Ground balance

[Elliot]

If the high pass filter on the front end of the PI4 eliminates EFE, why are we messing around with EFE pulses ?

Good question! Trouble is that they are not brick wall filters and have a first order roll off.

[6666]

If the high pass filter on the front end of the PI4 eliminates EFE, why are we messing around with EFE pulses ?

[Elliot]

Interesting that a 2C does a better job for you than a 1C. Do you have a EFE trim pot? Either 1C or 2C should cancel with a trim pot but the simulation suggests the 2C doesn't cancel if the sample timings are changed without retrimming. Including a schematic of the integrator I'm using. 1 turn of the 100 ohm trim pot adds 5R to one input and subtracts 5R from the other. 0.1% change, can see effect at integrator out on the scope when bouncing a magnet over the coil. Not suggesting integrator frequency response is ideal but it's what I'm using at the moment.

Hi Green, What are the advantages of having a 3 stage integrator circuit as you have ?

[ZED]

I use a micro-controller to adjust sample widths plus i store upto 16 individual timings,so using a trim pot doesn't help because its adjustment wont carry over to the next timing.
In the beginning i used a 3.9k input resistor on the diff integrator and adjusting EF was a finicky affair,i now use 5.1k or more and now EF adjustment is easier,also i use 0.1% resistors,this
helps when i move onto the next revision,no EF adjustments are necessary on the later revision.
i would like to edit my previous post,freq of interest is more along the lines of 0.5 to 5 Hz.

[green]

The integrator circuit i use is the same as the one shown in the 2C sim,except with different value components,frequency's of interest to me are from 0.1 to 5 Hz,in the past i used single ended integrators,i feel my current setup does a better job of tuning out the EF.
I use a ferrite speaker magnet when trimming the EF and like to get the magnet touching the coil without sounding off,some sample timings will lend themselves to that effect,others not so much.
Its not uncommon to bump into earth field effects when detecting out in the field,particularly with large coils,its most notable when raising the coil from ground level up to the 12 o'clock position,many retail detectors will exhibit this effect as well
I also wonder whether the high mineralization in this region of the globe would increase the density of the earths magnetic field and make EF noise more pronounced.

Interesting that a 2C does a better job for you than a 1C. Do you have a EFE trim pot? Either 1C or 2C should cancel with a trim pot but the simulation suggests the 2C doesn't cancel if the sample timings are changed without retrimming. Including a schematic of the integrator I'm using. 1 turn of the 100 ohm trim pot adds 5R to one input and subtracts 5R from the other. 0.1% change, can see effect at integrator out on the scope when bouncing a magnet over the coil. Not suggesting integrator frequency response is ideal but it's what I'm using at the moment.

[liekwang]

I've seen the ground balance 3 sample schema schema in handwriting in this forum too

[ZED]

The integrator circuit i use is the same as the one shown in the 2C sim,except with different value components,frequency's of interest to me are from 0.1 to 5 Hz,in the past i used single ended integrators,i feel my current setup does a better job of tuning out the EF.
I use a ferrite speaker magnet when trimming the EF and like to get the magnet touching the coil without sounding off,some sample timings will lend themselves to that effect,others not so much.
Its not uncommon to bump into earth field effects when detecting out in the field,particularly with large coils,its most notable when raising the coil from ground level up to the 12 o'clock position,many retail detectors will exhibit this effect as well
I also wonder whether the high mineralization in this region of the globe would increase the density of the earths magnetic field and make EF noise more pronounced.

[green]

Interesting experiment, when you rock the coil do you see much of a signal at preamp out ?

Preamp out is what I was looking at. Didn't see the change at integrator out until I adjusted EF trim pot.

[6666]

Connected the 8inch 300uH mono coil to Rx in. Tx disabled. Holding coil N and S sides stationary and rocking, minimum signal change. Holding E and W sides stationary and rocking, maximum signal change. Bouncing a 1/2 x 3/16 ceramic craft magnet over the coil, a little more signal than rocking. Bouncing a 12 x 3 mm neodymium magnet over the coil, a little more than the craft magnet. Lot stronger magnet, expected to see a higher signal. I have a trim pot to adjust circuit for EF cancel. A .1% change on either input resistor is easily visible looking at integrator out with a scope while bouncing the magnet. Still don't know how signal amplitude sweeping the coil compares with rocking the coil or using magnets.

Interesting experiment, when you rock the coil do you see much of a signal at preamp out ?

[green]

Any dB/dt cutting through the coil will produce a signal. If the Earth field is perfectly uniform and you swing perfectly level then there should be no EFE effect. Rocking/twisting the coil will maximize the effect. Amplitude depends on coil turns & area, gain of preamp, etc. A good bench test method is to use a magnet, but it must have no eddy (metal) response. Many ceramic magnets will work.

Connected the 8inch 300uH mono coil to Rx in. Tx disabled. Holding coil N and S sides stationary and rocking, minimum signal change. Holding E and W sides stationary and rocking, maximum signal change. Bouncing a 1/2 x 3/16 ceramic craft magnet over the coil, a little more signal than rocking. Bouncing a 12 x 3 mm neodymium magnet over the coil, a little more than the craft magnet. Lot stronger magnet, expected to see a higher signal. I have a trim pot to adjust circuit for EF cancel. A .1% change on either input resistor is easily visible looking at integrator out with a scope while bouncing the magnet. Still don't know how signal amplitude sweeping the coil compares with rocking the coil or using magnets.

[green]

Tried to calculate the minimum value I can detect with my detector. Maybe correct maybe not. Think 200uV maybe 100uV at integrator in if coil pickup noise is low enough(.2uV to .4uV at amplifier Rx in). 2C integrator reduces input by at least 30 so I would need at least 3mV EF to detect. Not sure I did it correctly. Interested in what other members get.

https://www.geotech1.com/forums/atta...6&d=1599429210 A scope picture of detecting a US quarter. Modified one of my spice simulations to match my detector circuit used to detect the quarter. 170uV in gives same signal as quarter. Should be able to detect 100mV signal at output, 85uV in. Similar to the 100uV in suggested with my first try.

[green]

Like waltr, my initial thought was that the circuit was sampling on different parts of the sine wave and this was the cause of the problem, but I was sidetracked into thinking there may be something wrong with either the simulation or the circuit itself.

The whole premise for EFE is that the early and late samples will contain the same amount of EF signal, and therefore a simple subtraction process can be used to remove the EF. You can prove that the cancellation process works in a simulation by using a dc voltage as the EF signal. However, in the real world the EF signal is not exactly the same during the early and late samples, and the EFE can become compromised. I suspect some circuits are better than others at providing good EFE, and green's use of a 1Hz sine wave seems good at exposing these limitations.

The question is: "How good does it have to be?".

1C integrator cancels 1Hz sine or dc voltage at different delay times(it samples the sine wave at different parts of the sine wave). 2C integrator doesn't cancel 1Hz sine or dc voltage at different delay times. The capacitors, feedback and +in to common effect error amplitude. The question is: "How good does it have to be?". Good question, how do we find the answer? ZED reply #55 seems to have a problem with EF.

[Qiaozhi]

Like waltr, my initial thought was that the circuit was sampling on different parts of the sine wave and this was the cause of the problem, but I was sidetracked into thinking there may be something wrong with either the simulation or the circuit itself.

The whole premise for EFE is that the early and late samples will contain the same amount of EF signal, and therefore a simple subtraction process can be used to remove the EF. You can prove that the cancellation process works in a simulation by using a dc voltage as the EF signal. However, in the real world the EF signal is not exactly the same during the early and late samples, and the EFE can become compromised. I suspect some circuits are better than others at providing good EFE, and green's use of a 1Hz sine wave seems good at exposing these limitations.

The question is: "How good does it have to be?".

[6666]

PI-4 also has a HPF at the input of the preamp.

Is that C2+R4

[Carl-NC]

PI-4 also has a HPF at the input of the preamp.