Electrical Wave Filters Employing Quartz Crystals as Elements

I L T E R S for communication systems must pass, without appreciable amplitude distortion, waves with frequencies between certain limits, and must attenuate adequately all waves with somewhat greater or smaller frequencies. To do this efficiently, the change from the filter loss in the transmission region, to that in the attenuation region, must occur in a frequency band which is narrow compared to the useful transmission band. At low frequencies, ordinary electrical coil and condenser filters can perform this separation of frequencies well because the percentage band widths (ratio of band width to the mean frequency of the band) and the percentage separation ranges (ratio of the frequency range required, in order that the filter shall change from its pass region to its attenuated region, to the adjacent limiting frequency of the pass band) are relatively large. For higher frequency systems, such as radio systems, or high frequency carrier current systems, the band widths remain essentially the same, and hence the percentage band widths become much smaller. Here separation by coil and condenser filters becomes wasteful of frequency space. For these filters, owing to the relatively low reactance-resistance ratio in coils (this ratio is often designated by the letter Q) the insertion loss cannot be made to increase faster than a 405