Mumbai University > EXTC > Sem 8 > Satellite Communication and Networks

Marks: 10M

Input back-off

When a number of carriers are present simultaneously in a TWTA, the operating point must be backed off to a linear portion of the transfer characteristic to reduce the effects of intermodulation distortion. Such multiple carrier operation occurs with frequency division multiple access (FDMA). The point to be made here is that backoff (BO) must be allowed for in the link budget calculations.

If the saturation flux density for single-carrier operation is known. Input BO will be specified for multiple-carrier operation, referred to the single-carrier saturation level. The earth-station EIRP will have to be reduced by the specified BO, resulting in an uplink value of

[EIRP]U = [EIRPS]U - [BO]i

Output back-off

Where input BO is employed a corresponding output BO must be allowed for in the satellite EIRP. As the curve of Fig, output BO is not linearly related to input BO. A rule of thumb, frequently used, is to take the output BO as the point on the curve which is 5 dB below the extrapolated linear portion, as shown in Fig. 12.7.

Since the linear portion gives a 1:1 change in decibels, the relationship between input and output BO is [BO]0 = [BO]I - 5 dB. For example, with an input BO of [BO]i = 11 dB, the corresponding output BO is [BO]0 = 11 - 5 = 6 dB.

AM/PM coversion

In Travelling Wave Tube (TWT), the absolute time delay between input and output signals at a fixed input level is generally not significant. However, 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.

Since phase delay is directly proportional to time delay, 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 figure as a function of input power. Thus, if the input signal power level changes, phase modulation will result, this being termed AM/PM conversion. The slope of the phase shift characteristic gives the phase modulation coefficient, in degrees per decibel. The curve of the slope as a function of input power is also sketched in Figure

Phase characteristics for a TWT. θ is the input-to-output phase shift, and S is the value at saturation. The AM/PM curve is derived from the slope of the phase shift curve.