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crystal filters

Crystal filters are electronic filters that use monolithic crystals. They are used in all kinds of electronic and radio communication devices. A monolithic crystal filter uses one large crystal rather than an array of several discrete crystals.

 

Crystal filters are usually made of synthetically-manufactured quartz. The quartz crystal is cut into blanks and then polished to produce a smooth surface. After the crystal is smoothed, the surface is chemically-etched to further refine the finish and minimize the effects of aging. In a monolithic filter, the single crystal is fitted with two sets of electrodes on either side.

 

The electrodes are deposited on the top and bottom surfaces of the crystal, and are usually made of silver, gold, or aluminum. The electrodes use the resonance qualities of the crystal to pass signals.

Crystal filters use the piezo-electric effect to transmit filtered signals from one set of electrodes to the other set. The signals on the originating electrodes cause the crystal to vibrate at a certain frequency. Only the signals within the bandpass of the filter are allowed across the crystal to the second set of electrodes. This crystal bandpass filter operates efficiently and cost effectively at a frequency at or below 30 MHz.

 

Filter Type

 

  • All pass - All pass filters (phase shift filters) do not affect the amplitude of the signal. Instead, they change the phase of the signal without changing its amplitude.
  • Band pass- Bandpass filters are used to attenuate frequencies above and below a range of frequencies called the bandwidth or passband of the filter. Any signal with a frequency within the bandpass range passes through the filter. Any signal with a frequency outside the bandpass is reduced.
  • Band reject- Band reject filters (band stop filters, notch filters) are used to attenuate a range of frequencies and allow all other frequencies to pass.
  • High pass - High pass filters attenuate frequencies above a particular frequency called the cut-off frequency. Frequencies higher than the cut-off frequency pass easily through the filter.
  • Low pass - Low pass filters attenuate frequencies below a particular frequency called the cut-off frequency. Frequencies below the cut-off frequency pass easily through the filter
  • Universal / programmable - Universal filters can be configured by the user to function as a low pass, high pass, band pass, or band stop filter.

Performance Specifications

 

  • Differential mode attenuation - The attenuation of the filter is the ratio, at a given frequency, of the amplitude of the input signal to the filter to the output signal. It is measured in decibels (dB). Differential mode attenuation is the attenuation obtained when the filter is operating in differential mode.
    Search Logic: User may specify either, both, or neither of the "At Least" and "No More Than" values. Products returned as matches will meet all specified criteria.
  • Operating temperature - Operating temperature is the value specified by a level (minTypMax) of the ambient temperature (in °C) in which this item was designed to operate.
    Search Logic: User may specify either, both, or neither of the limits in a "From - To" range; when both are specified, matching products will cover entire range. Products returned as matches will meet all specified criteria.
  • Cutoff or center frequency (Fc) - The filter type determines the specified frequency (Fc). For band pass and band reject filters, the specified frequency is the center frequency. For low pass filters, the specified frequency is the cut-off frequency. For high pass filters, the specified frequency is the cut-on frequency.
    Search Logic: User may specify either, both, or neither of the "At Least" and "No More Than" values. Products returned as matches will meet all specified criteria.
  • Bandwidth - Bandwidth is the range of frequencies that filters pass with minimal attenuation or, in the case of band reject filters, maximum attenuation. 

Applications

 

Crystal filters designed for two way radio communications include front end filters, which are designed with multiple pole discrete crystal filters. A front end filter is often used in radio frequency environments that produce strong signals, such as high power transmitters.

 

Monolithic crystal filters may also serve as a high frequency filter for use in wireless applications. A surface mount filter is a crystal filter mounted directly to an integrated circuit. A crystal ladder filter consists of several crystal filters of the same frequency and is often used to build a single sideband transmitter in amateur radio applications.

 

Standards

 

Crystal filters mu st adhere to certain standards to ensure proper design and function. A full list of crystal filter standards can be found on the IHS standards store.

 

BS 9602 - Gives ratings, characteristics, inspection requirements and mandatory layout for detail specifications for quartz crystal filters.

JIS C 6703 - Crystal filters.

 

References

 

Image Credit:

 

Syfer Technology Limited