INTRODUCTION

Radio transmitters typically consist of a modulator and a power amplifier, and usually also involve digital-to-analog conversion and frequency upconversion. Of these elements, we have previously covered modulation (Chapters 5 and 6) and frequency conversion (Chapter 14) in detail. In this chapter we address the remaining elements and the associated architectural considerations. Architectural considerations are presented in Section 17.2. Digital-to-analog conversion is presented in Section 17.3. Sections 17.4–17.6 address power amplifiers (PAs), which are a largely distinct enterprise from the small-signal amplifiers considered in Chapter 10. This chapter concludes with a discussion of methods for combining amplifiers in parallel (Section 17.7), which has applications for the design of transmitter PA stages.

ARCHITECTURES

Section 1.4 presented two broad architectural strategies for the design of radio transmitters. In this section we elaborate on this with some extra detail accumulated from the intervening chapters. In fact, there are at least six common transmitter architectures, with many more variants that are less commonly used. The common strategies are depicted in Figure 17.1. Essentially the only common element in all six architectures is the final stage, which is a PA.

In the first two schemes (Figure 17.1(a) and (b)), sinusoidal carrier modulation is generated using an oscillator operating at the carrier frequency. In Figure 17.1(a), AM (including ASK) is generated by varying the amplitude of an oscillator, or FM (including FSK) is generated by varying the frequency of the oscillator. In Figure 17.1(b), AM is generated by controlling the output power of the PA; this is a technique we shall have more to say about in Section 17.4.6. The principal advantage of these schemes is simplicity. These architectures appear primarily in low-cost/low-power applications such as wireless keyless entry; and in very high-power applications, such as AM and FM broadcasting.

In Figure 17.1(c), the modulator generates output at an intermediate frequency (IF), which is subsequently upconverted to the carrier frequency using a superheterodyne frequency converter. Once ubiquitous, this architecture is steadily declining in popularity due to the many advantages of digital baseband processing, which is employed in schemes (d)–(f). Nevertheless, such radios certainly exist and in fact are prevalent in some applications requiring simple, lowcost communications using AM, SSB, or FM. Examples of these applications include Citizen's Band (DSB-AM at HF), air-to-ground (“airband,” DSB-AM at VHF) and certain narrowband FM-based UHF-band mobile radio applications.