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Outout backoff(OPBO) vs Input Backoff(IPBO) in RF Amplifier

Both the Output backoff(OPBO) and Input Backoff(IPBO) are used to determine the operating power levels required in a satellite transponder TWTA or RF Power Amplifier to function in linear region.

Output Back-Off (OPBO) is the power level at the output of RF amplifier relative to maximum output level possible using the RF amplifier.

EXAMPLE:
Maximum output level = +40dBm
Measured output level of RF Amplifier = +34dBm
Output Backoff(OPBO)= 6dB

Input Back-Off (IPBO) is the power level at the input of RF amplifier relative to input power which produces maximum output power.

Example:
Input power level causes max. output power = -20dBm
Actual input power level = -25dBm
Input backoff(IPBO)= 5dB

OPBO is used even to calculate inter-modulation distortion power levels at the output of TWTA/SSPA. For SSPA output backoff is same as 1dB compression point and for TWTA output backoff is same as saturated output power.

In a satellite link calculation, input backoff is calculated by taking difference between actual flux density and the saturated flux density. Also calculated Output backoff help calculated actual EIRP of the satellite.

Following table mentions typical backoff requirements in SCPT (Single Carrier Per Transponder) and MCPT (Multiple Carrier Per Transponder) operations.

Number of carriers Mode Ratio of peak power to average power Output backoff in dBs
1 SCPT 4 0
2 MCPT 4 3
6 MCPT 3 4.5
20 MCPT 1 5
200 MCPT 0 5

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