifier which has the job of amplifying the small signals picked up by the antenna without amplifying the noise. Various kinds exist which all do the same thing, they provide enough signal level to demodulate the data from the carrier. The LNB is more than just an amplifier as it also handles polarisation selection and the first stage of down conversion to L-Band frequencies. Polarisation is just like a UHF TV or Terrestrial signal and can be horizontal. vertical or circular polarised.

The HPA (High Power Amplifier), otherwise known as a TWTA (Travelling Wave Tube Amplifier) or an SSHPA (Solid State High Power Amplifier), has one job. It amplifies a specific band of frequencies by a large amount, sufficiently large to enable the antenna to beam them up to the satellite. These can range in power from a few watts upto over 1000 watts in power. The bigger the dish, usually the bigger the power amp. The largest have to be cooled using liquid nitrogen and resemble electron microscopes. The smallest look more like a lump of metal bolted to a small heatsink.

1st & 2nd Down Converters.

The down converters 'do exactly as they say on the tin', they convert signals down in frequency. The signals arrive at the dish at anything from 10 to 40 GHz and are then filtered and amplified, they now need to be moved down the frequency spectrum so that the equipment can be made cheaper and easier. The 1st downconverter mixes the signals with another frequency, the result is both the sum and difference of the signals. By filtering out the original and the sum frequencies the result is that the original frequencies are now the difference frequencies - lower down in the frequency spectrum. An example would be the downconversion of 10 GHz to 1 GHz which is Ku band to L Band. The 2nd downconverter then downconverts the L Band signals to an Intermediate Frequency (IF) of around 70 MHz. this is then ready for the demodulator.

1st & 2nd Up Converters.

The up converters 'do exactly as they say on the tin' aswell, they convert signals up in frequency. The signals are sent to the up converters at at around 70 MHz. They now need to be moved up the frequency spectrum so that the HPA can amplify them and transmit them through the antenna. The 1st upconverter mixes the signals with another frequency, the result is both the sum and difference of the signals. By filtering out the original and the difference frequencies the result is that the original frequencies are now the sum frequencies - higher up in the frequency spectrum. An example would be the upconversion of 70 MHz to 1 GHz which is IF to L Band. The 2nd upconverter then upconverts the L Band signals to a Radio Frequency (RF) of around 10 GHz. this is then ready for the HPA to transmit through the antenna.

Demodulator / Modulator. (MODEM)

As mentioned previously, these two units are often combined as one and are known as modems. Just like the computer modem you may have at home, these units take digital data and modulate it onto a carrier and they demodulate the digital data from a carrier. Computer modems use audio frequency carriers but the end result is the same. The data isn't always provided in digital form to the modem. Often, especially in digital TV, the input is analogue and a converter inside them modem converts from analogue to digital. This is usually then compressed into one of many video compression formats. MPEG 2 is the most common at the moment and is the format used on Sky Digital. The outside broadcast cameras on the news plug into the modem as ordinary analogue video. This is then converted to MPEG 2 digital data and is modulated onto the carrier. The received data is converted from MPEG 2 into analogue video by another converter within the demodulator side of the modem. This video is then viewable on a video monitor. Our Sky Digiboxes are simply demodulators that do exactly that. the difference being that the downconversion to L Band takes place in the LNB on the dish. The L Band is further downconverted within the Digibox but other than that the principle is the same.