c h a p t e r 2Issues in RFIC Design: Noise, Linearity, and Signals2.1 IntroductionIn this chapter we will have a brief look at some general issues in RF circuit design. The nonidealities we will consider include noise and nonlinearity. An ideal circuit, for example an amplifier, produces a perfect copy of the input signal at the output. In a real circuit, the amplifier will add both noise and distortion to that waveform. Noise, which is present in all resistors and active devices, limits the minimum de-tectable signal in a radio. At the other amplitude extreme, nonlinearities in the cir-cuit blocks will cause the output signal to become distorted, limiting the maximum signal amplitude. At the system level, specifications for linearity and noise, as well as many other parameters, must be determined before the circuit can be designed. In this chapter, before we look at circuit design, we will look at some of these system issues in more detail. In order to design radio frequency integrated circuits with realistic specifica-tions, we need to understand the impact of noise on minimum detectable signals and the effect of nonlinearity on distortion. Knowledge of noise floors and distor-tion will be used to understand the requirements for circuit parameters. Finally, the chapter will conclude by studying some common digital modulations used in com-munications. In many cases the modulation used will determine the performance levels required by the various RFIC blocks to make the radio work properly.2.2 NoiseSignal detection is more difficult in the presence of noise. In addition to the desired signal, the receiver also picks up noise from the rest of the universe. Any matter above zero Kelvin contains thermal energy. This thermal energy moves atoms and electrons around in a random way, leading to random currents in circuits, which are also seen as noise. Noise can also come from manmade sources such as micro-wave ovens, cell phones, pagers, radio antennas, and so forth. Circuit designers are mostly concerned with how much noise is being added by the circuits in the trans-ceiver. At the input to the receiver, there will be some noise power present, which defines the noise floor. The minimum detectable signal must be higher than the noise floor by some signal-to-noise ratio (SNR) to detect signals reliably and to compen-sate for additional noise added by circuitry. These concepts will be described in the following sections.