Monolithic Crystal Filters Having Improved Intermodulation & Power Handling Capability

Recent studies of elastic non-linearity in quartz resonators suggest that power handling capabilities and in-band intermodulation performance could be improved by using BTor SC-cut resonators in filter realization. These improvements have potential advantages in spectrum cleanup applications, where increased power handling capability may offer better signal to noise ratios, and in communications system applications where receiver and transmitter exciter characteristics are limited by existing filter n ermodulation perf rmance. Improvements are not btained without penalty. Increases in impedance level, Co/C1 ratio and frequency change with temperature cause degradation of other performance parameters. These design trade-off's are discussed. filters in the frequency range 25 MHz to 125 MHz are Results on SCand BT-cut monolithic crystal presented. Data include measurements of conventional filter parameters as well as those relating to elastic non-linearities. Measurements of the motional and coupling parameters of SCand BT-cut two-pole monolithic devices are compared with theoretical predictions. Introduction Over the past decade or so advances in semiconductor technology have lead to the development of active devices with improved linearity and power handling capabilities. As active circuits have improved, the quartz crystal filter has become one of the limiting factors when low levels of third order intermodulation product (IM3) are required. In the past, improvements in IM3 generated in AT-cut quartz crystal filters have been achieved by improving processing techniques [ l ] and by the development of theory to assist in analyzing the anelastic properties of AT-cut quartz and its contribution to I M ~ generation [2,3,4], which in turn allows the optimization of design variables. However, the next generation of military H.F. radios is expected to demand even more stringent IM3 performance from crystal filters. Also, in order to improve exciter signal-tonoise ratios it is desirable to have as much signal amplification as possible before filtering. To achieve these objectives filters capable of handling higher power levels while still operating in a linear manner are needed. Concurrent with these pressing requirements are the demands of next generation instrumentation. As quieter, more linear measurement systems are required for the development of electronic warfare and global positioning systems the demands on spectrum clean-up filters l i k e wise become more stringent. Non-linearities in resonators can be considered as being of four types: 1) Non-linearity of effective series resistance, an effect which is often related to surface condition and surface contamination. This effect is largely process related and, providing units are fabricated in a similar manner and have similar surface motions, is expected to be similar for all cuts. The filter parameters most affected are out-of-band intermodulation performance, and variation in loss at low signal levels. 2 ) Non-linearity of motional parameters, an effect related to the anelastic properties of quartz and most apparent at high drive levels. Filter parameters most affected will be power handling capability and inband intermodulation products. This type of non-linearity will be dependent upon choice of cut and resonator design. 3 ) Thermal effects. Dissipated power is distributed norruniformly within the resonator in accordance with the squared amplitude of vibration, giving rise to thermal gradients which induce non-linear behavior. 4 ) Non-linear coupling to unwanted modes of vibration, which may affect both motional parameters and effective series resistance. While this effect is somewhat dependent on cut and is thought to be less prevalent in SC-cuts than in AT or BT it can often be eliminated by proper esonator design. Non-linear affects inband IM3 performance as well as filter coupling is most prevalent at high drive levels and amplitude and phase response. Non-linear resonance measurements suggested that the anelastic properties of SCand BT-cut quartz resonators might be superior to AT-cut; hence, in-band IM and power handling might be improved. Accordingly, a program to measure the relevant effective non-linear elastic constant for these cuts was instituted. Early results from that program are reported in a companion paper [5 ] . At he same time, asecond program, reported here, was undertaken to design and build SCand BT-cut monolithic filters and to measure their intermodulation characteristics and power handling capabilities. The Design of Coupled Resonator Structure on SCh BFCut Monolithic Dual Resonators Monolithic coupled resonator elements have r placed discrete resonators in most filter applications utilizing AT cuts. One of the major driving forces, along with economy and size, factors of prime concern to any user, is the effect of the hybrid coils required to construct filters using discrete resonators. These coils contribute excessive loss in the passband and will not give proper stopband attenuation unless manufactured with extreme care. In addition, especially at high impedance levels, there