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Thought For The Gurus
Searching for small low TC objects, like gold, requires low sampling delays to capture the target eddy currents before they fully discharge. Testing a coil for a self resonant frequency is a known way to determine the potential for a coil to operate at a low delay. The ML flat wound coil should have less capacitance than a bundle wound coil of the same number of turns. However, to achieve the same inductance a bundle wound coil would require less turns due the the increased coupling between the bundled turns. Adding a shield to the coil also adds capacitance due to the:
1. Distance of the shield from the coil wire
2. Area of the coupling
3. Dielectric effect of the spacer and wire insulation
If anyone makes one of these coils, try to measure the coil self resonant frequency with:
1. The coil alone no shield
2. The coil with shield
3. The coil at the end of the coax as it would attach to the Circuit board.
Based on my own research, the coax adds the most capacitance. If you want the highest sensitivity to small low TC targets you need
1. A shielded coil
2. A coil size optimized for your desired targets size and depth
3. Integrating many samples to increase the signal to noise ratio.
4. A higher value damping resistor to have a faster pulse discharge slope to better stimulate lower TC targets
The coil design is only one part of the overall issue to better detect smaller low TC targets.
I also have designed and built guitar pickups. They tend to have a self resonant point in the 2K Hz to 4k Hz range which has a noticeable effect on the sound we near since the human ear is most sensitive in this range. The self resonant frequency amplitude and frequency is highly dependent on the loading that the pickup sees such as:
1. Volume and tone pot loading typically 250K for single coil pickups and 500 K for humbucker pickups
2. Capacitance of a typical 10 foot coax about 300 pf
3. Typical amp input resistance load of 1 Meg Ohm.
When I plugged my guitar into an amp with a 2 ft coax I could hear a very different pickup tone due to much less coax capacitance.
Then I saw an internet post for a Tillman FET buffer that put a 5 Meg ohm load on the pickup because the FET was located in the guitar plug with the power being applied to the amp end of the guitar cable plug. Then the coax cable effect on the pickup tone was eliminated. As long as the output impedance of the FET was much lower than the passive pickup, the length of the coax had no tonal effect.
What if you applied this same logic to PI coils. If you built a very small and compact set of the minimal active and passive components needed to drive the TX and RX circuits near the coil in a shielded box, you could eliminate the coil from seeing the coil coax capacitance and wind up with a higher value of damping resistance.
The main point is that detecting small, low TC targets takes creative thinking not just in coil design but in all the places that control how low you could potentially sample and minimize unwanted noise.
Gurus, does this make any sence?
Joseph J. Rogowski
Searching for small low TC objects, like gold, requires low sampling delays to capture the target eddy currents before they fully discharge. Testing a coil for a self resonant frequency is a known way to determine the potential for a coil to operate at a low delay. The ML flat wound coil should have less capacitance than a bundle wound coil of the same number of turns. However, to achieve the same inductance a bundle wound coil would require less turns due the the increased coupling between the bundled turns. Adding a shield to the coil also adds capacitance due to the:
1. Distance of the shield from the coil wire
2. Area of the coupling
3. Dielectric effect of the spacer and wire insulation
If anyone makes one of these coils, try to measure the coil self resonant frequency with:
1. The coil alone no shield
2. The coil with shield
3. The coil at the end of the coax as it would attach to the Circuit board.
Based on my own research, the coax adds the most capacitance. If you want the highest sensitivity to small low TC targets you need
1. A shielded coil
2. A coil size optimized for your desired targets size and depth
3. Integrating many samples to increase the signal to noise ratio.
4. A higher value damping resistor to have a faster pulse discharge slope to better stimulate lower TC targets
The coil design is only one part of the overall issue to better detect smaller low TC targets.
I also have designed and built guitar pickups. They tend to have a self resonant point in the 2K Hz to 4k Hz range which has a noticeable effect on the sound we near since the human ear is most sensitive in this range. The self resonant frequency amplitude and frequency is highly dependent on the loading that the pickup sees such as:
1. Volume and tone pot loading typically 250K for single coil pickups and 500 K for humbucker pickups
2. Capacitance of a typical 10 foot coax about 300 pf
3. Typical amp input resistance load of 1 Meg Ohm.
When I plugged my guitar into an amp with a 2 ft coax I could hear a very different pickup tone due to much less coax capacitance.
Then I saw an internet post for a Tillman FET buffer that put a 5 Meg ohm load on the pickup because the FET was located in the guitar plug with the power being applied to the amp end of the guitar cable plug. Then the coax cable effect on the pickup tone was eliminated. As long as the output impedance of the FET was much lower than the passive pickup, the length of the coax had no tonal effect.
What if you applied this same logic to PI coils. If you built a very small and compact set of the minimal active and passive components needed to drive the TX and RX circuits near the coil in a shielded box, you could eliminate the coil from seeing the coil coax capacitance and wind up with a higher value of damping resistance.
The main point is that detecting small, low TC targets takes creative thinking not just in coil design but in all the places that control how low you could potentially sample and minimize unwanted noise.
Gurus, does this make any sence?
Joseph J. Rogowski
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