Radio Frequency Measurements

Abstract

AbstractMeasurement is fundamental to our understanding of any area of knowledge. The chapter starts with Frequency and time. Accuracy of measuring time grew beyond the wildest imagination, within the past few decades. Time has now become the fundamental unit of measurement even for length and mass since they are specified in terms of time. The accuracy of frequency and time far exceeds any other quantity that is measurable today. An important anchor in this area is “crystal oscillator” whose stability and reference to primary standard such as the Cesium beam is explained in detail.This is followed by RF power which is difficult to quantify accurately. The general approach is to take DC as reference point and micro-calorimeter as a reference standard to mark a stepping-stone in primary standards. RF power steers all wireless systems both in capacity and coverage as well as distance. Power meters with thermistor mount as secondary standard and working standard are described. Other network issues such as impedance matching and how it affects accuracy of RF power measurement are discussed, leading to the measurement of impedance. Thruline power meter, its simplicity and ability to read forward and return power is discussed to show how a field instrument can remain versatile for decades.Since RF impedance is a vector quantity that changes with frequency, an important tool—the Smith chart (uses first and fourth quadrants of cartesian system) is described. Since series and parallel circuits have their vision of resistance and its reciprocal admittance—these are discussed. RF—IV method of voltage ratio measurements using a fixed value resistor, is described. Such ratio-based methods provide better accuracy by referencing to a stable value.Scattering parameters or S-parameters are used by vector analyzers to characterize impedance. This is discussed in detail, indicating how they evaluate impedance and establish the nature of impedance in any network. Network analyzers test a wide range of active and passive devices—these are described with examples to show how Smith chart paved the way to automated impedance calculations for both capacitive and inductive loads—also showing how matching can be achieved in each case.Noise is inherent in all RF systems—it is a fundamental quantity that must be addressed seriously to make sure the system works. Starting with the power line from the utility, noise begins its systematic journey into all networks. Both precautionary measures and aggressive methods to reduce noise are described. There are many software packages available that simulate noise models well. They provide a better view of this complex, and random quantity.All digital systems use ADC (Analog to Digital Converter) since our world is analog in nature. Starting with design simulation indicating noise source and noise reduction techniques, other sources of noise in data converter—brings insight into the world of digital radio, where data converters and sources dominate baseband. A detailed investigation in terms of techniques to overcome noise in such areas—is provided. It is an important pointer to address noise in ADC and DAC. Noise Factor which is an important measurement—is discussed in terms of how it affects the first unit and subsequent network units.Other topics in measurements relate to transmitter and receiver parameters—the chapter describes how phase noise and modulation parameters and their measurement play an important role in establishing quality of a transmitter. Similarly, digital receivers relate to EbNo for bit error rate/frame error rate measurements. The most important parameter of receiver is “sensitivity”, which is described in detail showing how it is measured with the help of Digital Signal Processing (DSP) techniques.Field measurements are in the process of a major transformation given the speed with which cellular systems are changing. Starting with how cell site is established, measurements that provide an insight into their network deployment—particularly in 5G. The complexity is quite significant and sophisticated analyzers are required to set up a site. With the use of broadband systems, it has become necessary to address PIM—the Passive Inter-Modulation sources that are an inherent part of cellular systems. This is described briefly showing how PIM sources are identified in the field and eliminated.Many RF components are unique in their approach to design and operation. They are in widespread use in lab and field and are an integral part of all measurement set ups. A typical sample of such components is reviewed. Although majority of them are passive in nature, they contribute significantly in terms of deployment strategy, performance, and network behavior. Starting with lightning arrestors which are mandatory at all cell sites are described along with how surge due to lighting can be diverted by careful design. Other components such as couplers, duplexers, circulators, and attenuators are described indicating their design. Throughout this chapter the hierarchy of standards—primary, secondary, and working units are regularly described to emphasize the importance of calibration, precision, and accuracy.