Crystal Filters

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What is a filter? Filters are electronic circuits that emphasize certain signals and reject other signals. 

Crystal Filter Glossary/Terminology

Definition of a Monolithic Crystal Filter: A monolithic (Greek: one stone) crystal filter has 2 sets of electrodes deposited on the same quartz disc. This forms 2 resonators with acoustic (mechanical) coupling between them. If the acoustic coupling is correct a filter response will be achieved. 

Typical construction of a Discrete Crystal and a Monolithic Crystal 

Discrete Crystal                                                       Monolithic Crystal  

discrete crystal  monolithic crystal

Center Frequency

For bandpass or band reject filters, it is the arithmetic mean of the 3db point frequencies. Also used as the design midpoint of the passband. 

center frequency


Passband Bandwidth

In a bandpass filter, these limits, or passband edges, are generally the frequencies at which 3db of attenuation is measured. The number if hertz expressing the difference between the upper 3db frequency and lower 3db frequency of a bandpass filter.  

passband bandwidth center frequencyJPG


Insertion Loss

The amount of attenuation within the passband of the filter compared to the input signal level. May be measured at center frequency or within 1db passband limits.  

insertion loss center frequency


Ripple

The difference in attenuation between the highest peak and the lowest valley within the passband. It is measured in db.  

ripple center frequency


Ultimate Attenuation

The maximum attenuation guaranteed at the specified frequency range.  

ultimate attenuation center frequency

Spurious Response

Are produced by unwanted vibrational modes in the crystals. Every filter parameter can be moderately to severely distorted by them. They are generally located on the high frequency side of the passband, and in wideband filters, they can occur even within the passband.  

spurious response center frequency

Reasons to use Monolithic Crystal Filter
• Lower cost solution
• Less manufacturing time
• Smaller package size 

Crystal Filter Electrical Specifications

Standard/Required Specs.  Optional/Custom Specs. 
• Center Frequency (an absolute usually in MHz)
• Passband Bandwidth (3db or 6db in Hz)
• In-band Ripple (A maximum in dB)
• Insertion Loss (A maximum in dB)
• Stopband (Max. value in Hz @ 30dB, 60dB, etc.)
• Ultimate Attenuation (A minimum in dB) 
• Group Delay (in Seconds)
• Number of Poles
• Spurious Attenuation (A minimum in dB)
• In/Out Impedance (In ohms, typically +/-10%)
• In/Out Capacitance (Typically in picofarads) 

 

Crystal Filter Applications

Where/Who uses them? 
• Telecommunication Base Stations
• Radios (Receivers/Transmitters)
• Satellite modem applications (ground based side) 
• Radar Applications 

Crystal Filter SMD Package 

crystal filter smd package

Crystal Filter HC-49 Packages 

crystal filter hc 49 packages

Crystal Filter UM-1 Packages

crystal filter um 1 packages