A while back I mentioned I had an idea of how we could use a DD coil on the SPP. Well, Luke took on the task of building the box I figured we would need and has began some initial testing.
Well, at first it looked like all was a failure but after discussing just what was happening, I think the DD is useable providing people understand what is happening and how the detector has to be set up.
Using the initial suggestions didn't seem to work, especially if one tried to turn the gain up. However, with the gain much lower, it did seem to work but didn't do well on small gold. However, on larger objects like a copper penny, it did quite well, meaning the depth was impressive on the coin even at the low gain.
After giving the information I got from Luke some thought, I think I have figured out what is happening and most likely why it doesn't seem to work. There are still a couple of things I haven't figured out, but I think I will in time.
The problem lies in the wire used in the ML coils and how it affects the decay curve of the signal. For a coil to work right on the SPP, it has to decay properly to a suitable level before the 10 usec sampling. That isn't happening on the ML compatible DD coils. However, they are within range that the circuitry still does work. So, instead of acting normally, they act totally odd with, among other things, not being able to ground balance.
What isn't known is the coil sort of acts hypersensitive. This can happen when a coil is at the edge of working. When a coil is acting this way, it can have considerably greater gain, meaning what happens at a gain of 10 on a normal coil could happen at a gain of 3 or 4 or even less on a hypersensitive one. Turning the gain to max simply over powers all circuitry.
To make matters worse, the design of the overlap zone on a DD coil makes a big difference as to how small gold is detected.
So, in simple terms, this little project and the solution are more complex than initially thought. I am fairly certain people wanting the DD to find bigger objects will find it can be done. Small gold is a different story. The coil physical layout of the windings is what influences the small gold detection as much as anything. I am not sure just how much of an influence the idea of using a piece of lead tape might help after adding other parts in the form of a black box between the coil and the detector.
So, in simple terms, the problem isn't solved but the coil could be useable on coins, etc but still needs more done to use on small gold. We are working on it still. Well, the truth is Luke is doing all the work.
In the mean time, you might try the DD but set the gain at minimum and see if it GB's and if it does, then turn the gain up until it no longer GB's and then back it back down to just where it does. It might need the black box to even work at minimum gain. That is something I don't know just yet. My SPP is down for repairs, so I can't check.
Reg
