Application Note for Crystal Filters

Crystal filters are divided into integrated monolithic filters and separate filters according to their different structures.

The integrated filter is simple in structure, small in size and low in price, but its bandwidth and frequency are limited. The separate filter can make up for the deficiencies of the integrated filter and expand the achievable frequency and bandwidth.

       The development of digital communication technology has put forward requirements on the group delay characteristics and intermodulation distortion index of crystal filters, and separate filters can be easily solved.

       1. Impedance matching: When a filter with excellent performance does not match the impedance of the circuit it is connected to, the filtering characteristics will deteriorate, resulting in increased passband fluctuation and increased insertion loss. When the external circuit impedance is lower than the characteristic impedance of the filter, the center frequency will move down, and inversely move up.

The "signal source + level meter" function is completed by the network analyzer

Ri, R0: the internal resistance of the instrument: generally 50Ω

R1--External impedance at the input end of the filter, the impedance value is the matching impedance minus 50Ω.

R2--External impedance at the output end of the filter, the impedance value is the matching impedance minus 50Ω.

       2. Reasonable measurement level; just like the crystal requirements for the excitation level, the core element of the filter is still a crystal, so when the excitation level is not specified, generally choose 0dB as the input level.

, Good shielding: The input and output ends of the filter are well shielded so that the energy of the signal source cannot be directly coupled to the load end. For filters above very high frequency, the connection between the filter and the instrument should conform to the principle of coaxial line as much as possible. When the filter is on the line, it should be grounded as much as possible, and the input and output ends should be isolated to ensure the stop-band attenuation of the filter.

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