minimum sample delay

[KingJL]

What the H#!!, bought it anyway. If anyone needs some, let me know!

Maybe , would like to see a picture of the wire when you receive it thanks

Received my order of 5000' wire today.
Specs: 24AWG 19 conductor, tinned, 1 mm outside diameter, polyalkene primary jacket, polyvinylidene fluoride secondary jacket, 600 volt insulation rating.

I am super happy with it's performance. I wound a 1.5 mH (actually 1495 uH) coil for my Vallon test bench set-up... 10" mean diameter, 49 turns. With a 39 pf capacitor to isolate the injection source and scope probe, the coil resonates at 375 kHz. With an L of 1495 uH and and the 39 pf isolation cap, that results for ~80 - 90 pf for the coil. If anyone would have told me that I could make a coil of 1.5 mH, 49 turns @ 10" diameter and achieve less than 100 pf for the coil, I would have said they are crazy! I estimate that for a 10" 500 uH coil, I could achieve about 30 - 35 pf. With a 1 mm wire diameter, 49 turns make for a relatively thick bundle. But it fits well in my coil form.

I really do not have use for 4000' (1000' already spoken for)... anyone interested, PM me.

[green]

Still working on a coil for 1usec delay to chart AWG28 solid wire TC. I tried another coil, better but not 1usec. Tried another, not much different than last one(maybe 3usec delay). Some scope pictures of the last coil. Observations from scope pictures? Ideas on what to try next? More information about present coil? I'm lost.

[green]

Still working on a coil for 1usec delay to chart AWG28 solid wire TC. I tried another coil, better but not 1usec. Tried another, not much different than last one(maybe 3usec delay). Some scope pictures of the last coil. Observations from scope pictures? Ideas on what to try next? More information about present coil? I'm lost.

Another scope picture, same coil as reply #47. Reduced peak current from 1A to .5A peak and adjusted damping on Rx coil. Holding a ferrite bead near coil still causes a change out to 4usec or greater. Is Tx coil current causing this or some other reason? Could sampling where the ferrite bead is detectable cause a problem? Scope picture of IB coil.

I see the trigger is one division different on todays scope trace, maybe 1.5usec delay today vs 3usec delay yesterday.

[Ferric Toes]

Another scope picture, same coil as reply #47. Reduced peak current from 1A to .5A peak and adjusted damping on Rx coil. Holding a ferrite bead near coil still causes a change out to 4usec or greater. Is Tx coil current causing this or some other reason? Could sampling where the ferrite bead is detectable cause a problem? Scope picture of IB coil.

I see the trigger is one division different on todays scope trace, maybe 1.5usec delay today vs 3usec delay yesterday.

What I did was to reduce the coil current to 0.2A, then put up the repetition rate proportionally and adjust the integrator TC.

The ferrite bead could be giving a signal because of magnetic viscosity.

Eric.

[Bill512]

http://www.geotech1.com/forums/attac...4&d=1527807206 attachment from another thread.

Rx(two 1.5inch round coils connected figure eight) Tx(oval surrounding Rx). What is required to reduce sample delay to 2usec maybe 1usec? What measurements from existing circuit are needed if any? Maybe forget about existing circuit and define what is required to have sample delay at 1 or 2usec.

What is required to get the no target signal near zero volts at 1 or 2usec?

why not measure the "height" of the flyback voltage?
After all, in most PI designs, a significant percent of the "coil signal" is present in sampling times and the signals from targets are superimposed on the coil decay curve.
It is all there at zero time, coil, targets , ground - just the distribution of these, changes with time.
Of course, it will need a different circuit, but simpler than multiple sampling.
Done some quick experiments a decade ago on this subject (sensing the flyback voltage with a second coil ) and if I remember correctly, it worked and had discrimination as a bonus.
And where exists discrimination also there is Ground balance.
It was a sort range metal detector , something better than a pinpointer.
Technically not a genuine PI, but who cares.

[KingJL]

... The ferrite bead could be giving a signal because of magnetic viscosity...

And... not all ferrites are created equal!!

[green]

And... not all ferrites are created equal!!

When I recorded the imbalance caused with the ferrite core reply#47 I recorded imbalance caused by offsetting the Rx coils for the same imbalance during Tx on with constant rate(about 3600A/sec). The signal for the two methods during Tx off looked the same. Thought this might mean something, not sure what.

[green]

Finally got some charts for the AWG24 and 28 solid wire. Used 16 pieces 1inch long for AWG24 and 64 pieces 1inch long for AWG28(pieces laying flat not bundled). Added the TC lines from the projected chart, .8usec for AWG24 and .33usec for AWG28. Looks close. Had a hard time getting enough signal at amplifier out. Log amp not fast enough, charted amplifier out(target-no target)with Excel.

Appears TC chart might be correct. Wondering if TC vs wire size is charted anywhere else to compare.

[green]

Thanks for the reply, think I'm starting to figure it out. Still working on a coil for 1usec delay. Attempted to chart signal decay with fast decay and decay I think is similar to the impulse. Wondering if it makes sense. Target distance was adjusted to give a near full scale signal for fast decay with coins and foil(couldn't get full scale with ground). I'll have to try a full bridge circuit.

Thanks KingJL for posting it.

Thinking about Impulse again. Charted ground signal on log-log chart triggered at Tx off, Tx off +46 and 58usec. Ground charts fairly straight line on log-log chart with fast coil decay triggered at Tx off. Doesn't when triggered near zero current with slow coil decay. Probably only matters when charting signal decay? H bridge PWD13F60(N channel) was mentioned in another thread. What are the pros and cons of using a 4N channel vs a 2N and 2P channel bridge?

[KingJL]

What are the pros and cons of using a 4N channel vs a 2N and 2P channel bridge?

With the impulse and the low voltages involved, I am not sure there are any real pro's and con's... dealers choice. I've done both... my personal preference is 4N channel. I have found that I had to use diodes between the H-Bridge MOSFETs and the coil to insure there were not unwanted currents flowing back through the MOSFETS during coil current decay. In the case of the Impulse, they could be very low Vj Schottky diodes (like the ones steering the capacitor currents).

[green]

What is the objective

Question asked by KingJL reply #13. Been trying to answer with reference to trying the impulse. Are there targets that the impulse might be a better choice over a normal PI with fast decay? Been playing with spice and I have some thoughts, sometimes wrong. Want to order some parts, having an objective was a good idea. Any thoughts for an objective with an impulse?

[green]

What is the objective

Question asked by KingJL reply #13. Been trying to answer with reference to trying the impulse. Are there targets that the impulse might be a better choice over a normal PI with fast decay? Been playing with spice and I have some thoughts, sometimes wrong. Want to order some parts, having an objective was a good idea. Any thoughts for an objective with an impulse?

Some thoughts for the impulse. Reduced signal for low time constant targets so I'm thinking trying to detect high time constant targets only. Larger coil, maybe 16 to 20 inch diameter. Coil time constant greater than 2 maybe 3 times Tx time. Maybe 1000uH coil around one ohm. Around 300usec Tx time. H bridge with 12 to 18 volt supply. Does what I'm thinking make sense and are there other things I should consider? Think it's interesting that at the same time I'm ordering parts for a detector to detect high time constant targets only I'm ordering parts to try constant current damping again to try to get the sample delay down to around 3usec for very low time constant targets. Don't know if it makes any sense but it could be a challenge.

[KingJL]

Some thoughts for the impulse... Maybe 1000uH coil around one ohm. Around 300usec Tx time.

1 ohm might be a little tough with a 1000uH coil and a practical wire diameter (even 2 might be a challenge... should be able to hit about 3 ohm with 24 awg). Using the same timing methodology as the impulse would put the prt at 3000usec (333.33 Hz prf), if that suits your needs.

[green]

1 ohm might be a little tough with a 1000uH coil and a practical wire diameter (even 2 might be a challenge... should be able to hit about 3 ohm with 24 awg). Using the same timing methodology as the impulse would put the prt at 3000usec (333.33 Hz prf), if that suits your needs.

Was thinking of trying some AWG19 magnet wire I have for Tx coil. Don't have a need, just trying to learn something and have some fun with the impulse circuit. Would be nice to end with something worth building. Suggestions appreciated if what I'm thinking doesn't make sense.

[green]

Playing with minimum delay again. Trying to understand why ferrite beads cause a signal at low delays. Hadn't noticed a signal at longer delays until his morning. Charted signal decay for California ground and some ferrite beads in a plastic bag. To get a positive signal decaying to zero, beads needed to be added to the opposite Rx coil as ground and all other targets I've tried. Both charted a similar log-log decay. Tried a large ferrite bead, decay similar to the bag of smaller beads(orientation seemed to effect decay slope). Why do ferrite beads have a signal and decay with opposite polarity of other targets after Tx off? Slope change below 0v could be measurement error or not.

chart time scale usec

I've seen in other threads suggesting using a bed of ferrite beads to simulate ground. If beads decay from an opposite polarity is that a problem? Am I missing something?