SAW IMPEDANCE ELEMENTS

In the past few years a new class of SAW devices has been under develo pment based on the use of one port SAW resonators as lumped elements w ith their impedance strongly changing in a frequency range of effectiv e SAW excitation. These 'SAW impedance elements' can be connected in l adder type, balanced bridge type, or others types of networks to provi de desired filter characteristics. In this paper the admittance charac teristics of a single element are analyzed using coupling of mode (COM ) model simulations. It is found that resonant type characteristics of a long transducer can be described by very simple formulas. The frequ ency gap between resonance and antiresonance, important for filter des ign, is determined practically by the Delta V/V parameter of the piezo electric substrate used, while the minimum magnitude of the impedance at the resonant frequency is determined by the SAW attenuation. The in ternal reflections only shift the resonance to the left edge of the st opband not changing other characteristics. A case of a synchronous res onator comprising relatively short transducer and two reflectors place d on both sides of it is also analyzed. This type of impedance element has smaller frequency shift between resonance and antiresonance point s.