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I found another synchronization problem between the timer and A/D clocks. After I fixed the previous problem I started seeing the displayed phase angle occasional jump 2, 4, or 6° while I was typing commands. The problem was that the serial interrupt routine keeps other interrupts masked until it reads the character from the UART. That could delay the timer interrupt long enough for the timer to get out of sync by 1, 2, or 3 usec from the A/D clock, 4 usec would get it back in sync again.
Of course this was also happening before, but I did not see it then because the clocks were rapidly getting in and out of sync and the filters showed me the average phase. With the previous problem fixed the clocks could get out of phase and stay that way. So I gave up on the idea of a 1 usec timer and went back to 4 usec, which is the same as the A/D. Now they always have to keep the same relationship. If the timer does slip it has to be in jumps of 4 usec, so there may be one bad reading but the clocks stay in phase.
I think I am approaching the limits of what I will be able to do with this 10 bit A/D. With a 4 volt steady state ground signal from the high gain output of the preamp I can get a reasonably good phase angle from a 4 milivolt target signal (a penny at 5"). That is a 1000:1 ratio of ground to target signals. The 4 milivolt signal is 0.8 LSB. From the results I see, I would estimate that I am getting at least 3 bits below 1 LSB. That is about 13 to 14 bits of useable data.
I may be able to improve it a little by better filtering, but I think I am getting near the end. I could increase the preamp gain to get more distance in an air test, but I don't see much point in that. I am interested in studying ways of separating the target signal from the ground signal and improving the identification of targets in the presence of a strong ground signal. More gain is not going to help that. In fact If I increase the ground signal a little more the detector will automatically switch to the low gain preamp output to keep the signal within the A/D range. That drops the gain by a factor of 4 and the target signal is then just .2 LSB. At that point I cannot detect the penny at 5" any more. It is hidden in noise. One possible improvement is to have more levels of gain and to only drop the gain by a factor of 2 at each step. That would allow picking a more optimum gain for each level of ground signal.
I had concluded before I bought the microcontroller board that at least a 12 bit A/D would be required for good results. But I could not find a chip in the families that I was interested in that had a 12 bit A/D and flash memory. So I went ahead with 10 bits to see how far I could get. I am pleased with the results I am getting from this 10 bit A/D, but I think that it is going to max out with a depth of just below one coil diameter. From the results I have seen so far I would conclude that this design using a 12 bit A/D would get a depth of just over one coil diameter in weak ground. I think that with a 16 bit A/D it could equal the commercial metal detectors.
Robert Hoolko
Of course this was also happening before, but I did not see it then because the clocks were rapidly getting in and out of sync and the filters showed me the average phase. With the previous problem fixed the clocks could get out of phase and stay that way. So I gave up on the idea of a 1 usec timer and went back to 4 usec, which is the same as the A/D. Now they always have to keep the same relationship. If the timer does slip it has to be in jumps of 4 usec, so there may be one bad reading but the clocks stay in phase.
I think I am approaching the limits of what I will be able to do with this 10 bit A/D. With a 4 volt steady state ground signal from the high gain output of the preamp I can get a reasonably good phase angle from a 4 milivolt target signal (a penny at 5"). That is a 1000:1 ratio of ground to target signals. The 4 milivolt signal is 0.8 LSB. From the results I see, I would estimate that I am getting at least 3 bits below 1 LSB. That is about 13 to 14 bits of useable data.
I may be able to improve it a little by better filtering, but I think I am getting near the end. I could increase the preamp gain to get more distance in an air test, but I don't see much point in that. I am interested in studying ways of separating the target signal from the ground signal and improving the identification of targets in the presence of a strong ground signal. More gain is not going to help that. In fact If I increase the ground signal a little more the detector will automatically switch to the low gain preamp output to keep the signal within the A/D range. That drops the gain by a factor of 4 and the target signal is then just .2 LSB. At that point I cannot detect the penny at 5" any more. It is hidden in noise. One possible improvement is to have more levels of gain and to only drop the gain by a factor of 2 at each step. That would allow picking a more optimum gain for each level of ground signal.
I had concluded before I bought the microcontroller board that at least a 12 bit A/D would be required for good results. But I could not find a chip in the families that I was interested in that had a 12 bit A/D and flash memory. So I went ahead with 10 bits to see how far I could get. I am pleased with the results I am getting from this 10 bit A/D, but I think that it is going to max out with a depth of just below one coil diameter. From the results I have seen so far I would conclude that this design using a 12 bit A/D would get a depth of just over one coil diameter in weak ground. I think that with a 16 bit A/D it could equal the commercial metal detectors.
Robert Hoolko