05-01-2017, 10:22 AM
Mike, re: "The fundamental frequency is 2.92 times (almost 3times) the marked frequency", this is an error of -8% on the third overtone of the marked frequency which is a ludicrous figure for the accuracy of a crystal oscillator! The crystal seems to be active but something is badly wrong - thus my comment that the crystal is being over driven.
A different oscillator circuit may well highlight the problem.
"Expressing the test frequency as a percentage of the marked frequency for the top crystal on the right half gives a result of 100.0197316%": so it does, Mike, but what I am doing is calculating the error - think carefully about what an error of +100% really means!
The actual error is +0.0197316% but, for clarity, I'm only showing the result to 3 decimal places which rounds this figure up to +0.020% as shown.
In an ideal world, all the figures in the Error column would be 0% ...
Note also that I have manipulated the figures for the 3rd overtone crystals to show the error relative to the fundamental, rather than the marked, frequency. I don't think expressing the actual frequency as a -66.6% error on the marked frequency would be of much use to you or anybody else!
By the way, how accurate is your frequency counter and how are you measuring it? Any error here will have an effect on these results.
Finally, I note that all the crystals with a fundamental frequency in the Medium Waveband seem to show some attempt at activity but that's as far as it goes. Again, a different oscillator - or even different component values - might tell a different story. Why not try something with a MW tuned circuit in it, which is how I would expect it to be used in any micro Tx? I think it unlikely that all of these crystals are duff, so persevere: I'm sure you'll get the right answer in the end!
A different oscillator circuit may well highlight the problem.
"Expressing the test frequency as a percentage of the marked frequency for the top crystal on the right half gives a result of 100.0197316%": so it does, Mike, but what I am doing is calculating the error - think carefully about what an error of +100% really means!
The actual error is +0.0197316% but, for clarity, I'm only showing the result to 3 decimal places which rounds this figure up to +0.020% as shown.
In an ideal world, all the figures in the Error column would be 0% ...
Note also that I have manipulated the figures for the 3rd overtone crystals to show the error relative to the fundamental, rather than the marked, frequency. I don't think expressing the actual frequency as a -66.6% error on the marked frequency would be of much use to you or anybody else!
By the way, how accurate is your frequency counter and how are you measuring it? Any error here will have an effect on these results.
Finally, I note that all the crystals with a fundamental frequency in the Medium Waveband seem to show some attempt at activity but that's as far as it goes. Again, a different oscillator - or even different component values - might tell a different story. Why not try something with a MW tuned circuit in it, which is how I would expect it to be used in any micro Tx? I think it unlikely that all of these crystals are duff, so persevere: I'm sure you'll get the right answer in the end!
