A method and apparatus for measuring the frequency of a desired resonant mode of a crystal arrangement, or other two-port device, during an automated operation. The crystal arrangement, or other two-port device, is placed into a test circuit and subjected to a sinusoidal test signal of known frequency....http://www.google.de/patents/US6292002?utm_source=gb-gplus-sharePatent US6292002 - Crystal resonant frequency sensor
