Cytat:Disadvantages of Crystal Filters.

There are two basic problems associated with crystal filters: spurious responses and non-linear drive level responses.

The spurious responses are caused by, inharmonically related, modes of vibration in the crystal. They appear in the filter as narrow responses that degrade whatever band they appear in. For example, if they appear in the stopband, the attenuation is decreased (Figure 1) but if they appear in the passband they will introduce an unwanted notch. Generally, spurious responses are very narrow and don’t cause a problem unless an unwanted signal appears at their frequency. They have a minimal impact on the overall noise bandwidth. More on spurious later.

The non-linear drive level causes a series of problems. First it limits the drive level to a maximum of +10 dB with a recommended drive of –10 dBm max. Unless the crystals are carefully designed and manufactured the Q and frequency can change as a function of drive and the Q could halve as the drive level was changed from –10 to –60 dBm. Since crystal filters must operate over wide drive level ranges they should be tested over all expected drive conditions.

The non-linear drive condition is the main cause of Intermodulation distortion in crystal filters.