React to a ground

[CAC]

Hello Carl
Excuse for bad English
At me the device reacts to chernozem especially on the ploughed field, at fine tuning R31 or R42 sensitivity strongly falls. Working frequency - 600Hz.
The problem in working frequency can, and on what influences R45?
Theoretically the pulse metal detector should not react to a ground.
Share the reasons in this occasion

[lbt]

hello
excuse for my bad english but i'm french
i build hammerhead and have the same probleme in first time.It's because the coil react to vibration not to ground. You must fixe all the wire of the coil with epoxy or silicon.

[Reg]

React to ground

Hi CAC,

Sorry to take so long to answer you about the reaction to the ground.

A PI using a mono coil will react to the ground if the sample delay is very short. Usually, if the delay is less than about 25 usec then the ground will begin to cause a signal. If you reduce the delay even more, maybe down to 12 usec to 15 usec, the signal from the ground can become quite strong. It all depends upon the ground conditions.

Now, using a DD coil will help reduce this ground signal but not eliminate it.

The reason the ground signal is there is complex, but in simple terms, is because the ground contains levels of iron oxide can cause such a signal. The oxide is generally maghemite.

Make sure you shield your coil. The shielding becomes quite important at the shorter delays and can cause a much stronger ground signal if the shielding is not there or very poor.

Finally, about the only thing you can do without a lot of circuit changes to reduce the ground signal is to raise the coil above the ground a little more when searching. Fortunately, the ground signal will reduce dramatically while signals from buried objects reduce much less.

The circuit changes would require one build a form of ground balance circuitry to minimize the ground signal. This requires taking later samples, amplifying them and then subtracting this signal from the main signal with a ratio that subtracts out the ground response.

Now, if looking for copper and bronze objects is the objective, then simply increasing the delay to the point where the ground signal is not there is one simple way to minimize the problem. The signals from copper and bronze should still be very strongs signals.

I hope this helps.

Reg

[САС]

Thanks for advice{councils}. Now I complete "basket" the coil with the graphite screen, preliminary tests have shown absence of reaction to a ground.
One more question as it is possible to make stable a static mode ?.

[Carl-NC]

One more question as it is possible to make stable a static mode ?.

Do you mean non-motion?

[САС]

Do you mean non-motion?

Yes

[Carl-NC]

With DC blocking cap C19 installed and SW2 open, IC8 will need to have zero input bias current to have a stable non-motion mode. A CMOS opamp comes close, and should run 5-10 minutes between retuning (momentarily closing SW2). I don't recall exactly, but I think if you short C19 it's hard to keep the audio stable.

- Carl

[САС]

Carl result please the circuit decision.

[ÑÀÑ]

So will work ?