Testing crystals

[Terry]

Hi Terry
When I put your formula into cell I3 the result was 0.000197316. not quite the same as your answer of +0.0197316%.

Am I doing something wrong?

My fault, Mike. There is nothing wrong with the formula but I omitted to tell you to format the cells to display the result as a percentage. (If you look carefully, 0.000197316 is the same figure as +0.0197316% - it's just the position of the decimal point that is different.)

Select one block of cells, then look for a button on the ribbon or toolbar with % on it. Click that and then adjust the number of decimal places to suit. I used 3 but you might prefer 4. I wouldn't go any further because the additional jumble of digits isn't going to be of any use to you: it just makes the results harder to take in.

An alternative method is to select the cells as before, then press CTRL 1 to call up the format menu. Select 'Number', then 'Percentage' from the list provided. Select the number of decimal places in the box which appears, then click 'OK'.

It will then be possible to quickly sort out the wheat from the chaff. Any that indicate an error of more than about, say, ±0.05%, should be taken as indicating that either the crystal or the test method is suspect.

"What I want to understand is how to get the crystals to give out lower harmonics, so that they can be used as the manufacturer intended."

I'm puzzled by this as 'what the manufacturer intended'  was for the crystal to be used at the frequency marked on it!

If it is a third overtone crystal it can be made to oscillate at its fundamental frequency, as you have proved in your tests. If it has been made so that its marked frequency is also its fundamental frequency then it won't produce any usable sub harmonics.

From what Trevor had to say on the subject it looks as if would be a world first if you could come up with a circuit that did use sub-harmonics in a useful and repeatable manner!

But ask yourself why anybody would bother. It costs no more to have a crystal manufactured for the frequency you want as it does for a crystal for a frequency you don't want!

I don't know how much it costs for a one-off sample these days but in the days that I used such things they were remarkably cheap, I thought.

For a MW crystal, a parallel tuned circuit, as you suggested, should do very well. Either in the anode/collector circuit of the oscillator or AC coupled from the output. A ferrite rod aerial for the inductor would be ideal as you can then pick up the oscillator with a portable radio!