The return signal on metal in the ground is so weak that the TX field can mask it. You have two choices. Either you can do a RX reading while the oscillating TX signal is doing a zero crossing (that's when the voltage is at zero as it oscillates from positive to negative) or you can turn off the TX pulse for a short time. The latter is what is done most easily in homebrew designs. Doing an RX reading during this quiet time is the best way to sense the collapsing fields on the metal in the ground. That collapse starts just as soon as the TX goes silent and lasts until all of the energy in the metal is gone. It's not long. The timing between turning off the TX and starting the RX is critical. Do it too soon and the TX field will not have collapsed yet causing the RX to see a target which isn't there.
This, by the way, is the most common cause of the question, "My DIY detector makes a solid beep. What did I do wrong?"
If you turn it off too late, as may be the case with Xavier, the signal is sensed at a greatly diminished distance because the field resulting from eddy currents in the target has already diminished to a lower than optimum point. The F
o, commonly called the "F sub O," or frequency of operation has an effect on this collapse time based on three factors. The type of metal, mass of the metal and the shape. All three have different effects on the field that is there and those differences are how discrimination works. Thus, some frequencies are better for finding gold nuggets, some better for finding gold coins and some better for finding gold rings. Because shape and mass play a part, that explains why you can find a small gold ring and sometimes miss a big one. You can experiment with this if you are lucky enough to have a gold coin, nugget and various sized gold rings in your collection. Keeping a chart of each of your detectors and their response can improve your odds in the field.
Homefire, you already know about the relationship to timing in PI circuits. You once recommended a resistor change to me for my Sea Hunter which would have changed to timing relationship to one better suited to gold targets.
Xavier, a square wave is an infinite number of sine waves being switched on and off. It will have zero difference electronically.
Isn't this dry, boring theoretical stuff fun? As we continue to discuss these things, you can get a glimmering of why it is so difficult to see a newbie trying to build a metal detector from parts. It's a bit like watching a farmer try to build a tractor from iron ore. We all know it can be done but the question is, "Should the farmer really do it himself?"
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It's all about that moment when metal that hasn't seen the light of day for generations frees itself from the soil and presents itself to me.
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