Big Coil Problems on a PI


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Hi Reg,

sorry for using the wrong word. What I meant is CLAMPING diodes, with a C, yes am refering to the limiting diodes before the preamp. The pics are at the exit of the preamp, non inverting. Unfortunately I can not read the numbers on the glass diodes, most of my parts are some 20 years old. The forward voltage of the pair of diodes that give the good result is about 0.35 V or halve that of the other pair, that has about 0.7 V.

This is why I thought about trying Schottky diodes that have a low forward voltage drop.

There is also something called an ideal diode on the market that gives a forward voltage drop of 20 mV. I wonder if that would work? Since we measure the signal when it is very near 0V.

The coil resistance is about 2 Ohm, I added two resistors of 5 Ohm each to reduce the L/R time.

I have one more curious thing happening. When I conect the shield of the cable to ground, it reduces the overshoot. Could this possibly mean that the overshoot is due to some external noise? Btw, the coil's self resonant frequency is around 750 Khz with the cable, but the coil is not shielded and pottet yet.

ELDORADO.VE

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REG, I just tried what you said, on a Wallaby DD coil, first I just passed a nicle over the coil, it was load and would pick it up in an air test, out about 13 inches, Then I taped a pull tab off a soda can to one side, the left side, and it would sill pick up the nickel, but not as loud or deep, then I tape one on the other side of the coil, leaving the first one on also, the threshold smoothed out a lot, and then I passed the nickel back over the coil, I couldn't beleive the increase in depth, it blew me away, and the threshold was a lot smoother than normal. Now in laymans terms, tell me why. Looks like I might get deeper and smoother by taping two pull tabs to the coil, or am I nuts. Grubstake

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Hi Eldorado,

Ok, the two input clamping diodes probably could be about any diode, but the 1N4148 has proven itself to be fast and effective.

I have not tried a schottky but probably should some time in the future just to see what happens.

BTW, the most common preamp is the NE 5534. This particular IC has clamp diodes built in, but it is a good idea to use external ones also.

Personally, I have not tried many diodes so I can't say which is the ideal diode. I am working with sampling well below 10 usec so I haven't found a need to try to find anything better as of yet.

Now, I am not sure just what you are saying when you say you tie the cable shield to ground reduces the overshoot?

Normally, the cable used for a mono coil is a coax and the shield has to be connected because it is used as the ground conductor. Are you using different type of wire for the coil cable?

If you are using a two conductor shielded wire, then that wire is probably producing some extra capacitance that effects the coil's operation. Grounding the shield will cause that capacitance to change for sure as well as alter the noise characteristics. Now, if you have any other coils in the same room when testing, then the lack of shielding will allow those coils to have a much greater negative effect. In fact, everything will have a much greater negative effect.

So, I do not want to speculate any more at this time just what specifically is happening without running the experiment myself. Guessing can be bad at times because people have a tendency to assume it to be true all the time, and, unfortunately, that isn't the case.

PI's appear to be simple, but in reality, they are very complex.

I have not heard of the "ideal diode" Guess I should look for more info. Thanks,

Reg

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Hi Grubstake,

Glad you are experimenting and having fun. Strange just what happens when slight adjustments are made, huh.

Now, as I said before, guessing can be bad but unforunately because ML releases little or no info, so guessing is about all we can do.

Now, my best guess as to what is happening is this, when you place one tab, you are passing by the maximum zone and this causes an amp to saturate. Now, when you add the second one on the other side, there is some "balancing' on the transmit as well as the receive and this places things right on the edge of working. At this point, things are really quiet and if it is right on the edge, then any additional change causes the shift into the operating zone.

Now, on my PI's that slight shift that causes it to just shift back into the operating zone will also cause the coil to appear to be hypersensitive and this appears to be happening on your ML also.

My guess is if you experiment with real small nuggets, you might find that they do not display the same intense sensitivity. Some may, my most likely there will be some small ones that will be very hard to detect.

About the only exception I can think of is that you just happend to get real lucky and hit that ideal positioning spot where everything is right on the edge.

You might experiment with small pieces of gold and see how they react.

Reg

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REG, I tried a one Gram nugget, and it is very sens. to it, much more than just without the pulltabs. I'm still playing with this, but when I did it with the nugget, it has a really sweet spot right up at the front nose of the coil. And by the way my detector is a 2200sd. Thanks grubstake

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I have absolutely nothing to add to this thread because I only understand about 10% of it. OK I lied 7%.

I did get that part about the nickel, that is the coin that is worth more than a penny because it's bigger, but not worth as much as a dime, because the nickel is still bigger ???? OK, apparently I don't understand how that nickel thing works either.

Seriously when Reg, Eric Foster, and Capricorn Treasure get together it amazes me at the amount of knowlegde that these three men possess.

What also amazes me is their willingness to give so freely of their information to us guys.

On the down side, you guys really make me feel stupid. Make that stupider. :blink:

For me these posts truly give meaning to the saying "Ignorance is Bliss." I am very happy just knowing that when I hear that detector go WEEWOO, I should just dig until I find the cause. Knowing too much about why it goes WEEWOO gives me migraines and diarrhea. :huh:

Wonderful information! This forum and its readers are very fortunate to have you three as contributors.

BCOT!

DOC

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You totally right DOc, I still don't know why I get a better signal on the coil with the pull tabs on it, but it works. I figured it would cancel out everything. Baffling to me. I guess since they are stationary, you don't get a signal from the pull tabs. Grubstake

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Hi Grubstake,

I can understand why you might get a better signal to a point. What I don't know is exactly what is going on in the ML preamp.

I can also understand that placing a pull tab on the coil will change things and not be detected. However, if that tab moves it will cause a response. Just what type of response is determined by where it is located.

I sent you a pic of ML DD coil windings some time back and all DD type coils are generally built the same, so you have a good idea of just where the tab is setting. Now, on a basic PI, any metallic object that passes under either one of the two windings and not under the overlap zone will generate a signal that is opposite of the signal generated under the overlap zone.

Now, I have made a crude drawing of the windings of the DD coil and attached that pic to this post. When you look at the pic, you can see the long narrow strip down the center that is known as the overlap zone. This is the main detection area. Any other part of either oval is the remaining part of the winding and it is that area that will normally produce the opposite signal. This opposite signal is generally much weaker and may be difficult to hear, but it is this opposite effect that cancels part of the ground signal. So, if one thinks about it, the ground signal is very strong under the overlap zone and weak under the remaining part of each winding. It is also of opposite polarity of the signal under the overlap zone, so it effectively cancels out part of the overlap zone signal. This is why a DD coil is not as ground sensitive.

In very basic terms, this is why it is quieter in hot ground.

The DD coil will mainly detect with a positive signal when the object is directly under the overlap zone, so when that same object is under one of the windings only, then the signal should be a negative signal.

Now, if that object is stationary, that target signal will "tune out" so to speak. Because of the design of the electronic circuitry, the threshold signal will go back to its normal level, just like the object wasn't there. This will happen when the object is right under the overlap zone also if the object is small enough and made of the right material. If the signal is too strong, then it can cause things to not work right at all or not tune out if the object remains.

Ok, you mentioned that there is a "sweetspot" at the nose of the coil. I suspect that if you test carefully, you will find another spot at the heal of the coil also. Once again, if you look at the pic of a DD coil you will see the particular spot where the two windings overlap and just before that, the windings become closer. Things will get more sensitive at or very near that point just due to the nature of the design.

I hope this makes sense.

Reg

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Hi Grubstake,

Due to the extreme sensitivity required the signal from any PI would be too great if the circuitry were to be straight forward and direct. The slightest drift would cause dramatic voltage changes that could cause amplifiers to not work.

Now, to eliminate the slow drift problem, at some point, the signals are a/c coupled. This blocks the slow dc drift signal and sort of resets everything back to 0V. Ok, now when you place a target such as a tab on the coil and then tape it in place, that signal rises dramatically and remains in part of the circuit.

However, when that signal gets to the a/c coupling, any stationary or constant signal will now simply go away, just like the slow drift signal. This is why the coil has to be in motion for an object to be detected. It is also why the tabs that are taped on the coil do not cause an audio response.

The same a/c coupling technique is used on VLF's and is commonly called the SAT, the autotune, or simply the tune function. By changing just how fast the signal can retune can minimize the varying ground signal also. The down side of a fast autotune is some depth loss.

Now, one other little bit of wisdom is, the stationary signal from the tab is still there on one side of the a/c coupled stage but not on the other. So, the signal is still there, but not heard. Now, that same stationary signal is also causing other things to happen to the amplifiers before the a/c stage. If that particular tab signal is such that it causes an amplifer to almost saturate, then the machine will become much quieter and the threshold will become very smooth. Since the signal is opposite a normal target signal, then this smoothness is such that it will not cause a target signal to be eliminated.

If we think in terms of a basic teeter totter, maybe it will make more sense. Normally, the teeter totter is balanced so it can move easily either direction. At this point, any extremely small signal will cause a slight bounce one direction or the other. Now, if you place a target in one of or in both of the windings, then that same teeter totter may drop down on one side and rest on the ground. At that point, little signal or bounce can occur and can only occur in one direction. Fortunately, the signal can bounce upward by a target inside the detection zone, so suddenly a target creates a response. In other words, it is on the opposite side of the teeter totter and any target signal will cause the teeter totter to bounce the opposite direction.

Now, what is difficult to explain is just at that point where the signal can be detected, it is hypersensitive. The reason is too complex to try to explain in simple terms, but it has to do with just how the signal is created to begin with and what all is happening to the particular signal.

On a teeter totter, one can add a little weight to the opposite side and cause one side to drop a little. Now, you can also shift the pivot point and get the same effect. However, when you shift the pivot point, you will see that it takes less weight to create the same shift.

Ok, at the hypersensitive point on a coil, you are basically getting both a weight shift and a position shift and this combination is what causes the increase in sensitivity.

Sorry, but that is about as good as I can expain it and still make some sense. What happens is really more complicated, but the basics are similar to what I explained.

Reg

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DOC,

I agree with you, except I only understood .001% of what they are talking about :blink: . As with other things in our vast universe I am very glad there are folks in this world that understand these things & make them work. Because if it isn't on fire or laying there dying, I am of very little use to contribute anything other then take up space and look good :lol: .

REG - With all that said, if your back in Colo. wanna meet to beep for a day again? Thanks for all the tech. info. even though it went completly over my head by about ten miles.

Keep on beeping B) - Frosty

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DOC and Frosty,

I'm slack-jawed as well at the technical sophistocation on display in this thread.

I have read the posts and intuitively feel there is something important being discussed.

I reread the posts and get completely lost...my usual state.

The most confusing post however,

has been DOC's musing's about the nickle,

that one really put me away.

Seriously, I hope all of you keep going (you too DOC).

It looks like you are on to something that may ultimately,

benefit the rest of us.

An interesting and provocative line of thought.

thanks and regards,

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REG, I'm going out detecting on SAT with Shep, I will take my tape and pulltabs with me, and give it a real life test in the field. I will post on it here SAT night, and see if Shep will jump in also. I thank you for explaining in terms I can understand, when I was in grade school I got an A in teeter, but was slow in totter. Thanks Grubstake

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Hi Grubstake,

You might play around with positioning the tabs at different places to see just what happens. My guess is there will be places that are better than others.

Now, this type of experimenting does have merit and can enhance signals, as you have found out. So, one shouldn't be afraid of trying different things. As long as everything is done on the outside, nothing can be harmed.

Placing tabs or other metallic items on the outside of the coil does cause the coil signals to shift and remain shifted until the object is removed. Now this shifting can either help or hurt the sensitivity dependng upon how much is shifted and what direction.

Ok, here is something just posted on a different forum by Eric on how to compensate for strange signals or overshoot type signals.

"Try this little trick. Get some brass washers of different sizes, M2 - M5 range, and lay one M5 horizontally next to the coil. You will see the overshoot lessen somewhat. Try two washers and note the effect. By selecting a washer with the same eddy current decay as the overshoot, you can virtually cancel it out, and end up with a flat response. You may need two or three washers of the right size to get sufficient amplitude. Once you have got the response right, stick them down with epoxy, so they can't move relative to the coil."

Here is the link to the info" http://thunting.com/geotech/forums/showthread.php?t=11793

Now, on a mono coil, this is about all that can be done to correct a signal response.

Now, I have attached 3 pics, one that was posted earlier in this thread, one that was posted on the Geotech link just mentioned and a third is a simulation of a target signal. Without knowledge of what is happening, they will probably not make any sense. So, I will try to explain the important part in this discussion.

When you look at the pics, all will have a signal that drops down suddenly to a lower value and stays there for a while and then rises to a certain level or is already down and then ramps back up to a certain level. Now, the one posted in this thread earlier by Eldorado is the first pic. It has been flipped over so it looks more like the other two. This first pic is the normal signal. In this one, the signal rises fairly quickly and levels off to a horizontal line. Again, this is what a normal signal would look like.

Now, on the second pic, which is a simulation, the signal starts low and rises slowly up to the level off point. This signal is what you can have if you take the tabs you mentioned and add them to a normal signal. So, if you add the tabs to pic one then you would get something like pic two if you are using a mono coil. So, on a target signal, a target will cause the signal to rise slowly. The bigger the target, the longer it takes to rise. Also, the type of metal will determine how long it takes also.

Again, only worry about what happens as the signal rises from its lower point. Now, on the third pic, you will see the the signal doesn't stop rising but overshoots and then has to ramp back down to the typical level off point. This is the type of signal Eric was talking about when he said one could use washers to compensate for the signal.

If the ramp overshoot signal is great enough, then the detector can quit working properly and signals become muted. A threshold signal will suddenly become very smooth. Again, this is the only type of signal that can be compensated for on a mono coil.

DD coils are a little different and strange things can happen because one can take a normal signal and it can be "distorted" to look like either pic two or pic three, depending upon where the tabs are placed.

If we start out with an abnormal pic like pic 2 and we add the tabs in the area outside the main overlap detection zone, we will create a signal that looks like pic 1.

Ok, so if we just happen to have a signal that looks like pic 3 on a DD coil and we add the tabs, then the tabs will sort of "kill" the overshoot and the signal will suddenly look more like pic one, if the tabs are placed in the main detection zone.

Ok, to complicate things even more, lets say we start out with a pic that looks like pic 2 and we add the tabs to the zones outside the main overlap area, then we can create a signal that looks like pic 1, or make a normal signal out of a distorted one.

So, what does this all mean? Well, when coils are made and detectors are made, it is difficult to get a perfect match and end up with a signal that looks like pic 1, the normal signal. Depending upon just what is going on, the signal may end up looking like pic 2 or pic 3. Now, if the distortion isn't bad enough, the coil will still work and the problem will not be obvious. However, at times there can be changes that may cause a marginal coil to quit working simply because of continuous use. Let the detector and coil cool down and they may work right again.

Also, a DD coil is easier to modify to compensate for any improper signal. So, one can "cheat", so to speak to take a marginal coil and make it work.

With a DD coil, we can actually make a coil work that isn't working as long as the problem is minimal. We can make a noisy coil muted by driving the signal into a level that will cause one of the amps to go to its limit, or we can cause a coil to not work at all.

Once again, any external compensation such as using tabs will not harm anything and can be fun to play with.

Reg

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Yep REG, SAT when Shep and I go out, I will take a roll of tape and the pull tabs. I want him to be witnesss to this deal. I think its amazing how it works, and will play with it for a bit, and see if I can get it Maximized. Thanks your a smart man, and full of information, just like my old bud Jim Striaght. Grubstake

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Hi Reg,

I use the 5534, in the pix shown, in noninverting mode. The cable is double shielded audio 2 conductor cable (Belden) 17 pf capacitance.

I work in a very noisy environment with a 60Hz, 13800 V groundfault noise that is all penetrating. Do you have any advice how to filter this 60Hz noise out?

The "Ideal diode" from Linear, LTC4412 is supposed to have a forward voltage drop of 20mV. The way I understand it, using it instead of the other clamping diodes, it would reduce the saturation of the opamp, created by amplifying the 0.7 V forward drop of the diode many times. It also would help in the damping of the coil. However, possibly it will be so noisy that it can not be used for our purpose.

ELDORADO.VE

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Hi Eldorado,

I looked at the info for the diode you mentioned but haven't studied it enough to make a decision. As I said, the 1N4148 seems to work very well. If everything else is designed properly then you should be able to sample at less than 10 usec.

Going back and looking at your pics, it appears that you can sample at less than 10 usec now. That is good. Not many people can get the response to flatline on the output of the preamp at less than 10 usec.

Now, as for your 60 hz problem, it is difficult to minimize this noise if it is high voltage spikes or they are involved in any way.

What are you doing with the rest of the circuit? Do you have a differential amp where you sample early for the main signal and then later to reduce the earth field effect? If not, you might consider adding that feature. You can take a later sample with the same gain as the main one, invert the later sample signal and sum them. This should minimize much of the low frequency noise. The high frequency spikes will still be a problem though.

Now it is not easy to eliminate the higher frequency noise since much of it falls in the range of the signals of targets. So, simple filtering doesn't work that well.

You could try to design a noise blanker circuit and try to "zap" any fast pulse. This can be tricky to do because of the fact that the sampling by nature acts much like pulse signals.

The problem of trying to just filter such noise with caps is generally, such noise is a much higher voltage spike, that when filtered becomes a slower very large hump type signal. The spike energy is converted from a short high voltage spike to a large wide soft spike type signal. The can lead to "warbling" type signal among other things.

I use a little different technique and that is to try to minimize the filtering to the point that the signal level is such that a very loud signal has a voltage greater than the fwd drop of a diode. Then you can limit the o/a with diodes. They will "clip" the pulse signals and using an active method. This will minimize the problem but not eliminate it.

Reg

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Hi Reg,

thanks for your help. My 60hz problem does not have spikes, just all pervading 60Hz noise. I guess if I find a way to control that, my detector will become immune to powerlines.

I have made some further tests with diodes. between the 4148, 1N5817 Schottky and a generic Germanium diode, the latter gives the best result by about 2 us.

Analysing this, I feel that possibly the reason is that I am using the diodes outside their parameters. These small signal type diodes have a forward current limit.

I use a 1k ohm, 3 Watt damping resistor across the coil and another same resistor before the diodes. Both resistors run hot.

So far I am quite happy with the coil, but will see what I get after shielding it.

This setup works fine with 6 to 7 us first sample delay with pulses from 1 to 6 amps, 14 to 140 us Tx, at 6 Amps obviously I need to beef up the damping resistors and limiting diodes.

One more thing I learned: Using the mosfet avalanche diode for clipping the flyback, introduces a lot of noise in the 10 megaherz range.

ELDORADO.VE

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Hi Eldorado,

Interesting about the germanium diode. How did the schottky do? What battery voltage are you using? With the total resistance of the coil and series resistors being 12 ohms, I suspect your pulse current is less than you think. The avalanch current my be higher and in the range you metion, but it is of short duration so a diode like the 1N4148 can survive.

Are you just taking a single sample each pulse or are you using a differential integrator? You mention a 'first sample' which would indicate a dual sampling unit. If you are using a differential integrator, I would think it would nearly eliminate the 60 hz and maybe the noise is from something else.

One more thing, what is the gain of your preamp? The gain will drop off severely at sampling of less than 10 usec. This can effect how the differential amp works if the gain is very high. In other words, the gains may not be the same when your initial sample is taken with a very short delay and a later sample taken with a very long delay.

A good shielding technique should knock a lot of the noise you are experiencing, especially if it is noise pulses. What type of shielding are you going to use? Generally a non shielded coil will exhibit quite a bit of noise when used in about any environment. There has been a lot of discussion about this and other related subjects over on the Geotech forum. Do you frequent that forum?

Finally, what winding technique did you use on your coil. To obtain the clean and short preamp decay curve you did would indicate very little capacitance. I am impressed.

Reg

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Hi Grubstake,

You are probably on your way by now, so good luck. It will be interesting to see how the "tab" technique works in the field.

Did I mention that a technique such as you are using is being used by some friends in OZ? Actually, the technique is a little different, but similar in certain ways. I can't go into the details because I am not sure they want it distributed.

Anyway, as I said, don't be afraid of trying different things. Who knows you might come up with something that works super.

Reg

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Hi Reg,

the Tx voltage is 12 volt. I have tested the tx between 14 and 140 us and the coil between 2 Ohm with no additional resistor and with +5 Ohm and + 10 Ohm resistor meaning between 2 and 12 Ohm.

the first sample falls between 5.5 to 7 us throughout these ranges at 33 gain.

Here is the tricky thing. This is the only gain that this setup works. Kind of a sweet spot.

Neither more nor less.

From this point I take 7 samples spread at regular distance.

Store in Sample and hold amplifyer where I can average the individual samples over n readings. The output of this amplifier is a solid and clean dc voltage that is then amplified in a differential amplifier, or rather several differential amplifiers.

the storage and amplification of the seven samples individually should give me the possibility (by pic micro) to pick one specific sample to match with another specific sample, with various gains etc.

Each sample pair is developed into a different audio autput. I believe the human ear has the capacity to differentiate a very wide variety of audio signals.

I hope this will lead to a higher degree of target ID and ground elimination.

The design is still in developing stages and I apreciate all the help I can get.

Writing the source code for the PIC 16F817A is one place where I really could use help.

I have not worked with microchips or programming before, but this is where the future lies.

However, first of all I have to build a real good analog front end that produces a clean good S/N signal before I start shuffling it around digitally.

ELDORADO.VE

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Hi Reg,

some answers to your questions:

The 60hz keeps creeping in as fast as I take it out, but this is on the open breadboard I will look for a way to shield the whole system in the final box.

The Schottky was the worst, did not expect that. Want to try to get hold of different Germanium diodes to try. After all, 2 us difference at 6 us is 30% difference.

The winding is complicated but seems to be good because I used only magnet wire, hope to use Litz when I can get hold of it.

have tried many shielding schemes with whatever stuff I can get hold of. Will shield this coil with Graphite paint after potting it with polyurethane foam. I would prefere glass microbaloons with epoxy but cant find the microbaloons.

Yes I visit the Geotech forum a lot and much apreciate your frequent and very helpful posts.

ELDORADO.VE

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Hi Eldorado,

Well, this is my second attempt at trying to respond to your information. Lightening ate the first. Fortunately, the only fatality was my microwave.

Ok, thanks for the update on your system. That gives me an idea of what you are doing. As for the PIC programming, well, I programmed them quite a while back when I built the Stuart PI, but haven't done anything with them since, so I am not much help in this area. I could probably do it but there certainly would be better techniques that could be used.

Your idea of multiple samples activating different audio signals sounds interesting. Hope it works out.

Now, as for your unit only working at a gain of 33, be careful, my guess is your unit is doing something a little strange and that is there is a slight overshoot in the main xmit decay curve that is sort of countering the delay lag in the opamp. The result is a nice curve that is limited to the specifics involved. Change anything much at all and things don't work well. This appears to be more than common at delays of less than 10 usec.

Running a quick simulation indicates the gain of 33 should work fine at the lower delays and not display the gain loss that shows up when higher gains are used. BTW, Eric used a neat design to determine there was a gain loss at the short delays when using the NE 5534 IC. This gain loss was something that I hadn't thought of. So, I am really glad that he pointed it out. You should be able to find the info on it on the Geotech forum.

Now, as for the 60 hz, once you sum an early sample with a later one at equal gain, you should minimize the low frequency noise, which should include the 60 hz as well as the earth field effect. Also, like you said, placing the pc board in a housing will help, especially if the housing metallic or at least has shielding over the critical areas. Also, once you shield the coil, you should see even more noise reduced. A lot of noise is picked up by the coil acting as an antenna.

This brings up an interesting point you should keep in mind. One thing really need on a PI is a means of adjusting the pulse repitition rate. This will allow the operator to make adjustments to minimize external noise. Normally, it doesn't take much of an adjustment. Without it such an adjustment, noise can be a real problem.

I am sure that ML's manual tune button is basically an adjustment of the repetition rate and that is why it works well, as mentioned by Rob recently.

As for finding microballoons, they are a great way to reduce the weight of any epoxy project. I have used them in coils and added enough that the epoxy barely poured. This really reduced the weight. You can search with Google and find several sources. In fact, I found one interesting. Here is a link to that site: http://www2.towerhobbies.com/cgi-bin/wti0001p?&I=LXHZ15

Another site is Eager plastics. Here is their link:

http://www.eagerplastics.com/7962.htm

As for coil fillers, I now use two part foam, rather than epoxy. It is extremely light weight and quite strong.

As for Litz wire, you might want to check out the Surplus Sales of Nebraska. They have great prices on their Litz. I prefer the LW 12/36, which is about a 26 awg size. I also prefer the served type. This helps reduce the capacitance. Here is a link to their site:

http://www.surplussales.com/Wire-Cable/LitzWire.html

One way to reduce the winding capacitance is to use Teflon coated stranded wire for the windings. I think you will find this wire will reduce the problem significantly when using the normal winding techniques. Personally, I have found it to work better than Litz for my purposes and have been able to build coils that operate well below 10 usec. You can get some great deals on Ebay if you are patient.

Good luck with your design.

Reg

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Hi! Reg, well Shep and I went SAT. for a few hours, but it was sooo trashy that I went for the Platypus, and did not get to try the pull tabs in the field. But maybe this weekend. When I do, I will post pictures and my findings. Grubstake

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Hi Reg,

thanks for the good information. Had some thunderstorms here also. The lightning blew up a brand new lightning conductor on the 13800 V line in front of the house. Maybe the lightning conductor was the source of the groundfault that was plaguing me.

You are right. the sweet spot at 33 gain was due to 2 opposing kinks cancelling each other.

I have looked at Geotech for Erics post on the gain loss but could not find it.

I have noticed with some surprise that with a first sample at 6 us and only 33 gain the deflection of the decay curve is very visible for a Nickle. I was used to gains of 1000. So what is happening?

For my developing work I have set up a very flexible platform that lets me vary the Tx from 14us to 140us and all other timing accordingly.

The coil works well with 12 V from 14us to 140us Tx ( got some smoke from the 3 Watt damping resistors when I pushed the Tx to 200us) and 2 to 12 ohm with only slight changes in the delay time and slight changes in the damping resistors.

This puts me in front of the decision now between high power or low power TX. What is your advice?

ELDORADO.VE

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