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RFIC PA Development for Communication and Radar Systems

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RFIC PA Development for Communication and Radar Systems 6 The third-order (and fifth-order IM products at 2ω1 - ω2 and 2ω2 - ω1 are shown in Figure 7. If the difference between ω1 and ω2 is small, the IM products will be in the band of the amplifier and so understanding the nonlinear behavior of the amplifier is important. The corruption of signals due to third-order intermodulation of two nearby interferers is so common and critical that a performance metric called the third-order intercept point (IP3) has been defined to characterize this behavior. This behavior can be simulated with HB using a two-tone analysis. Figure 7: The third-order (and fifth-order) IM products as a function of swept input power. System Metrics Amplifier nonlinearities are especially critical in modern communication systems that utilize digital modulation techniques to enhance data rates. Specific metrics, which can be simulated with circuit-envelop techniques, are used to quantify amplifier behavior in the presence of digitally modulated RF signals, including error vector magnitude (EVM), adjacent-channel power ratio (ACPR), and complementary cumulative distribution function (CCDF), which are described below. EVM is a measure used to quantify the performance of a digital radio transmitter or receiver. A signal sent by an ideal trans- mitter or received by a receiver would have all constellation points precisely at the ideal locations, however, various imperfec- tions in the implementation (such as carrier leakage, low image rejection ratio, phase noise, and more) cause the actual constellation points to deviate from the ideal locations. Informally, EVM is a measure of how far the points are from the ideal locations. Noise, distortion, spurious signals, and phase noise all degrade EVM, and therefore EVM provides a comprehensive measure of the quality of the radio receiver or transmitter for use in digital communications. ACPR is a measure of the degree of signal spreading into adjacent channels, caused by nonlinearities in the PA. It is defined as the power contained in a defined bandwidth at a defined offset from the channel center frequency, divided by the power in a defined bandwidth placed around the channel center frequency. These metrics can be simulated for amplifiers defined as a Cadence AWR ® Microwave Office software circuit schematic in the AWR Design Environment platform and analyzed with the circuit-envelope simulator in Cadence AWR Visual System Simulator™ (VSS) system design software. AWR VSS software is a sampled time-domain simulator that uses a fixed-time step, which is set either by the default system settings for every system diagram or by individual blocks inside a system diagram (usually sources) and inherited by subsequent blocks.

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