I-ILTERS lOR (• 1 KKIEK SYSTEMS 205 



0.67 to 0.17 inch, in thickness from 0.119 to 0.012 inch and in lenj^lh frf)m 

 1.40 to 1.23 inches. The small variation in length is due to the fact that the 

 length is determined primarilx' !)}• the frequency of resonance of the plate 

 and this change is small across the tilter band. Tlie temperature coeflicient 

 of the -{-S degree X-cut quartz crystal element used in this filter is su[)erior 

 to the —18 degree X-cut longitudinal type which has been used in many 

 other crystal filters but otherwise they are similar in use and in manufacture. 



The filter attenuation distortion in the vicinity of the cut-ofTs is depen- 

 dent on the dissipation in the elements which resonate there. In order to 

 minimize this distortion, it has been found necessary to impose minimum Q 

 requirements of 80,000 on the high-impedance crystal elements which re- 

 sonate near the cut-ofifs. This high Q is realized by suspending the quartz 

 crystal plates from fme wires^ and operating them inside of evacuated con- 

 tainers. The low-impedance crystal elements which resonate at frequencies 

 removed from the cut-offs require a minimum Q of 15,000. This compara- 

 tively low Q is realized by quartz crystal elements vibrating in air at atmos- 

 pheric pressure. 



In the equivalent electrical circuit of a quartz crystal element the large 

 ratio of the shunt capacitance to the internal capacitance is a measure of 

 the poor electromechanical coupling of quartz. For the +5 degree X-cut 

 quartz crystal element this ratio of capacitances is about 140 for a plated 

 blank before fabrication. It is obvious that fabrication, wiring and para- 

 sitic capacitances which may be in parallel with the quartz plate will make 

 this ratio still higher and thus will reduce further the filter band width 

 obtainable. For this reason it is important to keep to a minimum any 

 capacitances which appear across any arms of the crj^stal lattices. One 

 method used to minimize these capacitances was to assemble the eleven 

 crystal elements required for each section in two containers instead of 

 eleven separate ones. The five high-impedance elements requiring mini- 

 mum ^'s of 80,000 are assembled in one evacuated metal container and the 

 si.x low-inductance elements having the lower Q's are assembled in another 

 hermetic sealed container filled with dry air. A photograph showing the 

 method of assembly used in given in Fig. 9. 



A method was found to reduce the ratio of capacitances of the crystal 

 elements. This method consists of dividing the plating on the surface of 

 the quartz so that the driving voltage is removed from the end portions of 

 the quartz plates. This plating division increases the equivalent induc- 

 tance of the quartz plate but also decreases the direct capacitance between 

 the plated surfaces. It has been found that the decrease in shunt capaci- 

 tance with removal of plating is more rapid than the increase in equivalent 

 inductance up to a certain point. If the plating is removed up to this opti- 

 mum point it has been found possible to reduce the shunt capacitance about 



