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when using an amplifier you need to allow for mini and maxi input power, which create the dynanic range of the amplifier.
therefor you need to know how the amplifier behaves at this two extreme depending of the modulation scheme it will used with.
As you drive an amplifier with more input power, the output compresses. That is, 1 dB more input power yields only 0.5 dB of output power increase. So any amplitude modulation gets somewhat stripped off. In addition, as the amplifier compresses, the angle of S21 changes also, so any phase modulation can get rotated momentarily.
So you have 2 problems.
1) If you overdrive an amplifier with certain types of modulation, it generates out of band spurious signals that can violate the emissions mask. "Non constant envelope" modulations do this.
2) If you overdrive an amplifier with certain types of modulation, the instantaneous phase and amplitude gets distorted by the amplifier, leading to bit errors
The level of Back off depends on the crest factor of the modulated signal.
Eg: pi/4Dqpsk modulation has a crest factor of 3.3dB (crest factor means peak to average value of the modulated signal envelope). CDMA signal has a crest factor of about 10dB.
So if you use a RF power amplifier for CDMA you have to operate at a power 10 dB less than its maximum power to avoid saturation or clipping of the signal envelope.
similerly in pi/4 DQPSK you have to back of by 3.3dB.
We need not back off for an FM system, since there is no information in amplitude.
Hope now have understood the concept.
As a PA designer myself, I hate the word "back-off".
Motorola used to define linear PA by PEP output power with certain
IMD level.
Somehow, the wireless industry used to talk about average power, which,
I believe, is misleading.
Suppose an Audio Power Amplifier. If you input long silence, there will be
no output, except small noise. As Audio PA can not control the content of music,
RF PA can not control modulation scheme.
No, I am not supporting the marketing buzword "PMPO".
I have seen too many cases that specify average output power without
defining PAR or even mentioning modulation scheme.
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