1.2 Lower Frequency Analog Design and Microwave Design Versus Radio-Frequency Integrated Circuit Design

Chapter 1.2 Lower Frequency Analog Design and Microwave Design Versus Radio-Frequency Integrated Circuit Design

Radio Frequency Integrated Circuit Design Second Edition Book
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Radio Frequency Integrated Circuit Design Second Edition Book

  • Introduction to Communications Circuits1.2   Lower Frequency Analog Design and Microwave Design Versus Radio-Frequency Integrated Circuit DesignRadio-frequency integrated circuit design has borrowed from both analog design techniques, used at lower frequencies [4, 5], and high frequency design techniques, making use of microwave theory [6, 7]. The most fundamental difference between low frequency analog and microwave design is that in microwave design, transmis-sion line concepts are important, while in low-frequency analog design, they are not. This will have implications on the choice of impedance levels, as well as how signal size, noise, and distortion are described. On-chip dimensions are small, so even at RF frequencies (0.1–60 GHz), tran-sistors and other devices may not need to be connected by transmission lines (i.e., the lengths of the interconnects may not be a significant fraction of a wavelength). However, at the chip boundaries, or when traversing a significant fraction of a wavelength on-chip, transmission line theory becomes very important. Thus, on chip we can usually make use of analog design concepts, although, in practice, mi-crowave design concepts are often used. Where the chip interfaces with the outside world, we must treat it like a microwave circuit. 1.2.1  Impedance Levels for Microwave and Low-Frequency Analog DesignIn low-frequency analog design, input impedance is usually very high (ideally infinity) while output impedance is low (ideally zero). For example, an opera-tional amplifier can be used as a buffer because its high input impedance does not affect the circuit to which it is connected, and its low output impedance can drive a measurement device efficiently. The freedom to choose arbitrary imped-ance levels provides advantages in that circuits can drive or be driven by an im-pedance that best suits them. On the other hand, if circuits are connected using transmission lines, then these circuits are usually designed to have an input and output impedance that match the characteristic impedance of the transmission line.1.2.2  Units for Microwave and Low-Frequency Analog DesignSignal, noise, and distortion levels are also described differently in low frequency analog versus microwave design. In microwave circuits, power is usually used to describe signals, noise, or distortion with the typical unit of measure being decibels above 1 milliwatt (dBm). However, in analog circuits, since infinite or zero imped-ance is allowed, power levels are meaningless, so voltages and currents are usually chosen to describe the signal levels. Voltage and current levels are expressed as peak, peak-to-peak, or root-mean-square (rms). Power in dBm, PdBm, can be related to the power in watts, Pwatt, as shown in (1.1) and Table 1.1, where voltages are assumed to be across 50W. æö=ç÷èøwattdbm1010log1 mWPP (1.1)