Chapter 10: Oscillators and Mixers

Figure 10

With the advent of modern radio and radar systems came the need to provide stable harmonic oscillations at particular carrier frequencies to establish the required modulation and mixing conditions. While the carrier frequencies in the early days mostly reached into the low to mid MHz range, today's RF systems easily surpass the 1 GHz point. This has resulted in the need for specialized oscillator circuits capable of providing stable and pure sinusoidal signals. What makes the design of oscillators such a difficult task is that we exploit an inherently nonlinear circuit behavior that can only be described incompletely with linear system tools. Specifically, the small-signal linear circuit models utilized to represent the active device afford limited capabilities to handle the complicated feedback mechanism. Moreover, since an oscillator has to provide power to subsequent circuits, frequency-dependent output loading often plays an important role. For these reasons, the design process for oscillators remains more of an art than an exact engineering design task. This holds particularly true for the high-frequency regime, where parasitic component influences can significantly impact the overall system performance. Affected in part by the additional resonance effects of the passive circuit elements, it is possible that the oscillator operates not only at the intended frequency, but also at lower or higher harmonics. Certain system realizations may even cease to oscillate completely.

In the first part of this chapter, we concentrate on the negative resistance and feedback harmonic oscillators. Once the fundamental idea of how to generate oscillations is mastered, we investigate the basic Colpitts and Hartley oscillators before moving to the modern RF circuit design approaches involving the S-parameters of the active device in conjunction with the various network configurations.

In the second part of this chapter, we turn our attention to the basic frequency translation tasks performed by mixers. Of the many different mixer implementations for a wide range of applications, the main emphasis in this chapter is placed on downconverters. A typical application of a mixer in a receiver system is to convert the RF input signal into a lower intermediate frequency signal that is generally more suitable for subsequent signal conditioning and processing. This conversion is accomplished by combining the RF input with a local oscillator signal as part of a multiplication operation that requires a nonlinear, at least quadratic, transfer function. Nowadays, transistors and diodes are in use, with present FET technology permitting the construction of mixer circuits up to 100 GHz, and diode mixers well exceeding the 100 GHz mark.

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