Greetings,
This is the 4th installment and I hope I am not boring to many. The nice think about a forum is that if something isn't of interest to you, you don't have to read it. The target sample and second sample timing circuit including the gate switch IC is basically the same as Carl Moreland came up with except for a slightly different hook up. I really liked the way Carl did it so I decided not to rebuild the wheel. I had first thought to build my timing circuit out of Cmos 555 timers wired as monostables but decided to stick with Carl's circuit. Reg Sniff's input here was very useful. The need to take the target sample right in the knee of the fly back spike discharge curve to detect small gold is very important. Reg also pointed out that small gold signals die out and are gone very fast so it is important to use a narrow sampling window. If the window is to wide the small gold signal will be averaged along with no signal so the average will be reduced and weaker. The target sample window that I am using is 10 us and I am sampling in 10 us from the end of the coil pulse to get the strongest small gold signals that I can. The pulse delay control determines where I take my target sample. It is adjustable from 10 us out to 90 us. At 90 us almost all target signals of coin size or smaller have died out and only iron signals are left. This makes sort of an iron test by turning the pulse delay out to max and seeing if the target still come in strong. If it does it is likely iron. If it is gone or weak, there is a good chance it is not iron and worth digging. I have attached a graph to give a rough idea of where various targets drop out. The second sample is also 10 us wide and since it is subtracted from the target information, I have it set at 90 us out. This gets it out far enough so as to have little effect on from good target signals. It is out there to eliminate common mode interference like the grounds magnetic field and power line noise and such.
This detector does NOT have a ground balance channel. My intended use for this detector is as a general purpose metal detector with good sensitivity to small gold. For the dedicated gold prospector interested is tiny gold nuggets and flakes a ground balance channel would be necessary. However, as you can see, for general purpose metal detecting and occasional gold prospecting this detector works pretty good. By reducing the chances of bad ground overloading the preamp and having an adjustable SAT (more on that later) allows the detector to be useful for finding small gold under some pretty bad ground conditions. I have a design with a ground balance channel and who knows, maybe I will go that route later.
I will give you my thoughts on the back end of the circuit next time
This is the 4th installment and I hope I am not boring to many. The nice think about a forum is that if something isn't of interest to you, you don't have to read it. The target sample and second sample timing circuit including the gate switch IC is basically the same as Carl Moreland came up with except for a slightly different hook up. I really liked the way Carl did it so I decided not to rebuild the wheel. I had first thought to build my timing circuit out of Cmos 555 timers wired as monostables but decided to stick with Carl's circuit. Reg Sniff's input here was very useful. The need to take the target sample right in the knee of the fly back spike discharge curve to detect small gold is very important. Reg also pointed out that small gold signals die out and are gone very fast so it is important to use a narrow sampling window. If the window is to wide the small gold signal will be averaged along with no signal so the average will be reduced and weaker. The target sample window that I am using is 10 us and I am sampling in 10 us from the end of the coil pulse to get the strongest small gold signals that I can. The pulse delay control determines where I take my target sample. It is adjustable from 10 us out to 90 us. At 90 us almost all target signals of coin size or smaller have died out and only iron signals are left. This makes sort of an iron test by turning the pulse delay out to max and seeing if the target still come in strong. If it does it is likely iron. If it is gone or weak, there is a good chance it is not iron and worth digging. I have attached a graph to give a rough idea of where various targets drop out. The second sample is also 10 us wide and since it is subtracted from the target information, I have it set at 90 us out. This gets it out far enough so as to have little effect on from good target signals. It is out there to eliminate common mode interference like the grounds magnetic field and power line noise and such.
This detector does NOT have a ground balance channel. My intended use for this detector is as a general purpose metal detector with good sensitivity to small gold. For the dedicated gold prospector interested is tiny gold nuggets and flakes a ground balance channel would be necessary. However, as you can see, for general purpose metal detecting and occasional gold prospecting this detector works pretty good. By reducing the chances of bad ground overloading the preamp and having an adjustable SAT (more on that later) allows the detector to be useful for finding small gold under some pretty bad ground conditions. I have a design with a ground balance channel and who knows, maybe I will go that route later.
I will give you my thoughts on the back end of the circuit next time
