Have you ever come across the term satellite transponder in your Satellite communications subject?
Actually it is nothing more than a frequency converter. As we know a satellite has an uplink frequency of 6 GHz and Downlink frequency of 4 GHz. Then the question is which device does all that ???
A transponder is a device which performs the function of frequency translation within the satellite. A satellite usually will have 12 transponders having 36 MHz of bandwidth each. So the total bandwidth of satellite is about 500 MHz. A satellite may be of many types depending upon the band of frequency it uses to communicate.
For C band satellite, 500 MHz bandwidth of satellite ranges from 5925 - 6425 MHz for uplink and 3700 - 4200 MHz for downlink.
For Ku band satellite, 500 MHz of bandwidth ranges from 14 - 14.5 GHz for uplink and 11.45 - 11.7 GHz for downlink.