Crystal oscillator

A quartz oscillator, an electronic circuit for generating oscillations of a quartz crystal as frequency-determining component uses.

In a narrower sense, a quartz oscillator is a ready made oscillator circuit installed together with the frequency-determining quartz in a housing and available as a standard component.

Quartz oscillators are in their frequency ( number of oscillations per unit time) very closely and have deviations of typically below 100 ppm. Another simple oscillator circuits, such as those with an LC resonant circuits are substantially less accurate, with deviations from the nominal frequency of more than 1 % (10,000 ppm). In practice, the crystal oscillator is often found as a clock for processors, microcontrollers, radios and watches. Increasingly, quartz oscillators are replaced by lower-cost ceramic resonators.

Technical structure

The oscillating crystals used in crystal oscillator circuits are generally crystal platelets, rods or forks (such as a tuning fork ), which can be brought to the mechanical deformations due to electrical voltage, which in turn generate an electric voltage. The reaction is given by the mechanical vibration modes of the piezoelectric crystal.

A quartz oscillator is excited with an alternating voltage of a certain frequency, its resonance frequency to particularly strong resonance vibrations (this property also possess piezoelectric transducer ). It is almost independent of environmental influences such as temperature or amplitude at a suitable crystal section and is therefore used as an accurate clock with a long-term stability better than 0.0001 %.

A vibrating quartz plates can be operated in the following electrical / mechanical modes:

  • In series resonance its apparent resistance to AC is particularly low and they behave like a series connection of an inductor and a capacitor.
  • For parallel resonance of the apparent resistivity is particularly large. Then they behave like a parallel connection of capacitor and coil with the particularity that no DC current can flow (quartz is a very good insulator ). This parallel resonance is about 0.1% higher than the series resonance may be slightly changed by a small capacitor connected in parallel, and is less stable in frequency than the series resonance due to the controllability. (so-called pulling the crystal )

A similar oscillation behavior is also found in the three-fold, five-fold, and so the fundamental frequency. A crystal having a resonant frequency of 9 MHz may be allowed to vibrate so also at 27 MHz or at 45 MHz. Specially suitable harmonic crystals have a corresponding suspension so as not to hinder these harmonics.

The frequency is slightly dependent on temperature. For larger demands on the temperature response there are temperature compensated oscillators (TCXO - Temperature Compensated Crystal Oscillator). In this case, thermistors are generally used, which generate a control voltage to counteract the temperature-dependent change in frequency of the crystal. The voltage thus produced is usually applied to a varactor diode, so that the change in capacitance characterized corrects the frequency of the crystal oscillator.

Is an even higher accuracy is required, a quartz furnace is used. Here, the quartz is installed in a temperature-controlled housing, to minimize the ambient temperature -dependent effects. This quartz is electrically heated to 70 ° C, for example. This design is called OCXO ( Oven Controlled Crystal Oscillator Data Sheet ). The "X" stands for Xtal, the short form of Crystal, respectively.


Integrated circuit

These crystal oscillators are manufactured with a metal or plastic housing available in increments of integrated circuits. They provide a logic - compatible square-wave voltage ( a clock signal ) with very well-defined frequency. They require an operating voltage and contain all the components required for an oscillator. The frequency of this quartz oscillator modules is normally printed on top of the housing in megahertz. The inaccuracy of the frequency is given in ppm ( parts per million in English ). The lower this uncertainty, the more complex ( and therefore more expensive ) is the component.

Common designs for through-hole mounting, DIP 14 ( rectangular, see picture) and DIP 8 (square). There are also quartz oscillators in smaller chip package as surface - mounted device ( SMD). Typical supply voltages based on the supply of digital circuits such as 1.8 V, 2.5 V or 3.3 V, and the formerly common in transistor-transistor logic ( TTL) used 5 V.

Pierce circuit

In particular, in the digital technique for generating clock signals logic gate, usually inverter with Schmitt - trigger inputs used. The circuit in question is referred to as a Pierce - circuit and is characterized by a simple structure without coils. The inverter stage U1, as shown in the illustration, can also be part of integrated circuits ( IC ), wherein the IC only the connector pins for the external quartz crystal to be connected X are then passed out. (typically labeled with terms such as XTAL ). This oscillator can also be implemented by means of CMOS inverter stages, which his great practical dissemination founded.

The quartz vibrates in this circuit in parallel resonance and vibrations permitted only in accordance with its fundamental frequency. The circuit can be used without change for all large crystal frequencies between about 30 kHz and 10 MHz, the frequency generated can be obtained by variation of the two capacitors C1 and C2 change slightly.

The second inverter stage U2 serves as an amplifier stage and the pulse shaping: At the output of a square wave is generated by U2, which can be directly used as the clock signal for digital circuitry, such as flip-flops clocked or microprocessors.

Discrete structure

This circuit is dimensioned and produced for higher frequencies - may vary depending on the resonant frequency of the resonant circuit - either 15 MHz or 45 MHz. The oscillation circuit has one vote about the frequency of the odd harmonic of the crystal, one wishes to produce. Synchronizing the frequencies of the quartz oscillation circuit, and can recognize the abrupt change of the voltage between the measuring points A and B. A striking feature of this circuit is the lack of a visible feedback. Still does this circuit, since the transistor has both internal capacity between collector and emitter and between the base and emitter.

Function: If one were to replace the quartz by a capacitor of sufficient capacity (a few nF ), we have a common base amplifier transistor, which is often used in FM amplifiers. This circuit does not have phase shift between the input and the output at the emitter to the collector. By a small capacitance (a few pF meet ) between the collector and emitter can be produced a feedback which makes the amplifier an oscillator.

In the adjacent circuit for sufficient internal capacitance of the transistor between C and E. However, this feedback produces a phase shift which is closer to 90 ° than for the required 0 °, as between the base and emitter of the input resistance of the transistor and not a capacitor. This is corrected with the small 10 - pF capacitor on the left. The frequency of this oscillator is determined by the resonant frequency of the resonant circuit to the collector.

Turning back the capacitor at the base - as drawn - replaced by a quartz oscillator, the oscillator can only swing if the base is AC "cold", so if the quartz element represents a particularly low AC resistance. This is the case with series resonance and all odd multiples. At all other frequencies, the gain of the basic circuit is to be small, and the vibrations are not fanned.

Adjustable quartz oscillators

The (English :) xtal variable ( = crystal ) oscillator, VXO short, is a crystal oscillator which can be changed by connecting a trim capacitor in its frequency on a small scale (a few ppm). Prior to the availability precisely manufactured quartz resonators such trimmers were needed, for example, to balance the accuracy of quartz watches. If the crystal oscillator can be trimmed by electrical voltage, it will VCXO ( voltage controlled crystal oscillator german, quartz oscillator with frequency over voltage trimmable ) called. TCVCXO and OCVCXO is temperature compensated oscillators and heated.

The control voltage can counteract eg temperature dependencies or aging. The frequency can usually be changed only in the order of 100 ppm. The field of application are often frequency generators, controllable oscillators in phase-locked loops and other high-frequency measuring and testing equipment.