| The functions performed by repeaters are | | | | and distortion. Present communication |
| amplification and frequency down conversion. | | | | satellite repeaters use dual mode filters |
| Repeater amplifies signal from a low power | | | | using cavity coupling and having centre |
| level of the order of negative hundred | | | | frequency to bandwidth ratio of about 100000 |
| decibels watt to an output power level of the | | | | and C band and about 10000 at about k band. |
| order of ten decibel watts thus providing | | | | Accordingly C band filter has bandwidth of |
| again of the order of hundred and ten | | | | about thirty six mega hertz. |
| decibel. Frequency down conversion avoids | | | | |
| interference between strong transmitted | | | | Primary function of carrier processing |
| signals and weak returning signal sometimes | | | | equipment is frequency translation that is to |
| these repeaters perform the additional | | | | bring the uplink signal to lower frequency |
| function of detection or demodulation then | | | | down link signal. Frequency translation can |
| the repeater or the transponder is called | | | | be done in the following four different ways. |
| regenerative transponder. | | | | Conventional repeater use either radio |
| | | | frequency-radio frequency translation or |
| Parts of a typical satellite repeater are | | | | radio frequency- Intermediate frequency-radio |
| satellite antenna, diplexer, front end | | | | frequency translation where as regenerative |
| electronic system, carrier processing system | | | | repeater uses either intermediate frequency |
| and power amplifier. In diplexer the same | | | | modulation method or demodulation-re |
| antenna is being used by the satellite for | | | | modulation method. Thus the difference in the |
| receiving the uplink signal and transmitting | | | | conventional repeater and regenerative |
| the downlink signal. This is made possible by | | | | repeater lies only in the structure of its |
| use of an electronic device called diplexer | | | | carrier processing unit. Last unit of the |
| which serves a two way microwave gate. Since | | | | satellite is the power amplifier. Traveling |
| two different frequencies are used for uplink | | | | wave tube amplifiers re popularly used as |
| and down link there is no interference | | | | power amplifiers. The size of traveling wave |
| between the two transmissions and the same | | | | tube depends upon the purpose of the |
| antenna serves satisfactorily. Front end | | | | satellite. Typical parameters of traveling |
| electronic system is made up of radio | | | | wave tube amplifiers are saturated output |
| frequency filter, equalizer and low noise | | | | power is eight to twenty watts, saturated |
| amplifier. Signal from the diplexer is fed | | | | gain is fifty five decibel, carrier to inter |
| to the radio frequency filter. It is an | | | | modulation noise power ratio at saturation is |
| integrated circuit microwave filter having | | | | ten to twelve decibel and amplitude |
| merits of better packaging, smaller weight | | | | modulation-pulse modulation conversion |
| and lesser power requirements. This filter is | | | | coefficient is 4.5 decibels. |
| designed to achieve requisite mask, noise | | | | |
| rejection and equalization. It should not add | | | | Tymon Hytem has worked in the electronics |
| any circuit noise. | | | | feild for the past 15 years. He enjoys |
| | | | helping people decide on electronic gadgets |
| Typical filters so used are Butterworth, | | | | from telephones to XM Radio and choosing the |
| Cheby Chev or Bessel filters. The delay | | | | perfect XM Satellite Radio system for their |
| introduced by the filter is cancelled by an | | | | needs. |
| equalizer without introducing any attenuation | | | | |