卫星通信第7章改.ppt

* * Chapter 7 The Space Segment 7.7 Transponder The advantage of the TWT over other types of tube amplifiers is that it can provide amplification over a very wide bandwidth. Input levels to the TWT must be carefully controlled, however, to minimize the effects of certain forms of distortion. * * Chapter 7 The Space Segment 7.7 Transponder At low input powers, the output-input power relationship is linear. The point of maximum power output being known as the saturation point. The saturation point is a very convenient reference point. The linear region of the TWT is defined as the region bound by the thermal noise limit at the low end and by what is termed the 1-dB compression point at the upper end. The absolute time delay between input and output signals at a fixed input level is generally not significant. At higher input levels, where more of the beam energy is converted to output power, the average beam velocity is reduced, and therefore, the delay time is increased. This results in a phase shift which varies with input level. Denoting the phase shift at saturation by θs and in general by θ, the phase difference relative to saturation is θ - θs. This is plotted in Fig. as a function of input power. Thus, this is termed AM/PM conversion. * * Chapter 7 The Space Segment 7.7 Transponder * * Chapter 7 The Space Segment 7.7 Transponder Frequency modulation (FM) is usually employed in analog satellite communications circuits. However, unwanted amplitude modulation (AM) can occur from the filtering which takes place prior to the TWT input. * * Chapter 7 The Space Segment 7.7 Transponder A TWT also may be called on to amplify two or more carriers simultaneously, this being referred to as multicarrier operation. The nonlinear transfer characteristic introduces a more serious form of distortion known as intermodulation distortion.Usually only the third-order contribution is hard limiter. At frequencies f1 and f2, these will give rise to frequencies 2f2 -fl and 2