Introduction

A frequency synthesizer is the most versatile piece of microwave equipment. Synthesizers come in a variety of forms ranging from tiny chips to complex instruments. Single-chip synthesizers are available in a die form or as surface-mount ICs. They include the key elements (such as RF and reference dividers, phase detector, and lock indicator) required to build a simple single-loop synthesizer. Such ICs are mounted on a PCB with additional circuitry. The PCB-based modules range from small, surface-mount, “oscillator-like” designs to more complex connectorized assemblies. Such PCB assemblies can be packaged into a metal housing and are presented as stand-alone, complete synthesizer modules. Connectorized synthesizer modules can be used to build larger instruments such as signal generators for test-and-measurement applications.

Not surprisingly, frequency synthesizers are among the most challenging of high-frequency designs. Many approaches have been developed to generate clean output signals [1–17]. This chapter presents a brief overview of today's microwave synthesizer technologies. It starts with general synthesizer characteristics followed by a review of the main architectures. Direct analog, direct digital, and indirect techniques are compared in terms of performance, circuit complexity, and cost. Synthesizer parameters can be further improved in hybrid designs by combining these main technologies and taking advantage of the best aspects of each. Finally, sophisticated test-and-measurement signal generator solutions are reviewed. The signal generators come with high-end technical characteristics and extended functionality including output power calibration and control, frequency and power sweep, various modulation modes, built-in modulation sources, and many other functions.

Introduction

In physics, power is the rate at which energy is transferred, used, or transformed. For example, the rate at which a light bulb transforms electrical energy into heat and light is measured in watts – the more wattage, the more power, or equivalently the more electrical energy is used per unit time [1]. Energy transfer can be used to do work, so power is also the rate at which this work is performed [2].

For systems or circuits that operate at microwave frequencies, the output power is usually the critical factor in the design and performance of that circuit or system. Measurement of the power (signal level) is critical in understanding everything from the basic circuit element up to the overall system performance. The large number of signal measurements that can be made and their importance to system performance means that the power-measurement equipment and techniques must be accurate, repeatable, traceable, and convenient.

In a system, each component in a signal chain must receive the proper signal level from the previous component and pass the proper signal level on to the succeeding component. If the output signal level becomes too low, the signal becomes obscured in noise. If the signal level becomes too high, though, the performance becomes nonlinear and distortion can result. The uncertainties associated with the measurement of power also play a very important role in the development and application of microwave circuits. For example, a 10 W transmitter costs more than a 5 W transmitter.

Introduction

The recent introduction of high-performance modulation schemes (e.g. (W)-CDMA and OFDM) provides the capability of realizing high-data rate communication links (i.e. up to 100 Mbps from 20 MHz spectrum)[1]. The broadband nature of those signals together with the large difference between the peak and the average power across the modulation bandwidth requires a large number of spectral components to accurately represent the signal statistics. The modulated signal should be amplified by the transmitting chain without loss of information (i.e. low EVM) and with little out-of-band-distortion to avoid interference with adjacent transmitting channels. The quality of the communication link can be translated into specification parameters of the active element of the transmission chain, in the case of the PA, through the device IM3 and ACPR level. In general, it is very difficult to link the technology parameters of an active device directly to its linearity performance, since the linearity achieved for a given PA is the result of its interaction with the surrounding circuitry. For this reason, most attempts to improve the linearity of PAs are currently made at the circuit level.

In order to properly compare different technologies (e.g. SiGe and III-V) or device technology generations, one must provide the optimum loading conditions, at fundamental, harmonic, and baseband frequencies, to the active device during the evaluation phase, ideally under the same driving signal of the final application. This measurement task is intrinsically complex since the broadband nature of the signal of interest conflicts with the narrow-band nature of the currently employed high dynamic range receivers (i.e. narrow IF bandwidth super-heterodyne receivers.

Introduction

Noise is one of the most critical issues in wireless systems because it is a fundamental limiting factor for the performance of microwave receivers. Industry requirements for increasingly higher performing communication systems require tighter noise specifications that make the noise figure measurement a critical step in the characterization of modern microwave circuits and systems.

Noise figure measurements of circuits and sub-systems have been traditionally performed with noise figure meters specifically developed for that purpose. A paradigmatic example is the HP8970 (and associated family) that was considered for years as the reference meter for noise figure characterization. This instrument, as well as other modern equipment, uses the popular Y-factor technique to compute the noise figure from the ratio of two power measurements (“cold” and “hot”). The scalar nature of the measurements allows an easy and straightforward characterization process. This simplicity is undoubtedly part of its large success. However, its accuracy is limited by the match properties of the device under test and measurement setup.

There are two factors that have been driving an evolution in the noise figure characterization schemes. One factor is a growing tendency in microwave instrumentation to integrate different types of measurements into a single instrument box. As a result, noise figure characterization is now available as an option in modern vector network analyzers (VNA) from different manufacturers. The other factor is that the accuracy requirements in environments that are not perfectly matched (millimeter wave and beyond, on-wafer setups, etc.) demand a noise figure characterization that takes advantage of vector measurements to improve scalar results.

Analog versus high-frequency circuit design

Analog ICs are characterized by the common use of:

• differential stages with active loads,

• DC-coupled broadband amplifiers (i.e. no DC-blocking capacitors are present between stages),

• strong impedance mismatch between stages,

• input and output impedance matching based on negative feedback or resistors.

At the same time, analog designers work with:

• currents,

• voltages,

• transistors,

• capacitors, and

• resistors,

while employing small signal AC, noise, and transient large signal simulations to analyze the performance of their circuits. In contrast, traditional microwave circuit design deals with:

• single-ended stages with a small number of transistors,

• AC-coupled, tuned narrowband or broadband gain stages,

• reactive components such as inductors, transformers, capacitors, and transmission lines, and

• lossless impedance matching to maximize power gain and minimize noise figure.

A study of phase noise

In this section we look into how phase noise arises from noise currents in an oscillator, using an analytic power series model. An analysis is conducted based on this power series model, and this is used to predict the phase noise of an oscillator. This circuit is then made to oscillate with transient simulation and is studied for its phase noise performance with the harmonic balance method using a proprietary simulator (ADS). The results are then compared. The basis of this study will be a half circuit test bench of a Colpitts oscillator as in Figure 10.14(a) and Figure 10.41 of Chapter 10, but with a HBT instead of a MOSFET.

The topic of phase noise was introduced in Section 10.1.4 of Chapter 10. Equation (10.15) simply assumed a phase noise existing at a frequency offset from the fundamental oscillation frequency, but does not explain how this phase noise arises from real physical noise current sources (e.g. resistors, lossy inductors, transistor shot noise, etc.) present inside the oscillator circuit. The latter is studied in more detail in this Appendix using a one-port equivalent circuit for the oscillator, as shown in Figure A10.1.

The inductor, Lpt , is assumed in parallel with a resistor Rpt, at the base input of the transistor. (An equivalent series L-R representation is also possible.) One end of this inductor is placed at the desired DC base bias voltage of the transistor. The value of Rpt is calculated based on the assumed Q of an actual linear inductor, at the oscillation frequency. The resistor Rpt is shown inside the “one-port” as illustrated in Figure A10.1. When this assembly is in steady-state oscillation, the input to the one-port must by definition be purely capacitive because the net negative resistance created inside the oscillator must balance any sources of positive resistance. Note that in the Colpitts oscillator the transistor inside the one-port does not have its emitter connected to ground, so the subsequent analysis of the one-port is of the prototype structure of this oscillator, not just of the proprietary device models included in it.

Wireless and fiber-optic communication systems

Communication systems transfer information between two points (point-to-point) or from one point to multiple points (point-to-multi-point) located at a distance from each other. The distance may be anywhere from a few centimeters in personal area networks (PAN), to a few thousand kilometers in long-haul optical fiber communication systems. The information can be conveyed using carrier frequencies and energies occupying the audio, microwave, mm-wave, optical, and infrared portions of the electromagnetic spectrum. In this book, we refer to the range spanning GHz to hundreds of GHz as high-frequency. Although optical frequencies do not fall into this category, the baseband information content of most current fiber-optic systems covers the frequency spectrum from DC to tens of GHz. This makes the circuit topologies and design methodologies discussed in this book applicable to the electronic portion of fiber-optic systems.

Wireless versus fiber systems

Figure 2.1 illustrates the block diagrams of typical wireless and fiber-optic communication systems. They both consist of a transmitter and a receiver, a synchronization block, and a transmission medium. The information signal modulates a high-frequency (GHz to hundreds of GHz) or optical (hundreds of THz) carrier which is transmitted through the air, or through an optical fiber, to the receiver. The receiver amplifies the modulated carrier and extracts (demodulates) the information from the carrier. In both cases, an increasing portion of the system is occupied by analog-to-digital converters (ADC), digital to analog converters (DAC), and digital signal processors (DSP), operating with clock frequencies extending well into the GHz domain.

What is a tuned power amplifier?

Tuned power amplifiers are key components in the transmission path of wireless communication systems and in automotive radar. They typically deliver the power required for transmitting information to the antenna with high efficiency and, usually, high linearity, over bandwidths of 10% to 20% relative to the center frequency of the amplifier. For battery-operated applications in particular, minimum DC power consumption at a specified output power level is required.

As shown in Figure 6.1, in its idealized representation, a tuned power amplifier consists of a common-emitter or common-source (very large) transistor operating under large signal conditions with large output voltage swing. The transistor drain/collector is biased through a bias T (which presents an infinite inductor to the power supply and an infinite capacitor towards the load) and is loaded with a parallel resonant tank (formed by RL, C1, L) at the frequency of interest. For the sake of simplicity, we will assume that the transistor output capacitance, Cout = Cds + Cgd or Cbc + Ccs, is absorbed in the load capacitance C1. In a similar manner, RL includes the loss resistance of inductor L1 and capacitor C1. The circuit draws DC power from the supply to amplify the input signal power and deliver it to the load. Ideally all of the DC power and the input signal power should be converted to output signal power. In practice, at least some of the DC power is dissipated as heat.