Resonance impedance testing

Transducer crystals are normally cut to a resonant frequency, the duckness being one-half die acoustic wavelength. A bond between die crystal transducer and the specimen matches the acoustic impedance, and carries the acoustic power into the latter. Backing layers may be fixed to the rear surface of the transducer. These layers are selected to reflect power forward into the crystal and specimen in some applications. On the odier hand, they may be selected to absorb power so as not to complicate signals received in material testing applications. 

Non resonant techniques are only of limited use to determine microwave losses with high precision, in particular when the losses are very small. Flowever, for the investigation of nonlinear absorption phenomena (i.e. rf power dependent on surface impedance or loss tangent) by intermodulation distortion measurements broad-band test devices are more common. Typically, a planar transmission line with an impedance of 50 Ohms can be employed for intermodulation... 

The advantage of network analysers is the possibility of impedance measurement near resonance with evaluation of the parameters R, L, C and C0 and test of the equivalent electrical circuit. However frequency response and network analysers are relatively slow with 1-10 s per measurement in typical experiments. A new generation of faster instruments has come to the market like the HP E5100 Network Analyzer with 40 (is per point in the impedance spectrum which allows us to obtain the impedance of the system in less than 10 ms. 

Because TSM oscillators have been around for over 50 years, quite a number of circuits to measure their response have been proposed, fabricated, and tested. The frequency of operation of TSM resonators (typically < 20 MHz) allows circuits to be constructed using ordinary components and printed circuit boards. Instruments and fixtures are commerciaUy available from a number of vendors (see Appendix D) that utilize fairly simple oscillator circuits incorporating the TSM resonator as the principal fiequency-control element. These systems are sold primarily for monitoring the deposition of metal films via evaporation or sputtering in a vacuum environment. The operator must typically input the density and acoustic impedance of the metal to be deposited, and the instrument then displays film thickness as deposition proceeds. These systems can also be utilized for gas-phase sensing applications, provided the TSM device is not coated with any particularly lossy materials these can cause so much damping that oscillation ceases. The systems provide information derived only from the resonant frequency there is no indication of damping except in the instance that oscillation ceases entirely. 

An rf power at 13.56 MHz was supplied to the coil (15) by a crystal controlled generator with a power amplifier (Electronic Navigation Ind., A-300) through an LC parallel-resonance type impedance matching network. The power and SWR were monitored with a through-line wattmeter (Leader Test Instrument, LPN-885). 

The high impedance of the parallel resonance circuit is transformed to 50 Q nominally by the second variable capacitor. Both variable capacitors are Johan son 5341 which are piston air capacitors with Be-Cu stators and Ag rotors. They work very nicely except for the problem of high voltage breakdown. The dc breakdown under test conditions is listed as 1200 volts at sea level but that number has to be severely derated for radio frequency and altitude. (The latter problem is serious at Los Alamos where the atmospheric pressure is 590 mm Hg and the altitude correction to the breakdown is about 10%.)... 

The harvested power from the IPMC is obtained by connecting it to a shunting resistor. We find that the power output is maximized when the shunting resistance matches IPMC internal impedance, the test structure is excited at its resonance frequency, and the IPMC is placed in the vicinity of the clamped side of the tail. The optimal harvested power is on the order of 10-100 pW per unit base angle. 

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