Similar questions

Explain why operation near the saturation point of TWTA is to be avoided when multiple carriers are being amplified.

Marks: 6 M, 10 M

Year: Dec 2014, May 2015

• 1 dB compression point is defined a point on the transfer characteristic of TWTA where the actual transfer curve drops 1dB below the extrapolated straight line
• At low input powers the output input relationship for a TWT is linear. At higher power inputs however, the output power saturates, the point of maximum power output being known as the saturation point
• The linear region of the TWT is defined as a region bounded by the thermal noise limit at the low end by the 1 dB compression point at the upper end
• In order to reduce intermodulation distortion, the operating point of the TWT must be shifted closer to the linear portion of the curve
• The TWTA in the satellite transponder exhibits both amplitude non linearity and phase non linearity. As the number of carrier increases, it becomes necessary to operate TWTA close to the saturation region in order to supply the required power per carrier to reduce the effect of downlink thermal noise.

Transfer Characteristics of TWTA

• But near saturation region the input/output amplitude transfer characteristic of the TWTA is highly non-linear and the level of the inter modulation products is increased which affects the overall performance
• These intermodulation products are close to the operating carrier frequency so cannot be filtered and when these carrier frequencies are modulated the intermodulation distortion appears as noise across transponder frequency band where f1 and f2 are the carrier frequencies and other are intermodulation noise.

• So in order to reduce the intermodulation noise the output power ratings of TWTA must be decreased i.e. the operating point must be shifted to the linear portion of the curve, so that it can be backed off from saturation region sufficiently to reduce intermodulation effects.