Basic Technology of Quartz Crystal Filters

1. Monolithic Crystal Filters vs. Discrete Crystal Filters

     There are two types of filter design technologies based on quartz crystal: Monolithic Crystal Filter (MCF) and Discrete Crystal filter.  MCF is a filter that realizes the characteristic of filter by mounting electrodes on a piezoelectric substrate (such as quartz blank) and using a mechanical combination occurring between the electrodes.  MCFs are usually smaller, more reliable and more cost effective than discrete crystal filters because they use fewer components and have fewer interconnections.  The majority of MCFs are symmetrical band pass filters.  In addition, in the VHF range, the MCF approach allows bandwidths on overtones that can only be realized with more costly fundamental mode resonators if a discrete crystal filter design is used.  On the other hand, discrete crystal resonator filters have better power handling capabilities than MCFs.  They may be a better design choice for very narrow or very wide bandwidth because of their more flexibility in the network topology.  This allows the design of networks that have sharply asymmetrical single sideband performance, which is difficult to achieve with monolithic designs.  MCF is first developed by Toyocom in Japan in 1962, and since then many kinds of high reliability filters in a wide frequency range in small packages have been developed.  Today, MCF is one of the most widely used frequency control components in radio communication equipments.

2. Crystal Filter Bandwidth Categories

     Because of bandwidth limitations imposed by the material properties of quartz (represented in the resonator equivalent circuit by the ratio of static to motional capacitance C₀/C₁), crystal filters, both monolithic and discrete, are commonly categorized according to fractional bandwidth as follows:

Narrow Band Crystal Filters

      A narrow band filter is one in which the network can be designed so that the crystal static capacitances can be accommodated without the use of inductors.  The capacitance ratio C₀/C₁ of the resonator equivalent circuit determines the maximum bandwidth for the inductorless narrow band crystal filters.  For filters using fundamental mode AT-cut quartz resonators, under ideal conditions this maximum bandwidth is approximately 0.32% of the center frequency.  If the required bandwidth exceeds this limit, the network design must be changed to incorporate inductors.

Intermediate Band Crystal Filters

      Intermediate band filters use inductors to remove excess capacitance presented by the resonator C₀ plus unavoidable stray capacitance.  Most intermediate band designs use discrete resonators but they may incorporate monolithic dual resonators. Bandwidths are between 0.3% and 1.0% for fundamental mode resonators.  Spurious responses will sometimes be present in the filter passband, as well as in the transition region, and will usually be fairly strong in the filter stopband.  Those spurious responses can be controlled by proper design tactics to achieve required performance.

Wide Band Crystal Filters

      Wide band crystal filters provide the final link between crystal and LC filters by using inductors to contribute poles to the filter response, while at the same time accommodating the resonator static capacitance.  For this reason, the filter response is quite sensitive to inductor and Q values and requires precise temperature compensation of inductors if performance is to be maintained.  Bandwidths are between 1% and 10% of center frequency.  Discrete resonators are generally used. When AT-cut crystals are used, spurious responses will often be present in the filter passband and transition region.

3. Attenuation Specifications

       Attenuation is the power loss (in dB) incurred by a signal in passing through a two-port network, such as a filter.  Absolute Attenuation may be defined as the attenuation relative to either a direct connection of load to source or the available source power.  For conjugate or resistive and equal source and load impedances, the two definitions are equivalent.  Relative Attenuation is the attenuation measured relative to a reference, usually the point of minimum loss (maximum transmission).  Guaranteed Attenuation is the maximal guaranteed attenuation at the specified frequency.