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With the current design one of the demodulators samples near the zero crossing of the ground signal and gets a good ratio of target to ground signals. But the other demodulator is sampling near the peak of the ground signal which gives a poor target to ground signal ratio. This puts a limit on the sensitivity that can be achieved.
As a test I set up conditions that gave a ground signal that would have been 12 v p-p out of the final stage of the preamp, if it were not clipping. In this case the detector has to use a lower gain so the signal does not get clipped. It has to sample the signal from the second stage of the preamp.
Then I added a sign wave signal into the preamp with the right amplitude and phase to cancel some of the ground signal. I reduced it from 12 volts to less than 4 volts so the highest gain of the preamp could be used. Under these conditions it could get a good ID from foil and a penny from 7".
This is not a practical solution because I could not adjust the amplitude and phase of the ground canceling signal for changing ground conditions. But it indicates that I could improve the sensitivity if I could generate the proper signal. Unfortunately, I cannot see any way of generating such a signal without adding more hardware.
Robert Hoolko
As a test I set up conditions that gave a ground signal that would have been 12 v p-p out of the final stage of the preamp, if it were not clipping. In this case the detector has to use a lower gain so the signal does not get clipped. It has to sample the signal from the second stage of the preamp.
Then I added a sign wave signal into the preamp with the right amplitude and phase to cancel some of the ground signal. I reduced it from 12 volts to less than 4 volts so the highest gain of the preamp could be used. Under these conditions it could get a good ID from foil and a penny from 7".
This is not a practical solution because I could not adjust the amplitude and phase of the ground canceling signal for changing ground conditions. But it indicates that I could improve the sensitivity if I could generate the proper signal. Unfortunately, I cannot see any way of generating such a signal without adding more hardware.
Robert Hoolko