Ground balance

[6666]

Which would explain why some simple designs like PI-4 do not appear to have an issue with the Earth field due to their much lower gain

No EFE for my PI-4

[green]

[QUOTE=Qiaozhi;280665
I suspect you may have the EF amplitude set too high in the simulation anyway. I've never measured the EF signal level in practice but it's probably much lower than 10mV. Which would explain why some simple designs like PI-4 do not appear to have an issue with the Earth field due to their much lower gain.[/QUOTE]

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.

[green]

If you change the input signal to 10mV dc, and do the sampling, it doesn't completely cancel.
There is a dc level at B of about 2mV. Since the inverting gain of the opamp is 100x there is clearly some cancellation going on. It's just not that good.
I suspect you may have the EF amplitude set too high in the simulation anyway. I've never measured the EF signal level in practice but it's probably much lower than 10mV. Which would explain why some simple designs like PI-4 do not appear to have an issue with the Earth field due to their much lower gain.

What do you get at b when 800us is changed to 589us?

Rocked about 1/2inch, my 300uH, 200mm diameter coil connected to my amplifier(gain=450). Easy to get 50mV peak, 100mV harder. Wondering what others get.

Try moving R3 and R11 to the input side of the switches. Cancels but I think other problems.

[Qiaozhi]

Changed the input to pulse. The +input has a sawtooth waveform(R11, C5). If the second sample is taken at the right time of the sawtooth the output is zero. Change the 589us sample time to 800us and see what happens. Think I see the problem now?

The strange thing is that Hammerhead uses a similar configuration and I know for a fact that it can cancel the Earth field. 2C integrator does reduce signal and cancels if sample delays are correct. Maybe good enough or just lucky.

If you change the input signal to 10mV dc, and do the sampling, it doesn't completely cancel.
There is a dc level at B of about 2mV. Since the inverting gain of the opamp is 100x there is clearly some cancellation going on. It's just not that good.
I suspect you may have the EF amplitude set too high in the simulation anyway. I've never measured the EF signal level in practice but it's probably much lower than 10mV. Which would explain why some simple designs like PI-4 do not appear to have an issue with the Earth field due to their much lower gain.

[green]

Changed the input to pulse. The +input has a sawtooth waveform(R11, C5). If the second sample is taken at the right time of the sawtooth the output is zero. Change the 589us sample time to 800us and see what happens. Think I see the problem now?

The strange thing is that Hammerhead uses a similar configuration and I know for a fact that it can cancel the Earth field. 2C integrator does reduce signal and cancels if sample delays are correct. Maybe good enough or just lucky.

[Qiaozhi]

As far as I can ascertain there is nothing wrong with the simulation. If you set the switches to permanently on and set the target signal to 10mV dc, then the circuit outputs a signal that is "near as dammit" zero volts.

Going back to a sine wave input and sampling the switches with main and EF pulses, if you move the 10k input resistors to the output side of the switches (instead of the input) then you can probe the switch outputs to see what is happening. Comparing the two outputs shows clearly that the differential opamp cannot possibly subtract the non-inverting input from the inverting and achieve a zero volt output as the two signals are in constant fluctuation.

The strange thing is that Hammerhead uses a similar configuration and I know for a fact that it can cancel the Earth field. The circuit used in the Minipulse is different in that the feedback resistors are connected to the switch inputs.
We will all need to think about this problem a bit more.

[Carl-NC]

With spice EF simulation. 1C integrator_ 5% mismatch of input resistors causes output to be about 5% of the input. Zero output if input resistors are matched. 2C integrator_ cancels at one second delay time only. 5% mismatch moves the second delay time that cancels. Signal out less than 5% of input at other second delay times?

Don't understand why, sorry.

Could someone explain EF. Connected a 8inch mono coil to Rx input to see what we are trying to cancel. Don't see a change at amplifier out when moving coil back and forth. Rocking coil causes a signal. What amplitude should I expect for EF signal at coil out(amplifier out/amplifier gain). Should I expect to see EF if coil remains flat while sweeping?

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.

[green]

The trimmer is not needed if you match the input resistors to, say, 1% or better. Also the resistors & caps in the integrator need to be well-matched.

With spice EF simulation. 1C integrator_ 5% mismatch of input resistors causes output to be about 5% of the input. Zero output if input resistors are matched. 2C integrator_ cancels at one second delay time only. 5% mismatch moves the second delay time that cancels. Signal out less than 5% of input at other second delay times?

Could someone explain EF. Connected a 8inch mono coil to Rx input to see what we are trying to cancel. Don't see a change at amplifier out when moving coil back and forth. Rocking coil causes a signal. What amplitude should I expect for EF signal at coil out(amplifier out/amplifier gain). Should I expect to see EF if coil remains flat while sweeping?

[Carl-NC]

Thanks Carl. I think I've got it now. I appreciate the patience and time you've taken to explain it. What sort of maximum relative gain might be required for the Ground channel in highly mineralised fields compared with the Target channel (in other words, should VR1 (refer attached) be say 4k7 or 10k)? Also, some say that you a should insert a trimmer (VR2) between the switches S4 and S2 to replace the link. But if your explanation is correct, I don't see the need for it. Am I missing something here? With respect to the "hole width" question at #39, Green has answered that for me at #44. My question was basically, if you adopt a GB solution that changes the width of the GB sample (as opposed to changing the gain (A2) approach), will the TC hole widen? It appears the answer is NO.
[ATTACH]53196[/ATTACH]

The GB gain required depends on the spacing between the Target and Ground pulses. The more spacing, the more gain. This will also alter the location of the target hole. For, say, 10us spacing VR1 should never need to be as high as 10k, probably not 5k, maybe 2k. Also, if you run the Ground pulses wider than the Target pulses then that will reduce the Ground channel gain requirement.

The trimmer is not needed if you match the input resistors to, say, 1% or better. Also the resistors & caps in the integrator need to be well-matched.

[waltr]

I was sampling 1000 times/second for 10 seconds. Looks like you are sampling once in 10 seconds? reply #54

Saw that also.

Oh bugger!
I thought it was an issue between using the "signal" symbol in the misc library and the independent voltage source, even though the netlist looked identical.
Unfortunately I then introduced a typo in the source parameters.
Back to the drawing board for another look.

It happens.
Still think my answer points to part of the issue, Op-amp input bias currents and Voffsets.

By the way, you don't need to explicitly put in the number of cycles. You only need to do that if you want to set a limit, otherwise the cycle just keeps repeating until the end of the simulation.

Saw that and learned something new. Thanks.

[ZED]

The hole as mentioned by some members (a point where the target TC is the same as the ground balancing null point) is not related to the TC of the ground signal,but related to the interplay between how the samples are adapted and arranged.I made a number of targets of different time constants,and each time constant progressively getting longer,this allows me to detect target holes and to map them as i use different sampling coefficients,i noticed as other members have that the target hole remains relatively constant as the ground balance is adjusted, but when a completely different set of sampling coefficients are used,that is to say dramatically different,then the target hole shifts to a different position altogether.

[Qiaozhi]

By the way, you don't need to explicitly put in the number of cycles. You only need to do that if you want to set a limit, otherwise the cycle just keeps repeating until the end of the simulation.

[Qiaozhi]

I was sampling 1000 times/second for 10 seconds. Looks like you are sampling once in 10 seconds? reply #54

Oh bugger!
I thought it was an issue between using the "signal" symbol in the misc library and the independent voltage source, even though the netlist looked identical.
Unfortunately I then introduced a typo in the source parameters.
Back to the drawing board for another look.

[green]

I was sampling 1000 times/second for 10 seconds. Looks like you are sampling once in 10 seconds? reply #54

[ZED]

The gain of A3 would be independently adjusted to fully cancel out EFE from the equation, regardless of the GB gain setting (A2).
I'm thinking if first delay is changed, GB(A2)needs to be readjusted. Think if A2 is changed A3 would also need to be adjusted. Maybe I'm wrong.

Your thread started me wondering again if changing gain or sample time to GB is better. Thinking it might depend on the target. Been using a 1C integrator and adjusting sample time to GB. Tried a 2C integrator awhile back, didn't work as good. Tried 2C circuit in spice and can't see why it wouldn't work. Thinking of making another integrator. Make delay times #1 and #2 adjustable. Make target sample time adjustable. Make GB sample time and gain adjustable. Test circuit to see if I can tell if adjusting GB sample time or gain is better. Any suggestions appreciated.

G'day green

I like to GB by changing the phase of a sample,changing the phase of the target sample (sample delay) works but it makes the detector sound off so its usually the second or third sample that i adjust,i dont like adjusting the gain cause it affects the earth field
balance,i aim for a very tight EF balance and i dont want anything upsetting it,the ground here with its remnant magnetism has me getting very fanatical about EF balance.Also tried and still experiment with adjusting the width of samples to GB,now this does work but curiously when the EF is balanced and the sample width is adjusted i can notice the samples going out of EF balance despite the sample widths remaining the same ,even down to an accuracy of 25 nano sec.

I will explain that a bit better,i have a very late sample (before the next TX period) that is my compensating sample (EF sample),so when i adjust the width of the GB sample it also adjusts the width of the late compensating sample,the resolution is
25 nano sec so its very accurate, but as the GB sample is increased or decreased and the compensating sample is adjusted accordingly it still throws out the EF adjustment,admittedly this has me scratching my head with puzzlement.