04-01-2017, 04:28 PM
Mike, I think you are getting a little confused here. You seem to be under the impression that all crystals can be used at sub harmonic frequencies. I am not a crystal oscillator expert but I cannot see any logical reason why they should.
I think the confusion stems from crystals made for higher frequencies that are cut to oscillate strongly on an overtone (harmonic) of their fundamental frequency: this is an entirely different situation, though, and does not apply to all crystals! A 30MHz 3rd overtone crystal might oscillate at 10MHz but there is no way it should be expected to generate 3.333MHz!
Looking at your results for all the crystals which are close to their indicated frequencies highlights an interesting inconsistency. Those that are slightly high show an error of, typically, +0.02% whereas many of those that are low are close to two orders of magnitude worse - around -0.65%. Some are very bad.
Could this be a difference between series and parallel resonant crystals or is your oscillator over-driving the under performing examples, I wonder?
The last column in your table doesn't make it easy to compare accuracy, so I've converted them all to percentages, to make it easier to see the wood from the trees. It is now easy to see that most of the third overtone crystals are very accurate indeed at 0.001% or better.
If you want to incorporate the same function in your own spread sheet, the formula I used is (this is in cell I3):
=IF(H3>2,H3-3,IF(H3>0.5,H3-1,H3-0.333333333))
(I had great fun doing an OCR on your graphic - virtually all of the numbers were correct but most of the decimal points vanished! A .csv file of the original will be useful if you post any more - change the .csv to .txt if it upsets the upload process.)
The crystals which are slightly above their marked frequency can probably be got spot on by adding a small value trimmer in parallel.
As for the bad and non performers, try a different oscillator design and see what happens!
I think the confusion stems from crystals made for higher frequencies that are cut to oscillate strongly on an overtone (harmonic) of their fundamental frequency: this is an entirely different situation, though, and does not apply to all crystals! A 30MHz 3rd overtone crystal might oscillate at 10MHz but there is no way it should be expected to generate 3.333MHz!
Looking at your results for all the crystals which are close to their indicated frequencies highlights an interesting inconsistency. Those that are slightly high show an error of, typically, +0.02% whereas many of those that are low are close to two orders of magnitude worse - around -0.65%. Some are very bad.
Could this be a difference between series and parallel resonant crystals or is your oscillator over-driving the under performing examples, I wonder?
The last column in your table doesn't make it easy to compare accuracy, so I've converted them all to percentages, to make it easier to see the wood from the trees. It is now easy to see that most of the third overtone crystals are very accurate indeed at 0.001% or better.
If you want to incorporate the same function in your own spread sheet, the formula I used is (this is in cell I3):
=IF(H3>2,H3-3,IF(H3>0.5,H3-1,H3-0.333333333))
(I had great fun doing an OCR on your graphic - virtually all of the numbers were correct but most of the decimal points vanished! A .csv file of the original will be useful if you post any more - change the .csv to .txt if it upsets the upload process.)
The crystals which are slightly above their marked frequency can probably be got spot on by adding a small value trimmer in parallel.
As for the bad and non performers, try a different oscillator design and see what happens!
