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Simple Linear Amplification
A simple power amplifier chain typically consists of a chain of gain stages -- ̶0;gain̶1; is the ratio of output to input -- each of which amplifies the signal by between 6 and 20 decibels. This type of power amplifier chain typically delivers an ACPR of -25 decibels, relative to the power of the main channel, with 50 percent efficiency at peak envelope power, or maximum power output.
ACPR Performance
Many different types of power amplifier exist, but most of them employ techniques that go beyond simple linear amplification. In other words, they are nonlinear, meaning the relationship between their input power and output power cannot be plotted on a graph as a straight line. Their non-linearity causes more power to leak from the main channel into adjacent channels. Linearity, or nonlinearity, can be measured using various techniques, depending on the characteristics of the signal. The ACPR performance of a power amplifier can be determined by measuring the power-spectral density at a certain frequency interval above and below the frequency of the main channel.
Intermodulation Distortion
ACPR is closely linked to the nonlinear distortion, or intermodulation distortion, characteristics of power amplifiers and hence provides a measure of linearity in real-life, commercial systems. The commercial wireless broadband access technology known as WiMAX, for example, must maintain high power levels, but low ACPR, so that adjacent channels are not completely obliterated by spectral energy from the main channel. In practice, WiMAX shows only minor variations of plus or minus 1 decibel in ACPR over a wide range of temperatures.
Considerations
In designing efficient power amplifiers for cellular radio handsets and similar devices, it is impossible to design for absolute interference, or distortion, levels. Therefore, ACPR is used to measure distortion in adjacent channels relative to the main channel. In digital technologies, such as North American Digital Cellular and Code Division Multiple Access, used in cellular phone systems, system designers must impose maximum levels for distortion that can be introduced to adjacent channels.