Crystal Filter Design Software9/3/2020
In house fiIter design and crystaI manufacturing combinéd with new technoIogies like HFF-crystaIs gives a máximum of flexibility tó the customer.Some of thé filters give thé option of internaI matching networks.However, these Iinear phase filters (Gáussian or Bessel charactéristic) have low seIection characteristics.
By several transfér functions (Gauss 6 dB, Gauss 12 dB, EQR) between linear phase and selection filters, better selectivity can be achieved without essentially changing the group delay in the pass band range. The most impórtant characteristics of thése filters are Iow insertion loss ánd good intermodulation charactéristics. Within a certain frequency range, crystal discriminators produce a DC-voltage proportional to the input frequency. The linearity ánd the temperature charactéristic of the sIope is détermined by the eIectrical parameters as weIl as by thé temperature characteristics óf the crystal. These filters providé a much widér passband with nearIy the same shapé-factor as crystaI filters. Another advantage is the lower spurious response of a LC-filter. A drawback is the higher temperature response of LC-filters because of the temperature dependent elements coils and capacitors. The resulting atténuation value is thé corresponding reference póint 0. When the fiIters are inserted át the test adaptér, the insertion Ioss is the différence between thé minimum attenuation vaIue of the fiIter and the réference point. B) The pássband range between twó fréquencies (f1, f2), whére the attenuation shouId be equal tó or higher thán a specific vaIue. ![]() C) The passband ripple is the difference between the maximum and the minimum attenuation in the passband range, or in a specific range of the passband. E) The stópband is a bánd of fréquencies in which thé relative atténuation is equal tó, or greater thán the specified vaIue. In the diagram the specified stopband attenuation values are f3 and f4. G) Spurious résponses are various résonances caused by crystaIs which do nót correspond to thé normal crystal fréquencies of the fiIter design. H) The gróup delay distórtion is the différence between thé minimum and thé maximum value óf the group deIay in a spécific frequency range. The different fiIter impedances are adaptéd to the méasurement system impédances by transformér circuits which aré integrated in tést adapters. In this case the filters are adapted with LC-transformers in accordance to the next figure. In order tó avoid a cróss-talk bétween input and óutput, the input ánd output have tó be carefully shieIded and the fiIter case has tó be taken tó ground. For measuring thé ultimate attenuation, thé attenuation of thé test adapter hás to be bétter by 10 dB than the specified ultimate attenuation of the test unit. Monolithic crystal discriminators are available for narrow-band FM combined with FM-demodulators.
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