MPP Coil And Preamp Speed

[green]

The pulse width has varied. Have used 40 to 240usec Tx pulse width. Starting out I varied the power supply voltage or added a resistor to keep peak current between 1 to 2 amps. The mosfet always avalanches. Lately coil current is in a servo loop, either peak current or constant rate, typically for 1 amp peak. Allows me to compare coils by keeping the same current profile. I trigger on the mosfet driver going low(zero time), usually takes about 4usec for the amp to come out of saturation. Had to use a 1.25 inch diameter coil for the .8 grain lead(.8usec TC) to get enough signal. I have compared decay curves with different coils and diameter doesn't seem to matter. Scope pictures of amplifier out with no target in reply#34. Some over shoot but taking the difference between a target and no target recording seems to solve it.

Looked at the .8 grain lead decay curve. Closer to 3usec when it came out of saturation. The 1.25 inch coil with 80usec on time, 1amp peak, peaked out at 430 volts, less than avalanche. Different than what I said above.

[green]

Thanks. 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.

The pulse width has varied. Have used 40 to 240usec Tx pulse width. Starting out I varied the power supply voltage or added a resistor to keep peak current between 1 to 2 amps. The mosfet always avalanches. Lately coil current is in a servo loop, either peak current or constant rate, typically for 1 amp peak. Allows me to compare coils by keeping the same current profile. I trigger on the mosfet driver going low(zero time), usually takes about 4usec for the amp to come out of saturation. Had to use a 1.25 inch diameter coil for the .8 grain lead(.8usec TC) to get enough signal. I have compared decay curves with different coils and diameter doesn't seem to matter. Scope pictures of amplifier out with no target in reply#34. Some over shoot but taking the difference between a target and no target recording seems to solve it.

[Qiaozhi]

Used 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.

Thanks. 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.

[green]

Can 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.

Used 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.

[Qiaozhi]

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.21

Can 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.

[green]

green. 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?
.
Thank you

I will try to get one, might be awhile. A couple targets, 1x1 inch aluminum can(5usec), .5x.5 inch aluminum can(2.9usec). I'll add to chart when I get a few more.

[WM6]

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.21

green. 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?
.
Thank you

[6666]

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.21

Thanks

[green]

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.21

[green]

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?

http://www.geotech1.com/forums/showthread.php?21931-Getting-ready-to-build-the-MPP&p=207715#post207715

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?

[6666]

I swap out my 10 NiMH battery pack for a 8 NiMH battery pack (thus lowering coil flyback peak voltage),

Why not use a coil snubber ?

[Qiaozhi]

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?

[green]

Hi 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...

Ran the time constant for a piece of lead, little over 2x2mm by1mm thick(.8 grains) TC=.8usec. Made another amplifier. Have been using LT6237(90 Mhz), used a opa1612 closer to a LM4562. Not as much gain as your MPP. Tried with nothing across the feedback resistor, back to back diodes and a 2N3906 emitter base across the feedback resistor. Don't see much difference. With spice I see some difference. Still need to work on the circuit to eliminate the ripple(repeats, maybe from the power supplies) . Connecting the scope to the coil effected #1 decay a little(top 2 scope traces). Circuit speed close to what I get with spice.

[KRinAZ]

HI 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.

Hi 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...

[green]

Hi 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.

HI 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.