EFE

[green]

Needed to add a trim pot to integrator in with my unipolar Tx to reduce EF signal. Checked EF signal this morning with bipolar Tx. Was seeing a signal at amplifier out and differentiator out when sweeping a magnet across the coil. After some playing I added a switch so I could disable Tx command to MOSFETS, signal from magnet field only. Added a pot to integrator inverted input, 11 ohms appeared to cancel magnet signal best. Removed pot, connected a 10 ohm resistor to amplifier inverted out and connected integrator invert in to 10 ohm resistor. Recorded sweeping a magnet across coil with 10 ohm and with 10 ohm shorted. A .2% change in integrator Rin is easily visible. With Tx disabled a ceramic or neodymium magnet could be used(don't over range the amplifier). I have read where a fridge magnet could be used. Tried with Tx disabled, no signal. With Tx enabled I get a signal with either of the magnets including the fridge magnet. Maybe because I'm sampling after a 4us delay. Is there a R signal with a ceramic or fridge magnet(can see a X signal at amplifier out and a signal at integrator out when magnet is stationary over the coil),or my circuit not good enough to sample that soon?

A pot was used to trim the unipolar integrator. Wiper to integrator in(pin 13)pot ends to switch outputs(pins 2 and6).

[Carl-NC]

Magnets can create a target response due to their composition and, like ferrite, the earlier the sample the worse it can get.

Sweeping a small magnet across the coil is a worst-case scenario. Normally you are swinging the coil through the Earth field with maybe a 1Hz bandwidth. The much smaller field of the magnet moving across the coil, even at the same sweep speed, produces a higher effective frequency, perhaps 20-30 Hz. This results in less attenuation in the EFE subtraction. The attenuation is given by



where td is the spacing between the main and EFE samples (or the period of the bipolar pulsing). Plug in f=1 for real life, f=20 for waving a magnet.

Finally, what are the tolerances of the resistors?

[green]

Magnets can create a target response due to their composition and, like ferrite, the earlier the sample the worse it can get.

Sweeping a small magnet across the coil is a worst-case scenario. Normally you are swinging the coil through the Earth field with maybe a 1Hz bandwidth. The much smaller field of the magnet moving across the coil, even at the same sweep speed, produces a higher effective frequency, perhaps 20-30 Hz. This results in less attenuation in the EFE subtraction. The attenuation is given by



where td is the spacing between the main and EFE samples (or the period of the bipolar pulsing). Plug in f=1 for real life, f=20 for waving a magnet.

Finally, what are the tolerances of the resistors?

Playing with EFE this morning, bipolar Tx circuit figure8 coil. Differentiator out connected to scope. Tx disabled so I could swing the coil without seeing target signals. Wasn't seeing much so I connected integrator invert input to common not amplifier invert out. Saw a signal when swinging the coil, larger signal when rocking the coil. Removed figure8 Rx and connected a mono coil. Similar results except higher amplitude, way over full scale when rocking the coil. Rocking the coil is higher frequency, maybe integrator and differentiator frequency response was reducing the lower frequency swing.

[Carl-NC]

Rocking the coil gives a higher signal because it creates a much higher dphi/dt than sweeping the coil in a level manner. Everything you're seeing makes sense; mono coil is worse than figure-8, rocking is worse than sweeping.