d might be on a frequency of 6 GHz. The satellite receives the signal and retransmits it. This might be on a frequency of 4 GHz on what is termed the downlink. This has to be on a different frequency to avoid interference between the two signals. If the transmission took place on the same frequency as the received signal, then the transmitted signal would overload the receiver in the satellite, preventing reception on the uplink. The use of satellites is very widespread and a wide variety of frequencies are used. While the figures included here are typical, many other frequencies are also used. The circuitry in the satellite that acts as the receiver, frequency changer and transmitter is called a transponder. As shown in Figure 8.7, this basically consists of a low noise amplifier, a frequency changer consisting of a mixer and local oscillator, and then a high power amplifier. The filter on the input is used to make sure that any out of band signals such as the transponder output are reduced to acceptable levels SO that the amplifier Satellites 233 (6 GHz) (6 GHz) (4 GHz) (4 GHz) Band-pass Low noise Mixer Amplifier High power Band-pass filter amplifier amplifier filter (2 GHz) Frequency coverage limited to the bandwidth required to be received Local Figures in brackets oscillator are from the example used in the text. Figure 8.7 Block diagram of a basic transponder is not overloaded. Similarly the output from the amplifiers is filtered to make sure that spurious signals are reduced to acceptable levels. Figures used in here are the same as those mentioned earlier, and are only given as an example. The signal is received and amplified to a suitable level. It is then applied to the mixer to change the frequency in the same way that occurs in a superhet receiver. As a result the satellite receives in one band of frequencies and transmits in another. In view of the fact that the receiver and transmitter are operating at the same time and in close proximity, care has to be taken in the design of the satellite that the