Just above f s, the reactance of L m becomes larger than that of C m and we observe that the crystal exhibits inductive behavior. Note that C o doesn’t affect the value of this frequency. This corresponds to f s in Figure 2 that is commonly referred to as the series resonant frequency of the crystal. At this frequency, the impedance of the lower branch and consequently, the total impedance across the crystal drops to zero. When L m and C m are in series resonance, their impedances cancel each other out. Hence, in the lower branch of the crystal electrical model, we have L m and C m in series. To gain some insight into the operation of a crystal, let’s assume that the crystal is ideal and R m is negligible. frequency curve of a typical quartz crystal unit as depicted in Figure 2: Image courtesy of STMicroelectronics.īased on this model, we can find the reactance vs. The equivalent electrical circuit for a crystal is shown in Figure 1.įigure 1. Armed with this knowledge, we’ll take a look at two different oscillator topologies and discuss how the circuit architecture forces the crystal to oscillate at a particular frequency.īased on this discussion, we’ll be able to look at the definition for parallel and series crystals-two technical terms that can sometimes cause confusion. Now, we'll look more closely at the operation of these devices. In the first part of this series, we looked at some of the important metrics that are used to characterize the frequency deviations of quartz crystals.
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