Crystal Filter
Theory

Application

Quartz crystal filters are the optimum choice where the application requires selective narrow band filtering at HF and VHF frequencies. Because of the extremely high Q of quartz resonators, crystal filters with a fractional bandwidth of the order of 0.01% to 0.5% can be realised with low insertion loss and high selectivity. Hy-Q International employs modern computer aided design techniques covering conventional crystal filter realisations and polylithic realisations. The conventional approach, utilising discrete crystal resonators, is employed at low frequencies and where the selectivity or bandwidth requirements are such that complex realisations are necessary, as in HF SSB filters. The polylithic approach, using cascaded monolithic dual filter elements, is used at higher frequencies where the selectivity and bandwidth requirements allow.

Terms and Definitions

Reference Frequency: a frequency defined by specification to which other frequencies may be referred. Normally, the reference frequency indicates the centre frequency (fo), the carrier frequency (fc) in SSB filters.

Relative Attenuation: the difference between the attenuation at a given frequency and the minimum attenuation in the pass band or the attenuation at the reference frequency.

Band Pass: bands of frequencies B1(kHz) in which the relative attenuation is equal or less than a specific value A1(dB).

Pass Bandwidth: the separation of frequencies between which the relative attenuation is equal to or less than a specific value A1(dB).

Pass Band Ripple: R(dB) the greatest difference between the maximum and the minimum attenuation within a pass band.

Insertion Loss: L(dB) for a specified frequency, the attenuation resulting from the insertion of a filter into a transmission system , i.e. the logarithmic ratio of the power delivered to the load before insertion of the filter to the power delivered to the load after insertion of the filter. The term ‘Insertion Loss’ is often loosely used to refer to the minimum value of the attenuation in the pass band.

Stop band: band(s) of frequencies in which the relative attenuation is equal or greater than the specified minimum values.

Stop Bandwidth: the separation of frequencies B2 and B3 kHz at which the relative attenuation first exceeds the specified minimum values A2 and A3(dB) respectively.

Spurious Response Attenuation: the minimum attenuation A4(dB) guaranteed for spurious response in the stop band. Spurious responses usually occur at frequencies higher than the centre frequency.

Attenuation Guaranteed: the minimum attenuation A2 or A3(dB) guaranteed in specific frequency ranges.

Terminating Impedances: the impedance presented to the filter by the source and load, usually expressed in terms of a parallel combination of resistance and capacitance.