A
Anonymous
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Hallo Eric and all detektorists on the net! some ideas to share:
1. pulse power vs. repetition rate: - doubling the pulse power should double the target response (noise remaining constant), othervise doubling the repetition rate will double the target response but increase the (uncorelated) nose at sqrt(2), so the pure win of doubling the rep. rate remains only sqrt(2). Therefore I think, if it doesnt affect the sampling delay, pulse power is the better choice. Eric, do you agree?
2.Shortening the pulse decay the fastest way is to keep the coil voltage highest (e.g.400V) until current reaches zero, meaning a variable damping resistor. Then it should switch quickly to the value needed to avoid ringing. Simulations (including coil capacitance of 40pF) show a possibility to get the voltage below 20mV 2us after starting to drop from 400V, (assuming very precise time control - resolution 10-20ns) In case of separated TX - RX coil there will be no no amplifier input resistor, and assuming also a balanced DD coils, the RX coil will not influence the decay of the transmitter. As far as back emv is hold on 400V, decay speed (starting drop below 400V) doesn't depend on pulse power. Now I am working on the whole cirquitry including time control and on coil design:
3. Eric, can you please tell us, what is the most promissing coil shape for balanced DD search coli? You mentioned rectangular coils to have better coverage - does it mean also detection distance? How should look a rectangular DD search coil look like? How did you come to this ratio of 3:1? And screening is also a problem because of the parasitic capacitance to the coil - do you have some good advice? What material do you use for screening? According to my calculations, I would use a 16x0.2mm stranded wire for lower coil resistance. Is there any disadvantage using 0.2mm instead of 0.1mm wires like yours? (e.g. eddy curents).
regards Walter
1. pulse power vs. repetition rate: - doubling the pulse power should double the target response (noise remaining constant), othervise doubling the repetition rate will double the target response but increase the (uncorelated) nose at sqrt(2), so the pure win of doubling the rep. rate remains only sqrt(2). Therefore I think, if it doesnt affect the sampling delay, pulse power is the better choice. Eric, do you agree?
2.Shortening the pulse decay the fastest way is to keep the coil voltage highest (e.g.400V) until current reaches zero, meaning a variable damping resistor. Then it should switch quickly to the value needed to avoid ringing. Simulations (including coil capacitance of 40pF) show a possibility to get the voltage below 20mV 2us after starting to drop from 400V, (assuming very precise time control - resolution 10-20ns) In case of separated TX - RX coil there will be no no amplifier input resistor, and assuming also a balanced DD coils, the RX coil will not influence the decay of the transmitter. As far as back emv is hold on 400V, decay speed (starting drop below 400V) doesn't depend on pulse power. Now I am working on the whole cirquitry including time control and on coil design:
3. Eric, can you please tell us, what is the most promissing coil shape for balanced DD search coli? You mentioned rectangular coils to have better coverage - does it mean also detection distance? How should look a rectangular DD search coil look like? How did you come to this ratio of 3:1? And screening is also a problem because of the parasitic capacitance to the coil - do you have some good advice? What material do you use for screening? According to my calculations, I would use a 16x0.2mm stranded wire for lower coil resistance. Is there any disadvantage using 0.2mm instead of 0.1mm wires like yours? (e.g. eddy curents).
regards Walter