RC (or capacitor plus current source) relaxation oscillators can easily attain stabilities approaching 0.1%, with initial predictability of 5% to 10%. That’s good enough for many applications, such as a vacuum fluorescent display (VFD)
in which the individual characters of a multi-character display are driven sequentially in rapid succession (a 100 Hz overall rate is typical); this is called a multiplexed display. Only one character is lit at any time, but, if the entire display is “refreshed” rapidly enough, your eye sees the whole display without obvious flicker. In such an application the precise rate is quite irrelevant – you just want something in the ballpark.
As stable sources of frequency, LC oscillators can do a bit better, with stabilities of 0.01% over reasonable periods of time. That’s good enough for non-demanding applications like an inexpensive radio. Both kinds of oscillators are easily tunable – with a variable R or current source (for the relaxation oscillator) and with a mechanically or electrically tunable capacitor, or a slug-tuned inductor (for the
But for real stability there’s no substitute for a crystal oscillator. This uses a piece of quartz (silicon dioxide, the primary ingredient of glass) that is cut and polished to vibrate mechanically at a certain frequency. Quartz is piezoelectric (a strain generates a voltage, and vice versa), so acoustic waves in the crystal can be driven by an applied electric field and in turn can generate a voltage at the surface of the crystal. By plating some contacts on the surface, you wind up with an honest electrical circuit element that can be modeled by a sharply resonant RLC circuit, pre-tuned to some frequency (which is the mechanical resonant frequency of the little slab of single-crystal quartz).
Figure 1: Quartz crystal packages. Across the top row are complete oscillator modules in DIP-8 and DIP-14 sizes; a much smaller alternative is the tiny 7mm×5mm surface-mount oscillator module at bottom center. The strange object in the middle is a bare crystal, shown with its spring-loaded electrode plates. You don’t see those anymore; instead, crystals come in the popular sealed packages known as (left to right, bottom row) HC49/U, HC49/US, and 3mm tubular. We were lucky to find that oddball glass case at the right: inside you can see the gorgeous quartz disk with its plated electrodes.
Quartz crystals come packaged as bare components or as complete oscillator modules; some examples are shown in Figure 1. The quartz crystal’s high Q (typically around 104–105) and good stability make it a natural for oscillator control as well as for high-performance filters. As with LC oscillators, the crystal’s equivalent circuit provides positive feedback and gain at the resonant frequency, leading to sustained oscillations.
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