Oscillators

Abstract

In many transceivers, oscillators are required for the up- and down-conversion of transmitted and received signals by means of a local oscillator (LO) signal as illustrated in Fig. 11.1a. The main function of an oscillator is the generation of the LO signal, which is a periodic output signal at a specific or tuneable frequency. Similar to amplifiers, the efficiency is given by the relation between LO output power and required DC power. In this context, the reader is referred to Eq. (9.3). We can identify the following general oscillator properties:

• 1. Based on a start-up signal, which can be the intrinsic oscillator noise, or a switched signal, e.g. a supply voltage ramp, the signal grows every oscillation cycle. This signal amplification requires an active device providing gain. Needless to say that an oscillator can not be purely passive.

• 2. To prevent from self-destruction, the oscillator must have an amplitude limiting effect. Most of the integrated oscillators employ transistors with IV characteristics as sketched in Fig. 11.1b. Fortunately, given a proper design, the lower and upper limits of the amplitude are already defined, e.g. for the FET by the threshold and linear (resistive) region exhibiting zero current and high damping resistance, respectively. Starting at the applied DC bias point, the signal swings along the load line with slope determined by the load resistance. Indeed, the corresponding properties are very similar to those of power amplifiers. One major difference is the fact that the RF input power of oscillators is generated internally. This is not the case for amplifiers.

• 3. Many oscillators feature frequency stabilisation based on the impedance and phase characteristics of a high-Q resonator.

Oscillators operate under large signal conditions, since the RF signal swing is large approaching parts of the IV curves exhibiting varying characteristics. Harmonics and nonlinearities are generated. Precise large signal models are required for accurate predictions of oscillators. Nevertheless, simple models can be used to explain the basic operation of oscillators.