allanhurst:
You can 'tweak' crystals to a very high tolerance.I used to design pagers, and we bought 2.5 ppm crystals from NDK by the million. Not good enough for the job.
The circuit included a series L ( a few hundred nH) and variable C ( a few pF ) tuned to around the wanted (3rd overtone) frequency, and the crystal could then be easily pulled to <0.01 ppm on test.
Allan.
- sounds like good job security there.
- can you explain more about your circuit? Is it some type of high pass filter? What is the 3rd overtone?