Originally posted by baum7154
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MPP Coil And Preamp Speed
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Originally posted by green View PostHi KR, Wondering what effect a 1N4148 diode across R13, the first amp feedback resistor would have. Cathode to pin 1. Should keep the output from going to the - supply.
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Originally posted by green View PostHI KR, Wondering if you have been doing any experimenting. I have been using a diode across the amp in my circuits. Been playing with spice. Want to try some things in hardware. Plan on taking some scope pictures with the different changes. Hope you are finding some nuggets.
...or...this MPP is working so well - even without GEB and on really bad ground - that I may build another MPP to experiment with - that way I don't adversely affect the one I use in the field. Currently the only thing I would desire on the MPP is to be able to detect smaller TC targets - would love to get down to detecting a .05 gram lead bit - probably a very lofty goal! - but the more sensitive it gets the better. I continue to experiment with coil builds and am thinking on ways to sample earlier - sub 1uS.
Since I brought up GEB & I've been thinking I should elaborate on how I use the MPP on high iron mineralized (bad) ground - when it comes to GEB - when I say the MPP is working well - this is what I mean:
First, if I pump the coil up & down as if I were ground balancing a VLF detector - it responds strongly to the ground of course - so I simply say - so don't do that!
I find that if I have a reasonably well controlled sweep (bobbing up and down as little as possible while sweeping, which is always a good idea) - and if I detune (just like the requirement to turn down sensitivity on a VLF detector when conditions dictate) then the GE sound is minimized A LOT and it is quite easy to tell a target sound from ground sounds. I can also to some extent also tell what kind of metal by the sound. For this reason I ~currently~ have no desire to add GEB to my MPP.
To detune:
I speed up my SAT a lot - threshold recovery is around 1/2 second or maybe a little less, Sensitivity is turned way down, I swap out my 10 NiMH battery pack for a 8 NiMH battery pack (thus lowering coil flyback peak voltage), and then turn up Threshold (or Audio per the build doc) a bit and it's amazing, I still detect targets well with little loss in depth but GE is quite diminished.
This of course requires the mods that I've done to my MPP...
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Originally posted by KRinAZ View PostHi green, been out in the field prospecting & using the MPP so haven't had a chance to experiment but I do want to try that diode mod & see the results. That will be the next thing I do when I open my MPP back up...
...or...this MPP is working so well - even without GEB and on really bad ground - that I may build another MPP to experiment with - that way I don't adversely affect the one I use in the field. Currently the only thing I would desire on the MPP is to be able to detect smaller TC targets - would love to get down to detecting a .05 gram lead bit - probably a very lofty goal! - but the more sensitive it gets the better. I continue to experiment with coil builds and am thinking on ways to sample earlier - sub 1uS.
Since I brought up GEB & I've been thinking I should elaborate on how I use the MPP on high iron mineralized (bad) ground - when it comes to GEB - when I say the MPP is working well - this is what I mean:
First, if I pump the coil up & down as if I were ground balancing a VLF detector - it responds strongly to the ground of course - so I simply say - so don't do that!
I find that if I have a reasonably well controlled sweep (bobbing up and down as little as possible while sweeping, which is always a good idea) - and if I detune (just like the requirement to turn down sensitivity on a VLF detector when conditions dictate) then the GE sound is minimized A LOT and it is quite easy to tell a target sound from ground sounds. I can also to some extent also tell what kind of metal by the sound. For this reason I ~currently~ have no desire to add GEB to my MPP.
To detune:
I speed up my SAT a lot - threshold recovery is around 1/2 second or maybe a little less, Sensitivity is turned way down, I swap out my 10 NiMH battery pack for a 8 NiMH battery pack (thus lowering coil flyback peak voltage), and then turn up Threshold (or Audio per the build doc) a bit and it's amazing, I still detect targets well with little loss in depth but GE is quite diminished.
This of course requires the mods that I've done to my MPP...Attached Files
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Green - Somewhere you stated that the decay constant for a US nickel was 10us. When I asked about a target versus time constant chart, I was really looking for something similar for different targets. For example: nickel, dime, pulltab, bottle cap, gold nuggets (or aluminium can squares), etc. I wanted to compare this with the LTSPICE simulation I created from John Alldred's paper. This would then provide a guide as to how practical it would be to detect certain low conductivity targets, and what settings may be required. In other words, the request is simply for a table with 2 columns: Target type and Decay Constant. Do you have such a thing?
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Originally posted by Qiaozhi View PostGreen - Somewhere you stated that the decay constant for a US nickel was 10us. When I asked about a target versus time constant chart, I was really looking for something similar for different targets. For example: nickel, dime, pulltab, bottle cap, gold nuggets (or aluminium can squares), etc. I wanted to compare this with the LTSPICE simulation I created from John Alldred's paper. This would then provide a guide as to how practical it would be to detect certain low conductivity targets, and what settings may be required. In other words, the request is simply for a table with 2 columns: Target type and Decay Constant. Do you have such a thing?
Made some observations in above reply, maybe not correct. Was going to list the time constants but didn't know how for a lot of the targets. What is the time constant for the dime, quarter, aluminum slugs, etc that don't have an exponential decay?
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A list of targets with exponential decay. Could add more if someone wants a particular target. Lead piece should be 2.1x2.1x1mm not .21x.21Attached Files
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Originally posted by green View PostA list of targets with exponential decay. Could add more if someone wants a particular target. Lead piece should be 2.1x2.1x1mm not .21x.21
Probably you have small titanium screws (as used with bicycle, material Ti-6Al-4V) dimensions M5x10 mm or smaller to test time constant?
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Thank you
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Originally posted by WM6 View Postgreen. Excellent work.
Probably you have small titanium screws (as used with bicycle, material Ti-6Al-4V) dimensions M5x10 mm or smaller to test time constant?
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Thank you
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Originally posted by green View PostA list of targets with exponential decay. Could add more if someone wants a particular target. Lead piece should be 2.1x2.1x1mm not .21x.21
I've scanned over your past posts, but cannot seem to find the details.
Thanks.
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Originally posted by Qiaozhi View PostCan you remind me how you were able to take these measurements, and what hardware you used?
I've scanned over your past posts, but cannot seem to find the details.
Thanks.
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Originally posted by green View PostUsed a 2 stage amplifier, Gain each stage(15 to 20) direct coupled, LT6237 amp(90Mhz) probably higher than needed, but when I started I didn't know better. Used a different coil diameter to get enough signal for the target if needed. Used DSO to record amplifier out with no target, record with target, subtract no target recording from target recording with Excel and chart. I found it's better if the scope doesn't over scale so I added two more stages of gain after the preamp x9 each for added resolution if needed when recording longer decay times, over 100usec.(record no target and target at each gain output before subtracting and dividing by the added gain before adding to the chart). Can chart linear-log to see if exponential decay and log-log if not exponential. I think it's easier to compare targets that don't have an exponential decay log-log.
What is the TX pulse width? I guess it must be quite narrow, otherwise the preamps will go into saturation and you'll miss the smaller targets completely.
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Originally posted by Qiaozhi View PostThanks. I understand what you're doing now. I'm surprised you can get any meaningful measurements on some of the smaller targets.
What is the TX pulse width? I guess it must be quite narrow, otherwise the preamps will go into saturation and you'll miss the smaller targets completely.
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