Subject : Satellite Communication and Networks

Topic : Satellite Links

Intermodulation Distortion is often found as an amplifier or pre-amplifier specification that quantifies the non-harmonic frequencies added to an input signal.

Similar to Total Harmonic Distortion, Intermodulation Distortion is measured and represented as a percentage of the total output signal. Lower numbers are better for improved performance.

Intermodulation Distortion can occur when two or more signals are mixed through a non-linear amplifier device. Each of the tones interacts with each other, producing modulated amplitudes. This results in the formation of frequencies (referred to as "sidebands") not present in the original signal. Since these sideband frequencies pop up at the sum and difference of the original tones, they are considered non-harmonic and highly undesirable due to the unmusical nature.

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). 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 below, 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 Figure.

Since the linear portion gives a 1:1 change in decibels, the relationship between input and output BO is $[BO]0 = [BO]I - 5 dB$.