1) Point of compression of 1 dB
- 1 dB compression point is defined as 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 and the point of maximum power output is known as the saturation point.
- The linear region of the TWT is defined as a region bound by the thermal noise limit at the low end by the 1 dB compression point at the upper end.
Transfer Characteristic of TWTA
- 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 in order to supply the required power per carrier to reduce the effect of downlink thermal noise. But near the saturation region the input-output amplitude transfer characteristic of the TWTA is highly nonlinear and the level of the inter modulation products is increased which the affects the overall performance of the satellite. The output power ratings of TWTA must be decreased so that it can be backed off from saturation sufficiently to reduce intermodulation effects.
2) Am to Pm conversion
- TWT amplifiers are designed to operate with constant amplitude input carriers to achieve cavity gain. Variations in input amplitude produce an unintentional phase modulation on the amplified carrier, referred to as AM/PM conversion.
- AM/PM conversion is a form of carrier distortion in which envelope variations on the total multiple-carrier waveform being amplified which is converted into phase variations on each individual carrier.
- These phase variations appears as additive waveform interference in angle modulated carriers. Its presence in cavity field being amplified cause a variable retardation on the field in the cavity.
- This time-varying retardation appears as phase delay variation or phase modulation in synchronism with the envelope variations on the cavity field.
- Thus, the resulting amplifier output waveform then has an additive phase modulation proportional to envelope variation.
- When the envelope variations are due to thermal noise, the additive phase variation appears as added random phase noise interference.
- When the envelope variation is due to baseband modulation, the AM to PM of the non-linear power amplifier may cause the modulated information to be coupled into the carrier as a form of intelligible cross talk.
3) Input and output back off ratio
- The input backoff is the difference in dB between the carrier input power at the operating point and saturation point which would be required for single-carrier operation.
- Output back off: The output backoff is the corresponding drop in output power. Output back off is about 5dB less than input backoff.