IB Rx coil at Tx off

[Ferric Toes]

The difference is subtle in that a coil on its own has its own self capacitance which can only be measured when the coil is connected, in which case the connecting wires are now considered part of the coil and not part of (belonging to) the measuring circuit thus implying that the self capacitance has been altered by the connecting wires.
Hence it is only practicable to measure srf and self capacitance of our pi coils when it is connected to the cable we intend to use as part of the final detector hardware.
That's my take away from it.
But I agree that the intrinsic self capacitance of a coil is independent of the circuit.

Here is a way of measuring the srf of a coil without having it connected to any instrument. Or at least it's connected wirelessly. That is to use a PI TX and RX board to excite a remote coil with the TX pulse and to look at the induced ringing on the output of the RX. The pictures show an active 8inch coil with an unconnected 10inch, 300uH coil, some 16inches below. The 10inch coil has a helically wound nickel plated copper fabric shield connected to one end of the winding.The next picture shows the decaying ringing picked up by the active coil. The following picture is with the timebase expanded to measure the period of oscillation. . Disconnecting the shield reduces the period to 1050nS. Strangely it also reduces the amplitude by a factor of 2.5.

Eric.

[green]

Here is a way of measuring the srf of a coil without having it connected to any instrument. Or at least it's connected wirelessly. That is to use a PI TX and RX board to excite a remote coil with the TX pulse and to look at the induced ringing on the output of the RX. The pictures show an active 8inch coil with an unconnected 10inch, 300uH coil, some 16inches below. [ATTACH]50804[/ATTACH] The 10inch coil has a helically wound nickel plated copper fabric shield connected to one end of the winding.The next picture shows the decaying ringing picked up by the active coil. [ATTACH]50805[/ATTACH] The following picture is with the timebase expanded to measure the period of oscillation. [ATTACH]50806[/ATTACH]. Disconnecting the shield reduces the period to 1050nS. Strangely it also reduces the amplitude by a factor of 2.5.

Eric.

Any thoughts why the amplitude reduced by 2.5 times when you disconnected the shield? Frequency increased so I would expect your amplifier would have some drop in signal but not that much.

Good way to test for SRF

[dbanner]

Perhaps there are transient (Eddy) currents being generated in the shield by the ringing which aids the amplitude at resonance. The shield being at right angle to the flow of current throughout.

[Ferric Toes]

Any thoughts why the amplitude reduced by 2.5 times when you disconnected the shield? Frequency increased so I would expect your amplifier would have some drop in signal but not that much.

Good way to test for SRF

I have no explanation at the moment, but a recent experiment has shed a bit of light on the matter. On the basis that no current flows in or out of the shield, it should not matter if some resistance was introduced between the ground end of the coil and the connection to the shield. This is normally a direct connection in the coils I make.

However, in my recent test, it did matter. I put a 10k pot between the coil ground and the shield and was surprised to find that there is a null point on the pot when the ringing of the open coil vanished. this turned out to be a value of 1.3k. Either side of this the ringing increased and maximised at the minimum pot resistance.

You can hold this open circuit coil right up against the drive coil and there is no response when at the null point of the pot. With the direct connection of the shield to one end of the coil, the two coils can be 24inches apart and ringing of the OC coil can be easily observed on the drive coil and observed on it's Rx test point.

This coil has no cable connected to it and for my next test about 4 feet of coax will be connected to see it the effect is still there with the added capacitance across the coil winding.

Eric.

[green]

https://www.geotech1.com/forums/atta...5&d=1522111240 a chart I posted awhile back.

Removed 133mm, 300mm and 450mm coils and added distance vs amplitude for a quarter with a 8in fig8 that I had and the 24in fig8 I've been making. The 8in fig8 parallels the 200mm fairly close. 24in fig8 parallels the 675mm coil fairly close and the 8in and 24in cross at about the same distance as the 200mm and 675mm. Tried the 30in DD, noisy and not stable. The DD and 24in fig8 are just taped together. Need to glue everything but wanted to post what I've seen so far if someone has a suggestion of something to try before I do. No changing after I glue them together.

[green]

https://www.geotech1.com/forums/atta...5&d=1522111240 a chart I posted awhile back.

Removed 133mm, 300mm and 450mm coils and added distance vs amplitude for a quarter with a 8in fig8 that I had and the 24in fig8 I've been making. The 8in fig8 parallels the 200mm fairly close. 24in fig8 parallels the 675mm coil fairly close and the 8in and 24in cross at about the same distance as the 200mm and 675mm. Tried the 30in DD, noisy and not stable. The DD and 24in fig8 are just taped together. Need to glue everything but wanted to post what I've seen so far if someone has a suggestion of something to try before I do. No changing after I glue them together.

Suggested 8in fig8 and 24in fig8 crossed at same distance as 200mm and 675mm coils. Shouldn't have since the inductances aren't the same. Looking at chart again, mm coils cross at 190mm. Inch coils cross at 150mm. 24in coil has higher inductance than the 8in causing it to cross sooner. Think I would use fewer turns with the 24in if I were to make another. Could sample sooner but might not make much difference with a quarter. Maybe fewer turns would help with a nickel.

[green]

Playing with 24in fig8. Noise at amplifier out. Disconnected Tx coil. Noise appears to be 60kHz, frequency display varies a little above or below 60kHz. Two scope pictures, first with coil rotated for near max signal. Second with coil rotated about 90deg for min signal. Looked like noise might be coming from scope, no effect when moving scope. Don't see what might be causing it. Hoping someone might have a suggestion since the frequency appears to be 60kHz. Testing in dining area near the kitchen.

[Carl-NC]

Is there a computer monitor nearby? Or flat-screen display?

[green]

Is there a computer monitor nearby? Or flat-screen display?

Thanks. Computer and TV are in the room next to the dining room. Wall between so I wasn't thinking. Computer maybe 15ft away doesn't cause the noise. TV and DISH receiver 4 to 5ft away. Unplugged TV, no effect. Unplugged DISH Hopper, no 60kHz.

The Hopper was plugged in not on with TV off when I did the scope pictures this morning, Maybe the TV could cause the noise also. Tried just the TV, signal looks more like a sine wave maybe a little less amplitude and not continuous like the Hopper.

[green]

I have no explanation at the moment, but a recent experiment has shed a bit of light on the matter. On the basis that no current flows in or out of the shield, it should not matter if some resistance was introduced between the ground end of the coil and the connection to the shield. This is normally a direct connection in the coils I make.

However, in my recent test, it did matter. I put a 10k pot between the coil ground and the shield and was surprised to find that there is a null point on the pot when the ringing of the open coil vanished. this turned out to be a value of 1.3k. Either side of this the ringing increased and maximised at the minimum pot resistance.

You can hold this open circuit coil right up against the drive coil and there is no response when at the null point of the pot. With the direct connection of the shield to one end of the coil, the two coils can be 24inches apart and ringing of the OC coil can be easily observed on the drive coil and observed on it's Rx test point.

This coil has no cable connected to it and for my next test about 4 feet of coax will be connected to see it the effect is still there with the added capacitance across the coil winding.

Eric.

Tried a simulation with different resistance from shield to ground. Looks similar to what you get except doesn't oscillate at higher R. Wondering if sim makes sense or should change something.

[green]

Glued 3/16in foam board painted with graphite paint on one side for shield to Tx side of coil. Increased needed delay time compared to when shield board was taped to coil board. Measured capacitance from shield to Tx coil, about 1500pf. Made a test capacitor, glued aluminum foil to each side of a piece of the foam board 4x2.4in. Calculates about 11pf*k, measured about 22pf (k=2). Tx coil .54x130in. Calculates 140pf if k=2. Measures 1500pf. Any thoughts why or is my thinking all wrong? I did use Elmers foam board and one less expensive, not sure they would measure same k. Need to buy one of the cheaper boards to test, Elmers calculates k=2. Picture of coil https://www.geotech1.com/forums/atta...8&d=1593546020 My meter charges the capacitor and measures time to charge if that makes a difference.

[Ferric Toes]

Glued 3/16in foam board painted with graphite paint on one side for shield to Tx side of coil. Increased needed delay time compared to when shield board was taped to coil board. Measured capacitance from shield to Tx coil, about 1500pf. Made a test capacitor, glued aluminum foil to each side of a piece of the foam board 4x2.4in. Calculates about 11pf*k, measured about 22pf (k=2). Tx coil .54x130in. Calculates 140pf if k=2. Measures 1500pf. Any thoughts why or is my thinking all wrong? I did use Elmers foam board and one less expensive, not sure they would measure same k. Need to buy one of the cheaper boards to test, Elmers calculates k=2. Picture of coil https://www.geotech1.com/forums/atta...8&d=1593546020 My meter charges the capacitor and measures time to charge if that makes a difference.

Your shield to coil capacitance seems much too high. The capacitance I measure is 178pf. This is on a 10inch mono coil of inductance 311uH; resistance 1.8ohms. shielding is nickel plated copper fabric tape helically wound on polyethythene cable wrap, which overlays the coil winding. The shield is very conductive, measuring 1.5ohms across the gap by the lead out wires from the coil. I measured the capacitance of 6ft of coax that I use (75ohm in this case) at 138pF. I used a Peak LCR meter.

Eric.

[green]

Tried measuring capacitance from shield to coil on two other flat spiral coils. Smaller about 8inch ID. Measured capacitance greater than calculated from area or resonance. Got resonant decay on the 24in fig8 Tx(no damping resistor) before I glued the shield, some overshoot but no resonance after I glued the shield. Still wondering why. Glued shield to Rx side of 24in fig8. Getting a lot of noise at output of AC coupled amplifier after integrator. Been trying to figure out why. Disconnected Tx and just looking at Rx pickup noise. Rotated coil and got a lot less, haven't found the noise source yet. Including pictures with coil rotated about 90 degrees(coil flat, ends rotate).

[Ferric Toes]

Tried measuring capacitance from shield to coil on two other flat spiral coils. Smaller about 8inch ID. Measured capacitance greater than calculated from area or resonance. Got resonant decay on the 24in fig8 Tx(no damping resistor) before I glued the shield, some overshoot but no resonance after I glued the shield. Still wondering why. Glued shield to Rx side of 24in fig8. Getting a lot of noise at output of AC coupled amplifier after integrator. Been trying to figure out why. Disconnected Tx and just looking at Rx pickup noise. Rotated coil and got a lot less, haven't found the noise source yet. Including pictures with coil rotated about 90 degrees(coil flat, ends rotate).

A couple of thoughts.

1) Regarding the high capacitance you measured between coil to shield. You mentioned that your tester works on the principle that it measures the time it takes to charge the capacitance. From my experience with graphite shielding, it has a high resistance compared to the metallised fabric that I use. Could it be that this fact increases the time to charge and which results in an erroneous reading?

2) I suffer from pickup noise in my workshop and have not yet identified all of the sources. One major source is the LED lighting which I have to switch off when taking measurements. The other source is the security alarm. Both systems run off voltage converters which are a source of radiated and conducted noise. I go round with a hand held AM band radio which pinpoints these noise sources. Even if switch off everything in sight that has a voltage converter, there is still noise which appears to be radiated by the ring main power wiring. Maybe what is needed is a heavy duty filter where the power comes into the building.

Like you, I can align the search coil for minimum pickup and that is what I do when doing range and sensitivity tests, even when known sources are switched off.

Eric.

[waltr]

Maybe the noise from the AC mains is due to this:
https://en.wikipedia.org/wiki/Power-line_communication

One of the parks I detect at has bad noise near the road where there is power lines.