Piezoelectric ceramic filter design
In many respects, LC filter due to the inductance element can not meet the requirements of limiting the development of modern technology. The piezoelectric ceramic having a mechanical resonance characteristics and energy conversion capability, and a piezoelectric crystal, as has been widely used in the filter art.
A piezoelectric ceramic oscillator
Currently, piezoelectric ceramic filters used in the transducer, the relative bandwidth of 0.5% to 20%, the quality factor QM in the range of 600 to 3000, mass production of ceramic oscillator frequency temperature range of 1KHZ ~ several tens MHz, but also the specific into audio (3KHZ below), very low t (3 ~ 30KHZ), low frequency (30 ~ 300KHZ), IF (300 ~ 3000KHZ), high frequency (3 ~ 30MHZ) and very high frequency (30MHZ or less), the lower limit of the audio reach 500HZ, which can reach the upper VHF 90MHZ. Use aluminum titanate ceramic material can be pumped into the acid 200MHZ oscillator and filter.
Ceramic filter center frequency and bandwidth is directly dependent on the oscillator frequency spectrum of the real and relative bandwidth, oscillator these characteristics also depend on the size of the transducer, the vibration mode electromechanical coupling coefficient and materials.
A) piezoelectric ceramic oscillator equivalent circuit elongated piezoelectric oscillator, in the vicinity of the resonance frequency, when the test filter to dielectric loss and mechanical equivalent circuit diagram of a loss. LCR series branch which reflects the piezoelectric ceramic oscillator nature of mechanical vibrations, R reflect oscillator mechanical losses; C0 and RO parallel branch biomass reflect dielectric piezoelectric material, RO is the dielectric loss resistance, C0 is the vibrator two electrodes the electrostatic capacity between the faces.
1.jpg
Ceramic oscillator equivalent circuit
Other shapes and vibration mode piezoelectric vibrator having a resonant frequency near the transducer and the long strip of the same form as the equivalent circuit, if there is no other vibration mode, the parameters of the transducer is equivalent regardless of the frequency in the use of small-signal, equivalent parameters constant, oscillator can be seen as linear devices. In general, a large dielectric loss resistance R0, considered can be seen, if the mechanical losses are ignored, it that, R = 0, the equivalent circuit of the piezoelectric vibrator can be equivalent to pure electric reactance elements L, C, C0 combination.
2) the resonance characteristics of the piezoelectric ceramic transducer that is, a piezoelectric ceramic vibrator mechanical resonator body, the resonator body is electrically, the impedance Z versus frequency equivalent to the variation of F shown in Figure 2.
Resonant frequency changes from low to high, the first occurrence is called the fundamental resonance after appearing as overtone. In discussing the properties of piezoelectric ceramic resonator oscillator, a frequency often encounter the following sense: the resonant frequency FR, the anti-resonant frequency of FA, the minimum impedance frequency FM, the maximum impedance frequency FN, FS, and the series resonant frequency of the parallel resonant frequency FP.
FS is the equivalent circuit LC, R = 0 in the series resonant frequency of the FP series and parallel resonant frequency of the LC parallel circuit C0.
2.jpg
Reality, when mechanical losses can not be ignored, take a first approximation, the relationship between the expression of their
3.jpg
2, ceramic filters
Several forms of structural ceramic filter ceramic filter being developed so far only about 20 years of history. Now that the filter contains a ceramic oscillator collectively referred to as ceramic filters, the typical structure of the following categories:
A) two-terminal oscillator consisting of filters, piezoelectric ceramic oscillator is used as a two-terminal element. Filter composed by these oscillators can be composed according to the circuit structure L-, T-, X-, T-and bridge-type bridge form.
2) electromechanical coupling type filter, and its structure has many forms, the basic working principle is to use piezoelectric electromechanical transducer surprising effect of mechanical resonance characteristics to complete the filtering effect.
3) monolithic filters can trap constitutes an essential part of vibrator use can trap theory, it is composed of a three-Jing element or section of a single or multi-mode filters filter on a ceramic plate.
4) SAW filters, since 1965, at home and abroad on the surface of the solid surface elastic wave propagation began to aggressively research and its application, a number of electronics devices using the SAW, SAW filters piezoelectric ceramic is one typical device its implementation is on the upper surface of a piezoelectric ceramic substrate, to set the input interdigital transducer output interdigital transducers, the surface of the electromechanical transducer is a piezoelectric effect, and band-pass characteristics of the interdigital transducer is completed filtering of the electrical signal.
2, T, X electrical characteristics of the ceramic filter is the most common type of ceramic filter is a T-shaped section, and each X-two circuit composed of three resonator body.
1) Impedance: the impedance characteristics of the ceramic filter characteristics only affects the attenuation in the passband, and the stability and reliability of the filter has a significant impact.
Characteristic impedance of T-type filter
5.jpg
Arms impedance
6.jpg
Wherein Z1 arm string FS1 to the series resonant frequency; FP1 series arms Z1 is a parallel resonance frequency; FS2 of the arm, and the series resonant frequency of Z2; FP2 is Z2, and a parallel resonance frequency of the arm; C01 series arm resonators of the electrostatic capacitance ; C02 and arm resonance body for electrostatic capacity.
X-type filter characteristic impedance
7.jpg
Where properties Z1, Z2 meaning and T-type filter impedance of the same formula.
2) the attenuation characteristics of T-type circuit and the X-type circuit, the stop-band attenuation B inherent impedance relationship with both arms
8.jpg
The arms impedances Z1, Z2 relationship into the above equation, there are
9.jpg
By the above equation, when F = FS2, F = FP1, the filter stop band attenuation two peaks. Filter attenuation characteristics of the transmission in the matched state.
10.jpg
Composed by a number of multi-section filters total attenuation is superimposed on each section of the filter attenuation is obtained.
