Being overly precise may not always be best!?

[Kev]

Hi,
I've been setting up a 15" DD coil, and decided to try balancing it correctly, rather than for ground rejection. I attached a 1.7kHz osillator to the Tx and checked the Rx on the scope. I was able to almost remove all but a very short 2mV ripple on the transient edges, by slightly moving the Rx coil tacking it down and then tweaking the damping resistance, I was able to sample at 4uS due to the fine balance I had achieved .

I then tested the coil with a half gram nugget and ziltch, it couldn't see it.
Extending the sample delay time made no difference at all.

Previously I was getting about 5 - 6 inches when the coils were just plonked in the housing willy nilly, critically damped, and sampling at 6uS. So I'm blowed if I know, there's got to be more going on than meets the eye.

Cheers
Kev.

 

[Reg]

Hi Kev,

Ah, the joys of working with PI's and the less than logical things that seem to occur.

I ran into what you did quite some time back and couldn't explain it either. That is why I deliberately leave my DD coils sort of aligned "willy nilly" as you would say. It simply works best for me.

What I didn't do is something you did and that was to try to minimize the signal from the offending ground (or rock) and then compare how a small nugget responds to a coil where the windings are just placed in the housing with more than overlap than necessary.

In other words, can one take and reduce the signal from the most offending "rock" without hurting the signal from small gold. That is something I need to try.

Again, what is frustrating is all of what you are finding now, is basically transparent if the delay is greater than 15 usec or so. It is only when working down in the 10 to 15 usec or less range that such things become really obvious. At least, that is what appears to happen when I experiment with it. As such, it is the really small gold nugget signals where most of the signal is less than 15 usec or so long that seem to suffer. Larger gold may suffer too, but it is not as obvious.

Reg

 

[Eric Foster]

Hi Kev and Reg,

I expect the geometry of the coil/target has something to do with it. The maximum range you can get with a mono coil is with the object on axis. I would guess that if you had an 11in mono coil you would be back to your 5

 

[Kev]

Hi Eric,
Yes I see, that makes perfect sense, it's not an electronic matter but one of physical dimensions, adding 2 coils, compounds the spatial relationships between coil and target.
Can't wait for a good read

Cheers Kev.

 

[Kev]

Hi Reg,
It's comforting to know that others have been over the same trail and took the same turns. I know from previous coils that I have stumbled upon electrical balance of the coils, because it's the point where the worst offending hot rocks disappear, but with the present coil it is also the point where little nugs disappear also.

As I make adjustments to the overlap I pass the rocks and nuggets over to find the compromise point, It's a point where I don't have to dig a particular hotrock because if they are detected at all, it's usually on the surface of the ground or just under, and it will still give a reasonable signal on a small nugget, but it's a compromise. This effect is most pronounced on flat wound coils, I'm not sure if they're called axial or radial, always get the two mixed up.

Cheers
Kev.