One of the most important specifications in a wireless transmitter is the adjacent channel power ratio (ACPR), which is used to measure the nonlinear distortion in the transmitted signal. ACPR, together with the modulation scheme, determines the maximum allowable nonlinearity of the power amplifier, the last active circuit block before the antenna. Although other measures of distortion such as harmonic or intermodulation distortion have been analyzed before, the relationship between the physical mechanisms in the transistors and ACPR is not well understood. Designers are also hampered by the difficulty of simulating ACPR, as carrier frequencies are often two orders of magnitude higher than the channel bandwidth.
In this research project, we will try to predict ACPR in linear RF power amplifiers required in wireless systems using non-constant envelope modulation schemes, such as CDMA. At first, frequency domain Volterra kernels will be calculated to model the nonlinear behavior of the power amplifier. Then the baseband equivalent of the transmitted signal will be fed into this model in order to estimate ACPR using MATLAB. As all of the simulations will be done in the frequency domain, the simulations are not expected to take long. This method can easily be used for processes from different vendors, once the basic SPICE parameters are known. Besides, it will also help designers during the initial design phase, because it does not require any amplifier to be fabricated and measured beforehand, as opposed to the empirical methods that utilize some form of parameter fitting. Vendors can also modify their processes to design special transistors for power amplifiers, once the main contributors to distortion and tradeoffs are identified.
The accuracy of this method will be tested by making measurements on single and two stage SiGe bipolar power amplifiers designed and fabricated using a commercially available BiCMOS process from Maxim Integrated Products. The measurements will be made using the IEEE 802.11b standard which operates at a 2.4 GHz ISM band.