How to make the best coil.

[bbsailor]

300 ohms will have low capacitance (good) but without any shielding may be more susceptible to EMI.

Carl and forum members,

If we as a collective group could derive how to use 300 ohm TV cable or a similar cable to replace a typical coax with about 100 pf of capacitance, we could find a new way to make faster coils. The main challenges are as follows.

1. Minimizing EMI and other noise pickup using this type of 300 Ohm wire.
2. Allowing the coil metal shaft length to be adjusted with the wire inside.
3. Finding a creative way to bring this new design into the control box to connect to the circuit board.
4. Find a new way to bring the coil wire leads out of the shell and up the plastic stem to make the connection to the 300 Ohm wire where the plastic stem meets the metal adjustable shaft.
5. Find a creative way to shield the coil wires going up the plastic stem using Scotch 24 wire mesh.
6. Use low dielectric foam to suspend the wire within the metal shaft.

With all the creative minds on this forum, this could happen!

Joseph J. Rogowski

[Stevedbk]

300 ohm twin lead has a capacitance of around 16pf/M. Certainly faster. Even using a 20mm conductive shaft with the hot wire down the centre is 35pf.
Perhaps a fixed shaft with a centre wire then adjustable non metalic for the last 300mm or so made from rg58.
This is a useful capacitance calculator.
http://www.mogami.com/e/cad/electrical.html
Steve

[David_1]

we could find a new way to make faster coils. The main challenges are as follows.
Joseph J. Rogowski

what benefit will a faster coil give ?

[bbsailor]

what benefit will a faster coil give ?

David 1

When the TX current turns off the stimulation of any target starts. However, the oscillations that normally occur need to be damped to allow the RX circuit to turn on being able to detect the induced energy in the target before it fades out. Small targets require faster coils to detect their induced energy while it is still present in the target.

Joseph J. Rogowski

[detectormods]

Carl and forum members,

If we as a collective group could derive how to use 300 ohm TV cable or a similar cable to replace a typical coax with about 100 pf of capacitance, we could find a new way to make faster coils. The main challenges are as follows.

1. Minimizing EMI and other noise pickup using this type of 300 Ohm wire.
2. Allowing the coil metal shaft length to be adjusted with the wire inside.
3. Finding a creative way to bring this new design into the control box to connect to the circuit board.
4. Find a new way to bring the coil wire leads out of the shell and up the plastic stem to make the connection to the 300 Ohm wire where the plastic stem meets the metal adjustable shaft.
5. Find a creative way to shield the coil wires going up the plastic stem using Scotch 24 wire mesh.
6. Use low dielectric foam to suspend the wire within the metal shaft.

With all the creative minds on this forum, this could happen!

Joseph J. Rogowski

I experimented years ago using 300 ohm tv antenna lead, to get no interference pickup into a differential amplifier i had to come up with a different shield design to equalise the shield loading on the coil. The shielding has to be made as a half cover and capacitively overlapped to keep it balanced, this will not work on single ended input stages, also a low noise differential input has twice the noise figure so win some lose some.

[Carl-NC]

If we as a collective group could derive how to use 300 ohm TV cable or a similar cable to replace a typical coax with about 100 pf of capacitance, we could find a new way to make faster coils. The main challenges are as follows.

1. Minimizing EMI and other noise pickup using this type of 300 Ohm wire.
2. Allowing the coil metal shaft length to be adjusted with the wire inside.
3. Finding a creative way to bring this new design into the control box to connect to the circuit board.
4. Find a new way to bring the coil wire leads out of the shell and up the plastic stem to make the connection to the 300 Ohm wire where the plastic stem meets the metal adjustable shaft.
5. Find a creative way to shield the coil wires going up the plastic stem using Scotch 24 wire mesh.
6. Use low dielectric foam to suspend the wire within the metal shaft.

Personally, I'm not a big fan of running the cable inside the rod. I looked into this extensively at White's and it just ends up creating more problems than it solves. Have you ever tried to swap coils on an Explorer or GPZ?

On the 300Ω twin-lead, this looks like another "more trouble than it's worth" effort. There are lots of other things to consider for earlier sampling that will would be easier. Right now, the Impulse Gold design is sampling at 3us with non-Teflon wire, a coax coil cable, and a monopolar NMOS switch. No need for esoteric methods.

[bbsailor]

Personally, I'm not a big fan of running the cable inside the rod. I looked into this extensively at White's and it just ends up creating more problems than it solves. Have you ever tried to swap coils on an Explorer or GPZ?

On the 300Ω twin-lead, this looks like another "more trouble than it's worth" effort. There are lots of other things to consider for earlier sampling that will would be easier. Right now, the Impulse Gold design is sampling at 3us with non-Teflon wire, a coax coil cable, and a monopolar NMOS switch. No need for esoteric methods.

Carl,

Is there something unique about the monopolar NMOS switch that allows early sampling?

Can this NMOS switch be used in other common PI metal detectors?

Is there something unique about the Impulse Gold design coil that allows it to sample at 3us delay?

Thanks
Joseph J. Rogowski

[Teleno]

Right now, the Impulse Gold design is sampling at 3us with non-Teflon wire, a coax coil cable, and a monopolar NMOS switch. No need for esoteric methods.

Interesting.The way I achieved such short delays was: SiC MOSFET in series with SiC Schottky, spiral-basket coil of less than 200uH, almost direct sampling (14 x gain preamp).

Besides the careful selection of materials and components, is there any novel circuit configuration involved in the Impulse Gold? any relevant patents published?

[Carl-NC]

No, the Impulse Gold is largely a TDI/Goldscan on steroids. It uses an SiC NMOS that does not avalanche; a faster preamp opamp; careful coil construction & shielding. Mostly attention to detail, including the PCB layout.

An easy place to improve speed is in the resistor that feeds the preamp input clamp. Usually 1k, increase it to 2k or 3k. Makes recovery way faster. Drawback is more thermal noise but I don't find that to be a problem.

[Teleno]

Carl and forum members,

If we as a collective group could derive how to use 300 ohm TV cable or a similar cable to replace a typical coax with about 100 pf of capacitance, we could find a new way to make faster coils. The main challenges are as follows.

1. Minimizing EMI and other noise pickup using this type of 300 Ohm wire.
2. Allowing the coil metal shaft length to be adjusted with the wire inside.
3. Finding a creative way to bring this new design into the control box to connect to the circuit board.
4. Find a new way to bring the coil wire leads out of the shell and up the plastic stem to make the connection to the 300 Ohm wire where the plastic stem meets the metal adjustable shaft.
5. Find a creative way to shield the coil wires going up the plastic stem using Scotch 24 wire mesh.
6. Use low dielectric foam to suspend the wire within the metal shaft.

With all the creative minds on this forum, this could happen!

Joseph J. Rogowski

EMI protection is not necessary during the flyback transient. A switch can be provided that disconnects the coax shield during the flyback and leaves it floating, then reconnect to ground right before sampling. I'm thinking of a small signal, high voltage MOSFET that has a relatively low Rdson and low output capacitance, such as the IXTU01N100 / IXTY01N100 ( Rdson < 80, Coss 6.9 pF). Perhaps a depletion MOSFET like CPC3980

[moodz]

You cant escape the physics of the damping resistance and reduction of capacitance folows the law of diminshing returns ( ie effort and cost exponentially inverse to benefits ).

I had a utility patent on active damping that escapes the damping resistance decay curve but that has now expired. ( so I am happy to talk about it )

A big manufacturer "over patented" my idea but they made a big mistake ... they left a damping resistance in the circuit across the current switch ( a diode ).

So their patent is not optimal and invalid as it still uses a damping resistor.

By using nonlinear damping and a trans-impedance / current amplifier as the preamp the damping resistance is eliminated.

I show a constructed version ( not simulation ) in this post https://www.geotech1.com/forums/foru...560#post408560

This demonstrates the "speedup" in damping of an off the shelf commercial mono coil from 4 microseconds at best to damping in less than 1 microsecond.

No need for "fast coils" using this technique.

moodz.

[David_1]

does the damping resistor have to match the coil ? why is it not in with the coil end of the detectors ?

[Teleno]

does the damping resistor have to match the coil ? why is it not in with the coil end of the detectors ?

R = 1/2 * sqrt (L/C).
Different coils require a different R.
C includes all the parasitic capacitances in parallel with the coil (self capacitance, MOSFET, coax, etc).

[Carl-NC]

does the damping resistor have to match the coil ? why is it not in with the coil end of the detectors ?

Ideally, yes. Most detectors put the damping resistor in the main circuit because putting it in the coil means there is cable and MOSFET capacitance that is not properly damped, although it's probably not a big deal. In any case, for a given PI design with a certain coil inductance, different sized coils will need about the same value damping resistor, or at least close enough. If you wanted to make it better, you could put most of the damping resistor in the main circuit and then a higher-value tweak resistor inside the coil, if needed.

[waltr]

Yes, on the PI I built I put a larger value damping R on the PCB. Then another R is parallel to tune the coil attached in the coil's cable connector.
Each coil then are tuned (damped) optimally and I can simply change coils.
I have a 10" Mono, an 11" DD and an 8" Concentric that all work with the same PI circuit.
Each coils has it pro & cons and uses in different tasks.