This question appears in Mumbai University > Satellite Communication and Network subject

Marks: 10 M

Year: Dec 2012

Intermodulation distortion:

The input output characteristic of non linear device is given as

$v_o=a+bv_i+cv_i^2+dv_i^3+⋯……….(1)$

where $v_i=k_1 cosω_1 t+k_2 cosω_2 t…….$ Input signal

a, b, c and so on are coefficients which depend on the transfer characteristic

$v_o$ is the output voltage

$∴v_o=a+b(k_1 cosω_1 t+k_2 cosω_2 t)+c(k_1 cosω_1 t+k_2 cosω_2 t)^2+d(k_1 cosω_1 t+k_2 cosω_2 t)^3+……………….. (2)$

Expanding above equation we get

$v_o=a+b(k_1 cosω_1 t+k_2 cosω_2 t)+c[\frac{(k_1 (1+cos2ω_1 t)}2+\frac{(k_2 (1+cos2ω_2 t)}2+\frac{2k_1 k_2 (cos⁡(ω_1-ω_2 )t+cos⁡(ω_1+ω_2 )t)}2]+d[k_1 cos3ω_1 t+k_2 cos3ω_2 t+cos⁡(2ω_1-ω_2 )t+cos⁡(2ω_2-ω_1 )t+⋯]$

From the above expression we get the following frequency components $ω_1,ω_2,ω_1±ω_2,2ω_1,2ω_2,2ω_1-ω_2,2ω_2-ω_1…$ It is seen that the frequency $2ω_1-ω_2,2ω_2-ω_1$ lie near the pass-band and it becomes difficult to filter out this frequency component.

The distortion caused by this frequency component is known as Inter-modulation distortion

The third order contribution is significant

For single carrier operation the device can be operated in the saturation(nonlinear) region. However for multiple carrier operation it must be below 1dB compression point to avoid intermodulation distortion. i.e. In order to reduce the intermodulation distortion, the operating point of the TWTA must be shifted closer to the linear portion of the curve, the reduction in input power being referred to as input backoff

Input backoff is difference in decibels between the carrier input at the operating point and the saturation input which would be required for single carrier operation.

The need to incorporate backoff significantly reduces the channel capacity of a satellite link because if the reduced carrier-to-noise ratio received at the ground station.