COIL NULLING

[Aiko]

Hi guys, lets discus here the secrets of the coil nulling. Probably this is the most critical procedure in the home metal detector building, so what techniques do you use?

[Henrikras]

I'm to much a newbie to say much. But maybee this help something! With a scope and measure at the RX ( I open the plug to measure stabile ). Finding the lowest amplitude at the RX, buy moveing the RX/TX, and if it can't balance in right sharpe, I try to take one Winding off or on again.
After finish at it came a little out of balance Null, I use a little ferrite piece and glue this If it Helped. Compair to ferrite, I Tryed with small foil or this thin paper you got in the kitchen, but that was without succes, I lost depth. ( piece like 2x2 cm ).
My problem was not to find the Null, but to keep it.

[Henrikras]

Idea to keep Null

Look at the picture

[Aiko]

Almost finished the DD coil for my IDX PRO, but I can not achieve better nulling then 10 mV. This is the lowest possible value and I do not know how someone can null at 4 mV, or lower.

[Pompey]

Almost finished the DD coil for my IDX PRO, but I can not achieve better nulling then 10 mV. This is the lowest possible value and I do not know how someone can null at 4 mV, or lower.

10mv - you mean 0.01v ?If so -no problem.

[Henrikras]

In my case... Moveing a small piece of ferrite arround the place where the RX/TX are crossing... If you find less that your 10mv, there is more to find and if the ferrite wouldn't help, you found the best Null. But do you got the shielding at this point in proces ? In my case the Null moved after shielding, and after finding the Null again after shielding, it got lower. Dont know wry that happen.

[Qiaozhi]

The correct position for nulling depends very much on the detector being used. In the majority of cases the correct position is not at the deepest null, but depends more on the correct phase-offset between TX and RX. As an example, take a Tesoro 5-pin coil and measure the residual amplitude at the RX coil and also the phase-offset. If you get the wrong phase-offset, you will most likely have problems getting the detector to ground balance, and the discrimination settings will be in the wrong place

[Henrikras]

Thanks .. Learned some again. Because ist difficult to know what phase-offset one Detector will work best with, how about If I (we) could measure a Stock coil with two prope at the scope... And then make the balance at our project of makeing a search coil as close as possible to that ?
Could that Be an idea ? Or would a change of size and form of the searchcoil, allso change the best working phase-offset / balance ?

[Qiaozhi]

Thanks .. Learned some again. Because ist difficult to know what phase-offset one Detector will work best with, how about If I (we) could measure a Stock coil with two prope at the scope... And then make the balance at our project of makeing a search coil as close as possible to that ?

If you are making a coil for an existing [working] detector, then that is exactly the way to do it.

Could that Be an idea ? Or would a change of size and form of the searchcoil, allso change the best working phase-offset / balance ?

The required phase-offset will be the same regardless of the size or shape of the search coil.

[Henrikras]

Thank you Very much...

[Aiko]

Hi Qiaozhi! Would you tell us what is the correct faze shift for the IDX-PRO and on which point to measure it? Thanks in advance!

[Qiaozhi]

Hi Qiaozhi! Would you tell us what is the correct faze shift for the IDX-PRO and on which point to measure it? Thanks in advance!

I do not know the correct phase-shift for this detector. You would need to get a commercial coil and measure it.

[golfnut]

Aiko - dont worry about the levels of the Null. What you have is way better than required.

I intentionally rubbished my DD null with ferrite slug to 800mV - in limited air testing I did not observe degradation.


In my limited experience the single most important build tweak for the builder/ user /experimented is the frequency tune of the Rx coil - this does affect sens depth in the ground and you should select the cap that gives best sens and best usability in you local soils.




S

[Gwil]

The "de-nulling" experiment using a ferrite slug seems to make sense. The way I see it, the signal at the input of the preamp is the sum of a number of signals, all at the transmitted frequency but with varying amplitudes and phases. They can be due to (1) inductive coupling directly between the Tx and Rx coils (this is the signal that is adjusted during the nulling procedure), (2) capacitive coupling e.g. between cores in the cable, (3) the ground, and (4) a target. And possibly other things I haven't thought of.

Each signal is chopped on and off by the synchronous detector and produces a voltage at the output of the following low pass filter. The total voltage at this point is the sum of the voltages caused by all the signals. Assuming the hardware behaves linearly, each signal acts independently - if it remains constant then its contribution to the total is unchanged. A varying signal means the total voltage varies (unless the sync. det. is adjusted to ignore it of course - ground exclusion is an example).

Adding a ferrite slug changes signal (1) to a new, constant, value but has no effect on any of the other signals. In particular, target signals will pass through the preamp/sync.det./low pass filter exactly as they did before, and sensitivity will be unchanged.

At least that's how it seems to me.

G

[Aiko]

Guys I will be very grateful, if someone of the experts here can describe in details the procedure of nulling of a concentric coil. I decided to make a separate shield from a thin aluminum foil for each one of the coils - TX, RX and the feedback coil. The coil will be for the IDX-PRO. I already have the coils wired and shielded with the proper inductance and resistance. Only for the nulling processes I do not know haw to proceed.

And one more question - is there a problem that my shield is from aluminum foil? I know that for DD coils the aluminum foil is perfect, but do not have an experience with concentric coils.