Ceramic resonator

Ceramic resonators and piezoelectric ceramic resonators are electronic devices made ​​of piezoelectric ceramic, a ferroelectric material. They use the piezoelectric effect for frequency -determining or frequency be selected applications and can be used similar to quartz crystals, but without their frequency accuracy and stability. Ceramic resonators have over quartz crystals to smaller dimensions and a reduced need for external electronic devices are robust against mechanical stress and cost less. By mechanical coupling of two or more resonators also simple electrical filter can be produced.

• Types of ceramic resonators

SMD ceramic resonators with capacitive connection

Operation

If certain anisotropic crystals or ferroelectric ceramic materials mechanically deformed so electric charges are generated on its surface. The piezoelectric effect, the piezoelectricity is also reversible, that is, an electric field is applied to this material, it is deformed (inverse piezoelectric effect). When the electric field is no longer applied, the material regains its original shape, wherein an electrical voltage is generated.

Once induced by applying a voltage to the mechanical deformation of ferroelectric material of a ceramic resonator generates an electric signal by switching off the voltage. By means of a feedback circuit, this signal to produce a mechanical resonance oscillation of the material are used, a stable clock signal with relatively accurate frequency and a defined amplitude arises.

Ceramic resonators oscillate as a thick shear mode along the longitudinal axis of the ceramic. For frequencies above about 8 MHz, the third harmonic of the resonant clocking for the circuit is used

Design and production

As the basis of ceramic resonators are mixtures of finely ground granules of ferroelectric materials. These are often mixtures based on lead-zirconate- titanate, lead magnesium niobium titanate or potassium sodium niobates, which small additions of niobium, strontium, barium, lanthanum, and antimony for the modification of the properties are attached.

The production of piezoelectric ceramics can be demonstrated using the example of a lead zirconate titanate ceramics. A first thermal treatment is used to bring the starting material, a multiphase powder mixture into a desired chemical compound. This takes place in the reaction of the multi-phase powder mixture into lead zirconate titanate at about 800 ° C. It is obtained by solid-phase reaction in the chemical reaction which takes place at the atomic diffusion at temperatures below the melting points of the Rohstoffkomponeneten. In the decomposition of raw materials also gaseous by-products (eg, CO2, O2) are released. The reactions in this first thermal treatment is called calcination or pre-sintering.

After calcination, the material is again finely ground. Thus, the homogeneity of the ceramics increased. The particle size of this powder is then typically 3-6 microns. By means of a binder, the powder is then added to a plastically deformable mass, which can be pressed into any desired shapes such as discs or square and sizes. In the second heating process of the fabrication, carried out in two subsequent heat process, the pellets are first pre-fired at a low temperature to remove the binder, and then is then fired, the ceramics of the resonator cell in a sintering process at high temperatures of the pellets. The ceramic is chemically inert and resistant to moisture and other atmospheric influences.

The thus prepared sintered ceramic is now in its microstructure, a polycrystalline material having crystallites contain white areas (domains ), the elementary dipoles are namely aligned parallel, but their orientation is not statistically distributed throughout the material. If this material is now subjected to mechanical stress, so charge displacements found in her place as a result of deformation, moving free charges occur on the boundary surfaces of the crystallites. Because of the statistical distribution of the direction of the domains, however, the sum of all the charge transfers would be equal to zero when a mechanical load of the entire sintered body. Therefore, in the production of ceramic resonators, after sintering the material is polarized, in other words, the domains are in total in the same direction. This is done by applying a strong external dc bias field at temperatures that lie just below the Curie temperature of about 340 ° C. This polarization also has a (slight ) change in length of the sintered body result. After cooling, the electric dipoles retain their field imparted by the direct polarization ( residual polarization ), and the material has its desired piezoelectric characteristics.

From the sintered ceramic polarized and then the individual resonator cells are cut and polished to the desired size. Then be on the end faces of the electrodes to which the voltage generated by the vibration of the ceramic is removed, metallized. Leads or pads that are electrically conductively connected to the electrodes form contact with the subsequent circuit.

The wrapping of the ceramic resonator has a special feature. The sintered body must be mechanically free and can oscillate without damping as possible. For this, the piezoelectric ceramic provided with electrodes and terminals may be first coated with a wax layer. Furthermore then a porous plastic coating is applied and cured on the body. In a subsequent heating of these absorbs the wax and the resonator can oscillate freely within its sheath.

A special feature of ceramic resonators is considered the lead out of two load capacitances. For the metallized faces of the ceramic form with a centrally arranged on the ceramic body lying metallization two series-connected capacitors, the central port can be brought out.

Electrical behavior

Electric behaves like a ceramic resonator, an oscillator circuit, consisting of a lossy series resonant circuit with an inductor, a capacitor, the electrical resistance and loss to a (static ) parallel capacitance.

As a ceramic resonator is a vibrating mechanical system, the mechanical characteristics into corresponding electric values ​​of the resonant circuit to be converted. The oscillating mass of the resonator corresponds to the inductance in the mechanical system of dynamic capacity and the mechanical friction losses are represented by the ohmic resistance. The static capacitance caused by the metalization electrical connections to the ceramic. It is called in the data sheets " load capacity " and is performed at many resonator designs using a centrally disposed third terminal to the outside and can be connected accordingly, thus corresponding external components omitted. Resonators intended for use in discriminators are generally in this 3-pole construction. Resonators, which are intended for use in oscillators are usually 2- pole and are wired as quartz crystals.

• Diagram, equivalent circuit and basic circuit of an oscillator circuit

Electrical equivalent circuit of a ceramic resonator

Basic circuit of a Pierce oscillator circuit with a ceramic resonator with accessible capacitive connection

The frequency response of a ceramic resonator can be represented in the impedance curve. | In this curve, the magnitude of the impedance drops | frequency increases initially to a minimum, the point of the series resonance, in which cancel out in a series resonant circuit, the capacitive and inductive reactance, and only the ohmic losses are effective. With further increasing frequency, the impedance increases to a maximum, the point of the parallel resonance, also called anti-resonance frequency. In the area between the two resonance points, the resonator behaves inductively, outside this range, it behaves capacitive. Ceramic resonators are basically operated in the inductive region between the two resonance frequencies. By inverting amplifier or inverting logic gate with amplifier function that produce a phase shift of 360 °, the resonance is started and maintained.

Properties

Housing and dimensions

Ceramic resonators are encapsulated in many industry-standard enclosures, plastic coated or ceramic, delivered. Naturally, the resonators for the smaller frequencies in the kHz range, the largest dimensions, and are also usually offered only in radial- leaded form for PCB mounting. For frequencies above about 2 MHz outweigh SMD types for surface mounting on printed circuit boards or substrates.

A major advantage of ceramic resonators are in some cases significantly smaller dimensions compared to quartz crystals, because by the mechanical properties of the piezoceramic they reach a desired resonance frequency even at smaller body sizes. They can also fulfill mounted without any volume glass or metal casing their function. There are (3.2 mm × 1.2 mm area) offered eg SMD ceramic resonators for 10 MHz in the 0805 format.

A corresponding SMD quartz crystal for example, has the dimensions of 3.2 mm × 2.5 mm footprint. This crystal oscillators require about twice as much floor space as corresponding ceramic resonators. In addition, most ceramic resonators have built-in load capacitors. Thus, the entire cost components and hence the entire space in the oscillator circuit can be reduced.

Resonance frequency

Ceramic Resonators for electronic devices are manufactured for the resonance frequencies of about 300 kHz to about 70 MHz. So you cover a very wide range of operating frequencies in electronics.

In addition to the available standard ceramic resonators to about 70 MHz can be covered up to about 3 GHz through special production process even the much higher frequency range.

Frequency Tolerance

In oscillator circuits, it is generally common practice to specify the tolerance of the resonance frequency of the quality factor or the Q short. The quality is derived from the values ​​of the inductance, the capacitance and leakage resistance of the equivalent circuit.

With the aid of the quality can be calculated from the bandwidth at the resonant frequency:

The bandwidth of a frequency curve, the smaller the higher is the quality or the quality factor.

For example, a quartz resonator with a resonance frequency of 10 MHz is specified with a Q factor of 105. The bandwidth is then 100 Hz Usual frequency tolerances for ceramic resonators are between ± 0.1 and ± 0.5%. They are specified as initial tolerance at a defined temperature and have over a much broader bandwidth quartz crystal at its resonant frequency. The deviation of the resonance frequency is given as either a percentage or parts per million (ppm). 0.1 % corresponds to this statement then 1000 ppm. The percent value but is equipped with the sign "±", so that ± 0.1 % results in a total width of 0.2% or 2000 ppm. Based on the resonance frequency of 10 MHz, this means that in this example, the resonator has a bandwidth of 5 kHz, which corresponds in this case the 50-fold value compared to the quartz crystal.

The specified frequency tolerance of a ceramic resonator is a measure of the accuracy of its output frequency at a specified temperature is delivered. But the overall frequency tolerance is composed of the material properties determined by the initial tolerance, temperature tolerance dependent on the use temperature range, and aging. The sum of all three tolerances results in the value, which is decisive for the desired application.

In nowadays usual ceramic resonators results from the temperature-dependent material properties additional to the initial tolerance, a total tolerance of the resonator, the values ​​at the ± 9.000 ppm or reach over it. This compared to quartz crystals rather high value, however, for many applications in electronic devices, eg, in the timing of microcontrollers, still quite acceptable. However, the frequency tolerance of ceramic resonators is significantly lower by more recent developments. Values ​​are ± 3000 ppm, characterize this trend and new materials with better temperature characteristics for ceramic resonators, the frequency tolerance of ± 500 ppm offer, let now even the direct comparison with quartz crystals to.

Jitter

Under jitter (English for " fluctuation " or " fluctuation " ) refers to the unwanted fluctuations of the clock signal, usually caused by thermal noise or phase noise. Jitter is undesirable as an interference in the normal case. Since the frequency tolerances in quartz crystals are usually much lower than with ceramic resonators, it is generally assumed that the jitter of ceramic resonators is greater than that of quartz crystal (quartz resonators). However, since comparative tests between quartz and ceramic resonators have shown that, for example, the short-term jitter (periodic jitter) with about 10 ppm at 8 MHz between ceramic and quartz resonators is approximately equal. Also, the long-term jitter in both types of resonators has since proven to be largely identical.

Aging

The change in the resonant frequency of the ceramic resonator is called aging. It is based on that some of the dipoles are aligned parallel dielectric disintegrate over time in their dielectric domains due to lack of stability. That is, they change their orientation and then no longer contribute to the piezoelectric effect. This ultimately changes the resonant frequency of the "ages" resonator. In the first hour after the polarization of the ceramic, the change of the frequency is the strongest, and then it follows a logarithmic law. Several days after the polarization measurable frequency change over time is relatively low. It is given in the data sheets leading manufacturer with about ± 0.3 % per 10 years. However, higher temperatures above the rated range can also accelerate aging.

Coupling factor

The efficiency of conversion of electrical into mechanical energy and vice versa in each piezoelectric material is indicated by the electromechanical coupling factor. This value is a factor of 5 higher than that of quartz materials in ferroelectric ceramics.

Another source estimates the coupling factor of ceramic resonators even greater than 1000 times larger than the crystal units. Ceramic resonators therefore deliver the same driving voltage, a signal with a much higher amplitude than quartz crystals.

Turn-

Due to the high efficiency in the conversion of mechanical energy into electrical energy is the transient response of ceramic resonators, the so-called start-up time, with correspondingly faster feedback, in some cases significantly faster than quartz crystals. At the start time is the time that passes after the application of the driving voltage until the oscillation has reached 90 % of its full amplitude. Ceramic resonators run up to twenty times faster than high quartz resonators and settling velocities can reach down to 0.4 ms.

Protective

In crystal oscillator circuits frequently an external resistor for protection against overvoltages are used, which could lead to a loss of the piezoelectric properties of quartz. Because of this risk does not exist with ceramic resonators, the components cost is lower in oscillator circuits with ceramic resonators by waiving the protection resistor.

Applications

Ceramic resonators find due to their low cost and their small dimensions, in quite many electronic devices their use. They are used for example as clock generators in such a microprocessor circuits where the frequency accuracy is not critical. They are also in intermediate frequency filters in radio receivers, as a clock or signal generator in inexpensive watches circuits, televisions, VCRs, home appliances, mobile phones, copiers, digital cameras, speech synthesizers, remote controls and can be found in toys.

Recent developments in the field of ceramic resonators have a significantly better frequency accuracy and lower temperature dependence, so that they can also challenge existing typical applications of frequency-stable quartz oscillators, such as USB ports, or in LA networks in automotive electronics. These new developments are based on improvements in processing techniques and new materials and make it possible SMD ceramic resonators in the 0805 format to offer (3.2 mm × 1.2 mm) in about half the size as appropriate quartz components.