Out of the box

[Anonymous]

Ok another out of the box thought and this one is really out of the box.
I am pretty sure I read something about this on the forum way back, but don

 

[Anonymous]

My guess would be because of audio feedback from the
transmit & recieve from the circuit board.

 

[Anonymous]

Ray,
Let's assume that we are using cable that is 20pf per foot. If the control box is mounted on the shaft of your detector, the bottom of the control box is 30" from the coil. Now let's assume that we can mount the MOSFET driver in a small box 6" above the coil or 2 feet below the control nox. That means that you are reducing the capacitance in the cable by 2 X 20 pf or 40 pf. Compared to the other capacitance in the coil, coil shield, internal components including the MOSFET driver which has several hundred pf equivalent capacitance, the 40 pf savings may only offer 1uS or less shortning of a delay. When doing optimization look at the parameters where larger gains can be made first!
Right now, looking at new MOSFET coil drivers with low capacitance seems to offer a much more practical solution than a remotly mounted coil driver. Granted, once you do everything else to reduce the delay, you may want to mount the MOSFET in or near the coil, if you don't create other problems.
I believe that Minelab has the right idea by mounting a preamp in the Sovereign receive coil. This technique may be a way to squeeze extra performance out of a DD PI receive coil with a gain of 20X or so then feeding a lower gain circuit in the PI control box to preserve bandwidth with quick delay turn-on speeds.
Your out of the box thinking is on the right track but think receive circuit processing in a DD coil first. But this is just my opinion and others may be able to shed more light on the subject or prove me to be wrong.
bbsailor

 

[Anonymous]

Hi Ray,
This is an idea that is very much in the box, as far as industrial detectors go. The picture shows a complete PI circuit board that is mounted just above a 3in coil. This completely eliminates the cable and its capacitance, enabling the unit to sample at 1uS delay. Obviously, many other aspects of the circuit have to be speeded up and special attention paid to the way the coil is wound.
For treasure hunting, such a short delay is impractical, as the numerous signals from almost invisible bits of metal would drive you mad. 5uS delay is useable and I have had a couple of prototype machines which I have used in the field. These, however, used a belt mounted control box with the normal 2.25 metres of cable, without too much difficulty in achieving the short delay.
Having the transmitter and front end amplifier half way down the shaft would help a bit, in that because of the lower cable capacitance, you may be able to add a turn or two to the coil.
I tried a IRF840LC (low gate capacitance) instead of the standard 840, and its performance was worse. However, it is the output capacitance that is more important.
Eric.

 

[Anonymous]

Hi BBSailor,
Running a Goldquest SS electronics right down at the coil, gains me 2uS i.e. I can sample at 8uS instead of 10uS. This is significant with very small targets, such as nuggets of 1gm or less. The damping resistors have to be increased in value as the coil resonance is at a higher frequency. The coil, in fact resonates at 250kHz without any cable and is 370uH inductance. The coil is fully shielded, so its total self capacitance is round about 1000pf. The cable adds another 200pf and the Mosfet 50pf, which drops the frequency to 234kHz, so getting rid of the cable has a significant effect.
Eric.

 

[Anonymous]

Eric,
Now that I am retired, I set up an electronics lab in my basement where I am tinkering with PI coils, taking measurements of self resonant frequencies and looking at ways to optimize PI coil performance.
1. I just picked up some 12 strand 36 gauge served litz wire to experiment on some new coils. Granted, I should have gotten 42 gauge strands to be efficient at the 250Khz self resonant range, but this is all I could find cheaply. Any thoughts on how this 12/36 litz wire might perform? This 12/36 litz wire is equivalent to AWG 25 wire at 34.56 ohms per 1000 ft. This gauge seems to be in the approximate range for a Hammerhead PI coil.
1. I read in one of your previous posts that you use twin coax that is 70pf/meter and is 6.9mm diameter. You must use about 2.5 meters of cable on your coils to shave 200 pf off the total capacitance. Please confirm you cable length?
I am using a 5 foot piece of Archer mega cable 6 mm diameter, OFCA/V Cable, catalog number 278-1271. This 5 foot piece measures out at about 100 pf.
2. You must be using a new breed of MOSFET coil driver with low output capacitance? Please let me know the part number so I can look up the data?
3. With your coils having an 11 inch diameter, you must be winding them with 21 or 22 turns of wire. I am using a Hays Electronics 11" coil form and must wind my coils 10.6" mean diameter to make them fit. My coils are 18 turns at about 300 uH. I spiral wrap them and have just started to use something to shield them that I found for free by accident.
I ordered a sandwich at a local fast food market and it came wrapped in thin aluminum foil lined paper. I asked for a few extra clean pieces (about 16" X 16") to shield my new PI coils. I plan on using a 1/4" inch wide by 2 inch long piece of conductive adhesive copper foil tape to attach the aluminum coil shield. My next experiment is to test the conductivity of that copper foil attachment before shielding the coil. This aluminum foil is about as thin as the foil on a gum wrapper. How do you think this will work as a shield?
I will need use two strips of foil 1" by 16" long (limited by size of paper wrapper) and attach two pieces, one going in each direction, at the cable connection with 2" of copper tape and leave the gap on the other end of the coil. I think it is best to put the foil side inside and secure it to the spiral wrap with vinal tape.
4. One final question. What is the value of the damping resistor at the 234 KHz resonant frequency and at the higher 250 KHz? I started to make a chart with a series of coils each having one less turn. It seems that the optimum damping resistance goes up between 20 and 30 ohms per turn removed from the coil. Is that your experience? Granted, this will vary depending on the coil wire insulation thickness.
It's good to have a place to get a little feedback about PI coil matters.
Thanks
bbsailor

 

[Anonymous]

Hi bbsailor,
If the foil turns out to be too conductive, you could try what I'm doing for shielding. I get two pieces of cable from my scrap bin, just a bit longer than the circumference of the coil. One is alarm wire and the other co-axial.
I split the sheaths of both of these cables and remove the wires. The alarm wire sheath fits snugly over the coil windings, and the co-axial sheath fits snugly over the alarm wire containing the coil.
I then half overlap wrap the whole lot in spiral fashion with 3/4 inch paper masking tape. I then wrap two turns of unsheathed 0.25mm kynar around the coil near where the wires emerge and twist off to secure, then the kynar is connected to the coil wire I've selected to be the gnd return. I then wrap a 1/4 inch ring of tape around the coil to provide a shield gap.
The whole lot is now spayed with nickel-loaded paint, giving special attention to the kynar wrap.
After the paint is dry remove the 1/4 inch ring to provide the gap. Because there is a contiguous length of painted tape leading away from the kynar, the coil can be bent a little without affecting the resistance. Thew weak point is where the co-axial sheath joins, a good thickness of tape is required here.
Since I make DD's mostly, I make one shield gap at alternate ends of the coils which correspond to the crossover points, this aids in electrical insulation between the two coils.
Initially I was spraying the insides of my coil shells, but this really required a lot of paint, where this new method requires very little. It is surprising how little is needed to provide sufficient shielding.
I have found that when I coated the shells, I have points on the base of the shell along the line where epoxy and shell meet where small amounts of paint are exposed to the outside world, and hence continuity is provided when contacting objects.
The shield works fine until I put the coil down on wet beach or immerse it in water, it does though stabilise in time and tune to the new conditions but is somewhat annoying. I think it may have a slight advantage under water though? I've yet to confirm this.
Happy experimenting bb
Kev.

 

[Anonymous]

Hi BBSailor
It is amazing what development goes on in basements and attics/lofts. I

 

[Anonymous]

Thanks for the time you spend in reviewing my comments and responding. I feel priviliged to be be able to have a dialog with you.
I have picked up a piece of test gear that has been very helpful in my PI coil rearch. It is an old (1960s vintage) tube-type Tektronix 130 L-C Meter which measures reactance of coils and capacitors. It works at a frequency range of 125KHz to 140KHz which provides more accurate readings than those new type LCR digital meters that work at 1KHz. I also pickup, on ebay, some standard inductors and capacitors to calibrate the Tek 130. It's measurements are very close to the ideal calculated values +/- 5% (from published specification).
Maybe it is more accurate than new digital LCR machines because the readings are being made at a frequency range that is closer to the critical operating frequencies of the PI coil self resonance oscillations. It has a limitation in that the maximum size of inductor is 300uH, but that can be scaled by placing an accurate 300uH inductor in parallel with an unknown value above 300uH and use the traditional parallel inductor/resistor formula to calculate the unknown test inductor value.
These Tek 130s seem to be available on ebay from time to time. This is a good machine for anyone who is serious about making PI coils and measuring coax capacitance.
Please do a simple experiment for me, based on your recommdation (in you previous responde). Take a piece of foil gum wrapper layed-out flat, and see at what delay point you can detect it. This will be very helpful for me and all those who are tweeking their homebrew machines to achieve an optimum delay value.
Thanks for your help and have a happy Holiday Season.
bbsailor

 

[Anonymous]

Hi BBSailor,
I like tube gear. I had a big collection of tube radios till my last move, when I sold most of them on Ebay. Had a couple of those large Tektronix scopes too, that kept the room warm like a fan heater. I still have a lot of tubes and enjoy making tube audio amplifiers by way of a change from metal detectors. The first experiments with PI, way back in 1956, used tube amplifiers for the receiver and a relay for the gating. The big problem was the transmitter, and to simplify things, the coil switching was done manually with "an iron clad mains switch". It was the army's way of doing things <img src="/metal/html/biggrin.gif" border=0 width=15 height=15 alt=""> At least it showed that the principle worked.
I'm not a gum chewer, but my son is coming over from Ireland on Sunday for a visit. I think he indulges, so I will get a wrapper from him and pass it over my 1uS PI unit. By looking at the receiver output, I can look at fast decay curves that can't be observed on a standard PI. I'll post a picture of the signal on this forum some time next week.
Have a good holiday too, and all others who visit this forum and make it one of the best technically oriented detector forums on the Internet.
Eric.

 

[Anonymous]

I noticed the plate of metal under the circuit board
why is this not interfering with the coil operation being that the metal so close to the coil? Could you explain to us in layman's terms?
appreciate it
Buzz(ID)

 

[Anonymous]

Kev,
Thanks for the information. It sounds like you have a pretty good way to reduce the capacitance of the shield to the coil.
What is the inductance, self resonant frequency (including coax lead) and value of the damping resistor on your coil?
When you measure the self resonant frequency using a scope, make sure you use the 10X range on the probe as it adds less capacitance load and does affect the self resonant frequency. My latest coil is 9.25" dia., 25 turns, 380 uH, 2.6 ohms, using 12/36 (25 AWG equivalent)litz wire and has a 566 KHz self resonance on probe 1X and 753 KHz with the scope probe on 10X. This measurement was made directly at the coil with no coax cable or coil shield.
I am using some 1/4" of spiral wrap which just fits snugly around the 25 turns of 12/36 litz. I used waxed cord (like dental floss) to secure the wire bundle each 1/2 inch before removing it from the coil form made of L shaped pins mounted on a board in a circle.
The key to making fast coils, from everything I have read on this forum and experimented with, is to take an inventory or all the things that add capacitance to the coil circuit and try to minimize them: the coil itself, the shield, coax wire, MOSFET output capacitance, other circuit components. One big factor is a fundamental design strategy. Are you using high power coil with low resistance or a low power coil with a higher resistance and series resistor to minimize the main spike to allow faster sampling?
The optimization process starts with those things you can control the easiest. Look at the things that are already fixed or close to optimized and look elsewhere for the larger gains.
bbsailor

 

[Anonymous]

Hi Buzz,
Just above the coil is a piece of blank copper pcb. This acts as a shield between the coil housing and the electronics. The coil housing is coated with nickel paint and grounded by the lead foil "straps". The coil does not see the copper pcb because the pulsed magnetic field only comes out on the underside. How is that done? Ah well, I'll let you do some head scratching.
Eric.

 

[Anonymous]

[No message]

 

[Anonymous]

Hi bbsailor,
Sorry for delay, I've been away. I'm using a GQ so I try to keep the parameters similar to the stock coil. This also requires a low power coil with high PRR as the FET has a low max voltage (low input C to boot) 300 to 350uH, 6 Ohms, 39 Ohm series resistor, and the damper varies, around 15k Ohm for TX (PCB has trimmer that is set for stock mono) and around 850 Ohm for RX (I usually use a couple in parallel to achieve accuracy)
I use waxed dental floss for initial binding too, I find it works very well.
I'm not sure what the self-resonant frequency is. I have access to all sorts of good gear 20GHz scope, TDR, SA etc., but no LCR bridge in sight.
May I ask how you are setting up your resonance measurements bb?
Are you hoping to adjust the resonant frequency to enable maximum power transfer from target to coil?
I've often wondered about how best to achieve this.
I see you're using Litz. It's interesting that Candy used this initially to minimise the coils ability to see itself, and with DDs mutual coupling. Do you find that it's low resistance increases the decay time constant as it does the rise, and thereby cancel out any advantage gained by the low self-inductance?
Interesting posts bb.
Cheers
Kev.

 

[Anonymous]

Kev,
To measure coil self resonance, use a variable frequency signal generator capable of generating output in the range of 300KHz to 1 MHhz. Attach the generator to the coil through a 10K to 100K ohm resistor. This isolates the coil from the low impedance of the signal generator and allows the coil to self resonate easily. Attach scope leads directly across the coil (after the resistor) and sweep the frequency between 300 Khz and 1 Mhz and look for the coil signal scope display to rise quickly at the self resonant frequency. Adjust the generator output level and scope level to see a good range of signal on the face of the scope. The resonant point is very distinct and cannot be missed. At the very highest voltage level (the peak point) indicated on the scope, read the frequency on the signal generator and that is the self resonant frequency of the coil.
Just to give you an idea how sensitive the coil is to capacitance, switch the scope probe between 1X and 10X positions and you will get two resonant frequencies about 100Khz or more apart. This is due to the difference in capacitance imposed on the resonant circuit by the scope probe. Some probes impose 90 pf more on the 1X probe switch position compared to the 10X probe switch position.
The main point in doing these tests are to have some quantative results with which to compare different homemade coils. Does using thicker insulation wire give me less coil capacitance? Does shortening my coax lead make a difference? Does spacing the shield from the coil have any effect? Does looking for that 17pf/ft coax make a difference? Well, now you can wind two identical coils, change shield spacing, use different coax and see for yourself.
When you start playing with coils and using coil calculators, you will find that as the turns are spread out due to using thicker insulation, the inductance drops some. Thus, two coils with exactly the same number of turns and being the same diameter will have slightly different inductances but a lot more difference in capacitance.
Knowing the self resonance frequency allows you to calculate the effective distributed capacitance in a coil. Measure the coil self resonance by itself (no cable) and again with the coax cable and you will see the impact of the cable on the self resonant point being lower with the coax attached.
Here are some key parameters to measure and keep notes about for comparison.
Wire gauge used:
Wire type: Magnet wire, Litz, Stranded hookup, etc
Number of turns:
Coil diameter:
Coil Inductance Calaculated (from turns and diameter):
Coil Inductance Measured:
Coil Self Resonance: (no cable, no shield)
Coil Self Resonance with shield:
Coil Self Resonance with shield and coax:
A scope, 2MHz signal/sweep generator with frequency counter and a LCR meter are primary tools for playing with coils and capturing the critical parameters for each of your coils.
After a while, changing some of the parameters become intuitive after seeing the results from many measurements.
bbsailor

 

[Anonymous]

[No message]

 

[Anonymous]

Hi BB,
Good to see some detailed work being done on coils. Your method of measuring the resonant frequency, is exactly the method I use; and I am still puzzled by some of the results. For example a shielded 8in Goldquest coil wound with the equivalent gauge Litz wire shows a resonant frequency 150kHz lower that the normal wire. This is just the coil and shield with no cable attached. The normal wire shows an inductance lower by 20uH so that may account for some of the difference. Certainly, the Litz is a smaller bundle cross section, which puts the inductance up a bit, probably winding capacitance too. In spite of the lower fr, the Litz coil can be sampled earlier, due to elimination of wire cross sectional eddy currents.
Two other things that make a difference to the winding capacitance. One is the insulating material (dielectric properties) on the wire. i.e. PVC insulation is different to Kynar. The other is potting material. I am very careful not to let any potting resin creep into the coil windings, as this can have a dramatic effect on coil capacitance.
Eric.

 

[Anonymous]

Eric
According to the litz wire vendors, thinner gauge strands in the AWG 40 to 42 range work better at the frequencies of about 500Khz. I'll bet that using litz wire strand sizes not in the recommended frequency range produces less optimum results. One quick way to test that theory would be to wind two identical coils, one with the strand gauge appropriate for the resonant frequency and one somewhat thicker. The litz wire that I use, 12 strands of 36 gauge, is most appropriate for 50 to 100 Khz and is not optimum for the self resonances that I obtain.
Here is the data on one of my latest 9 inch diameter coils, no shield, no coax wound with 25 turns of 12/36 served litz wire: 750 KHz self-resonance and measures out at 390 uH suspended in free space. My litz wire has the kind of insulation that melts with the heat of the soldering iron, so getting a good connection is easy. Ensure that all your strands are connected or that could account for the lower resonance.
If you want, I'll quickly wind a coil with my litz or Kynar 30 AWG to a diameter and number of turns that you specify, and let you know the results to help you unravel the mystery.
bbsailor

 

[Anonymous]

Hi,
Another factor not yet mentioned, is supply voltage. In/Out capacitance of many FETs can be dramtically reduced with increasing supply voltage (i.e. Drain/Source. However this in turn could increase the decay curve time in other quarters.
Talk about a balancing act.... Taking Reg's advice and increasing my GQ supply from 9.8 to 12 volts by the use of 10 x 2.3Amp NiMH cells has made my unit a lot more sensitive and responsive, also I'm now able to run my DD at 8.5us with a stable threshold. The supply increase of 2.4 volts will have reduced the FETs capacitance by up to 25%, so this may have contributed to the improved coil performance.
BTW for anyone else contemplating this, the pot that sets the low battery warning should be adjusted if changing your unit from 8 to 10 cells, otherwise the batteries could be discharged beyond the cells minimum end point voltage, a sure way to stuff em.
Cheers
Kev.