Denoting thephase shift at saturation by S and in general by , the phase differ-ence relative to saturation is Ϫ S. Since phase delay is directly proportional to time delay, thisresults in a phase shift which varies with input level. However, at higher input lev-els, where more of the beam energy is converted to output power, theaverage beam velocity is reduced, and therefore, the delay time isincreased. The absolute time delay between input and output signals at a fixedinput level is generally not significant. The selection of the operating point on the trans-fer characteristic will be considered in more detail shortly, but first thephase characteristics will be described. This is the point where the actu-al transfer curve drops 1 dB below the extrapolated straight line, asshown in Fig. The linear region of the TWT is defined as the region bound by thethermal noise limit at the low end and by what is termed the 1-dBcompression point at the upper end. The Space Segment 191Figure 7.18 Power transfer characteristics of a TWT.The saturation point is used as 0-dB reference for bothinput and. (From Hughes TWT and TWTA Handbook courtesy of Hughes Aircraft Company, Electron Dynamics Division, Torrance, CA.) TLFeBOOK The saturation point is a very convenient ref-erence point, and input and output quantities are usually referred to TLFeBOOKġ90 Figure 7.17 Schematic of a TWT and power supplies. At higher power inputs, the out-put power saturates, the point of maximum power output being knownas the saturation point. At low input powers, the output-input power relationship is lin-ear that is, a given decibel change in input power will produce thesame decibel change in output power. The worst of these results fromthe nonlinear transfer characteristic of the TWT, illustrated in Fig.7.18. Input levelsto the TWT must be carefully controlled, however, to minimize theeffects of certain forms of distortion. The advantage of the TWT over other types of tube amplifiers is thatit can provide amplification over a very wide bandwidth. Because of thiseffective reduction in phase velocity, the helix is referred to as a slow-wave structure. This component is less than the velocity of light approxi-mately in the ratio of helix pitch to circumference. The wave actuallywill travel around the helical path at close to the speed of light, but itis the axial component of wave velocity which interacts with the elec-tron beam. Under these con-ditions, an energy transfer takes place, kinetic energy in the beambeing converted to potential energy in the wave. The average beam velocity, which is deter-mined by the dc potential on the tube collector, is kept slightly greaterthan the phase velocity of the wave along the helix. In some regions, this field will decelerate the electrons in thebeam, and in others it will accelerate them so that electron bunchingoccurs along the beam. Theelectric field of the wave will have a component along the axis ofthe helix. The rf signal to be amplified is coupled into the helix at the endnearest the cathode and sets up a traveling wave along the helix. The compar-atively large size and high power consumption of solenoids make themunsuitable for use aboard satellites, and lower-power TWTs are usedwhich employ permanent-magnet focusing. ![]() For high-powertubes such as might be used in ground stations, the magnetic field canbe provided by means of a solenoid and dc power supply. A magnetic field is required to confinethe beam to travel along the inside of a wire helix. In the TWT, an electron-beam gunassembly consisting of a heater, a cathode, and focusing electrodes isused to form an electron beam. Figure 7.17 shows the schematic of a traveling wave tube(TWT) and its power supplies. Traveling-wave tube amplifiers (TWTAs) are widely used intransponders to provide the final output power required to the trans-mit antenna. Because this vari-able attenuator adjustment is an operational requirement, it must beunder the control of the ground TT&C station. The variable attenuation is needed to setthe level as required for different types of service (an example being therequirement for input power backoff discussed below). The Space Segment 189being made during assembly. (Courtesy of CCIR, CCIR Fixed Satellite Services Handbook, final draft 1984.) TLFeBOOK The Space Segment 187Figure 7.15 Input demultiplexer.(Courtesy of CCIR, CCIR FixedSatellite Services Handbook, finaldraft 1984.) TLFeBOOKġ88 Figure 7.16 Typical diagram of the relative levels in a transponder.
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