Many of the earlier metal detector circuits made the search transmitter coil a part of the transmitter oscillator. The problem with this is that lots of soils are para magnetic and lowering the search head to the ground had the effect of an unwanted shift in the oscillators frequency which also affects signal phase. One solution is to make the transmitter xtal controlled to eliminate this unwanted frequency shift. Unfortunately the kind of circuits I prefer (PIC micro-processors) Have a square wave output. However a square wave is made up of an infinite number of sine waves, the trick is to extract only the one you want, ie the fundamental sine wave.
There are many ways to do this but most get complicated. The series LCR circuit is very simple and can work well. If you look at the picture of the sine wave, it even looks good and obviously the filter has done its job in getting rid of most of the other harmonics.
I am doing three metal detector designs at the moment and the progress is posted on my workshop web site at ....
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http://www.thunting.com/smf/index.php/topic,5305.msg34768.html#msg34768
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« Last Edit: May 31, 2009, 09:21:26 am by moonshadow »
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