We present the analysis and measurement of spurious responses generated at the ends of interdigital transducers (IDT). Filters fabricated on LiNb03 show an unwanted pass band ripple whose period indicates additional generation of acoustic waves at the IDT end. As this effect cannot be explained by methods of analysis based on the infinite array approximation, an exact analysis of the complex-valued, frequency-dependent electric charge distribution on the finite IDT structure is required. Utilizing the method of moments our analysis is based on a Green's function concept and a spectral domain representation. Three effects are shown: The first is the charge accumulation on grounded guard fingers located closely to the IDT end, resulting in unwanted end radiation. The second is acoustic end reflections in split-finger IDT's, occurring at the transition from the periodic finger structure to the free substrate. The third is the finger charge induced by the metallic ground plane when the transducer is driven unbalanced to ground. Computer simulations based on our method agree well with measurements.

The structure of a QAM radio system is explained. Some novel designs that demonstrate the current status of spectral shaping SAW (surface acoustic wave) filters are presented. Compensation of second-order effects is discussed. Experimental results for a filter for a 140-Mb/s 16 QAM cochannel system with a roll-off factor of 0.19 are shown. Two other examples of filters for a 140-Mb/s, 64 QAM system with a 35% relative bandwidth are also presented.

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The passband slope of a SAW filter with lar e fractional bandwidth critically depends on the value of t f e operating capacitances of the input and output interdigital transducer (IDT). To reduce the number of design cycles required to achieve a flat passband response, a program for the computation of all relevant contributions to the IDT capacitance was developed. For the analysis of the apodized transducer, a track division approach is employed. The capacitance contributions from the different tracks are computed by means of an interfinger nodal capacitance matrix. As this matrix is calculated from the Green's function of the boundary value problem, the presence of the grounded back-plane and any non-conductive adhesive is automatically taken into account. The operating capacitances of two different SAW filters with center frequencies of 70 MHz and 140 MHz fabricated on YZ -LiNbOs were caltulated and compared to measurements. The dependency of the capacitances on the thickness of a non-conductive adhesive is also shown. Even small variations in adhesive thickness cause noticeable changes in the transducer capacitances.