Characteristics of piezoelectric filter

Views: 38 Author: Site Editor Publish Time: 2018-11-02 Origin: Site

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After the electrical signal passes through the filter, not only its amplitude changes with frequency, but also its phase changes with frequency. In order to understand the effect of the filter on the operating state of the circuit after the circuit is involved, it is necessary to study the three characteristics of the filter, namely the impedance characteristic, the attenuation characteristic and the phase characteristic.

Impedance characteristics of piezoelectric ceramics are input impedance and output impedance. The filter is a device connected to the four-terminal network and the input of the filter. The signal is transmitted to the filter and is regarded as the signal source. The internal resistance is Zi; the device connected to the output of the filter, the output of the receiving filter. The signal, considered as a load, has an impedance of ZL; both Zi and ZL are the termination impedance of the filter.

Characteristic impedance

When the termination impedances Zi and ZL of the piezo electric ceramic changes, its input and output impedances Z and Z are also changed, which affects the working state of the filter. For a filter, it can terminate a unique termination impedance at each end. This impedance enables the input impedance at the end of the study to be exactly equal to the load impedance at that end. This characteristic impedance is the characteristic impedance of the filter. The characteristic impedance is related to the structure and parameters of the filter itself, and also to the frequency of the input signal. Therefore, for a certain filter, it is not a fixed value, but it is independent of the termination impedance. If the final impedance of the filter is equal to its characteristic impedance, the filter will get the best working condition. This connection is called the matching connection of the filter. Since the characteristic impedance is frequency dependent, it is impossible to make a perfect match. When the termination impedance of P-41 material pizoelectric ceramics is equal to its characteristic impedance, the input impedance seen from the other end is equal to the characteristic impedance of that end.

Attenuation characteristic

About the attenuation characteristic or the loss characteristic, It reflects the attenuation of the signal by the filter for different frequencies, or the ability of the filter to select different frequency signals. The characteristic of the piezoelectric ceramics manufracturing as a function of frequency is called the attenuation characteristic of the filter. The attenuation of the filter can be divided into transmission loss, interference attenuation (insertion loss), and work attenuation. In the use and measurement of filters, the most common is the interventional attenuation.

Phase characteristics and phase shift characteristics

The electrical signal passes through the filter not only with its amplitude but also with its phase. The phase change caused by the filter is not only related to frequency, but also to the structure and parameters of the filter. The characteristic of the phase caused by the filter as a function of frequency is called the phase characteristic. The characteristic that the phase difference varies with frequency is called the phase shift characteristic. For a certain attenuation characteristic, there is a phase characteristic corresponding thereto. For the interventional attenuation, there is an intervening phase shift, that is, the voltage (current) on the load has a phase change before and after the filter is interposed. For transmission attenuation, there is a transmission phase shift, ie the transmission signal of the filter has a phase shift relative to the input signal. Since the output signal of the filter has a time delay with respect to the input, it is referred to as delay. The delay is also a function of frequency.

Principle and electrical characteristics of piezoelectric ceramic filters

The principle of the PZT5 material piezo disks is that the piezoelectric ceramic filter is designed by utilizing the resonance characteristics of the piezoelectric ceramic vibrator. When a piezoelectric ceramic vibrator is connected in series between the signal source and the load ZL, it has the attenuation characteristics shown in the following figure.

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As can be seen from the figure, when the signal frequency is near the resonant frequency of the ceramic oscillator, its impedance is the smallest, the output signal is the largest, or the signal has the least attenuation. So a monolithic ceramic vibrator has the property of filtering, which is equivalent to a 2-terminal filter. Although monolithic piezoelectric ceramic vibrators have filtering properties, the attenuation characteristics are poor and often fail to meet the requirements for use. In order to obtain a filter with good performance, it is necessary to combine the piezoelectric vibrators in a certain manner.

Design and calculation of piezoelectric ceramic filter

The design calculation of the piezo ceramic filter is based on the performance index of a given filter, such as the center frequency f0, the bandwidthΔf, the rectangular coefficient K, the insertion loss Bon, the defensive degree Bz, the matching impedance Z, and the electromechanical selection of the piezo ceramic material by theoretical calculation. The coupling coefficient, the vibration mode, the geometry of the vibrator, and the number of nodes and structure of the filter create a filter that meets performance requirements. The design method of the filter can be generally divided into two types: analysis method and comprehensive method. Both methods have their own advantages and must be applied in specific situations. Generalized methods are generally used for narrowband filters with bandwidths less than 2%, while high pass low pass and larger bandwidth filters are typically designed using analytics.Taking the T-filter as an example to illustrate the calculation steps of the analytical design.The known conditions for design calculation are generally center frequency f0, bandwidth Δf, rectangular coefficient K, stopband defensive (or stopband attenuation), matching impedance, and stability. Final requirements for design:

1) Material properties: frequency coefficient of temperature, relative dielectric constant, electromechanical coupling coefficient and piezoelectric coefficient of porcelain
2) Vibrator size: vibrator vibration mode, structure size
3) Number of filter sections and number of vibrators