Ground balance

[Qiaozhi]

There is a mystery here to be solved.

Have a look at the attached image. Green's simulation is now working!!!
See if you can spot the difference.

Ignore the fact that I swapped out the universal opamp model for an LT1007, and changed the pulse rise and fall times to 10ns as that's not the fix.
Also, I set the input resistors to the switches to 10k. That's not it either.

[Elliot]

EFE cancellation is achieved when S1-S3 = 0 (Target channel) and S2-S4 = 0 (Ground channel). Then when you apply whatever gain to each channel and subtract, EFE cancellation is maintained. That is to say, each channel is independently "EFE clean" before you ever apply gain or combine them. This is done at the integrators by simply making sure the EFE sample has the same pulse width as the main sample (S1=S3 and S2=S4).

What you are suggesting will work but it's the hard way to do it. You will need a separate integrator stage with a separate gain stage, and then you need to do a GB adjustment followed by a separate EFE adjustment. The way I suggest (which is the way all commercial non-bipolar ground balancing PI detectors do it) is simpler and automatic... EFE cancellation is always maintained throughout the full range of GB.



Sorry, didn't understand this question.

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.

[Carl-NC]

What I think is going on is the second Sample is on a different part of the 1Hz Target 'sine' so is at a different Voltage.
If both sampling switches pulse at the same time then output is zero since there is NO Voltage difference between Sample1 and sample2.
I don't think using a Sine wave input is a valid 'ground' signal although is seems to make sense. Or does it???

He's simulating a slow (1Hz) signal to emulate EFE. This is valid.

[waltr]

The sine wave is for EFE, sweeping the target with the coil

Then I am not clear on what you are trying to do in that Spice sim.

A 'real' target has an exponential decay and is a straight line when plotted log/log.
'Ground' is Not exponential, it plots as a straight line when plotted Lin/log.
Think you have them reversed, should be Lin/log then log/log.

Correct, I have them reversed.

Did you try setting the Target signal source to 0VDC??

[green]

green,
Tried the 2C integrator sim you showed. Get similar results.
Then swapped the delay on SW1 with Sw2 and the offset flipped polarity.

What I think is going on is the second Sample is on a different part of the 1Hz Target 'sine' so is at a different Voltage.
If both sampling switches pulse at the same time then output is zero since there is NO Voltage difference between Sample1 and sample2.
I don't think using a Sine wave input is a valid 'ground' signal although is seems to make sense. Or does it???

A 'real' target has an exponential decay and is a straight line when plotted log/log.
'Ground' is Not exponential, it plots as a straight line when plotted Lin/log.



Can not explain how to make it work but this is my thoughts as to why this simulation is not working as expected.

Playing more with Sim.
Measure Current into the op-amp inputs and find, as expected, they are NOT this same.
In real GB circuits, there is a pot to adjust Gain of GB. This would compensate fro op-amp input current differences.

Also try setting Target generator, V3, to 0V DA 'real' target has an exponential decay and is a straight line when plotted log/log.
'Ground' is Not exponential, it plots as a straight line when plotted Lin/log.
C. There will be offset at output due to op-amps.

Sorry this wasn't a 2x4 but hope it helps some.

I don't think using a Sine wave input is a valid 'ground' signal although is seems to make sense. Or does it???
The sine wave is for EFE, sweeping the target with the coil

A 'real' target has an exponential decay and is a straight line when plotted log/log.
'Ground' is Not exponential, it plots as a straight line when plotted Lin/log.
Think you have them reversed, should be Lin/log then log/log.

[waltr]

green,
Tried the 2C integrator sim you showed. Get similar results.
Then swapped the delay on SW1 with Sw2 and the offset flipped polarity.

What I think is going on is the second Sample is on a different part of the 1Hz Target 'sine' so is at a different Voltage.
If both sampling switches pulse at the same time then output is zero since there is NO Voltage difference between Sample1 and sample2.
I don't think using a Sine wave input is a valid 'ground' signal although is seems to make sense. Or does it???

A 'real' target has an exponential decay and is a straight line when plotted log/log.
'Ground' is Not exponential, it plots as a straight line when plotted Lin/log.

Can not explain how to make it work but this is my thoughts as to why this simulation is not working as expected.

Playing more with Sim.
Measure Current into the op-amp inputs and find, as expected, they are NOT this same.
In real GB circuits, there is a pot to adjust Gain of GB. This would compensate fro op-amp input current differences.

Also try setting Target generator, V3, to 0V DC. There will be offset at output due to op-amps.

Sorry this wasn't a 2x4 but hope it helps some.

[Carl-NC]

No thoughts why the 2C integrator cancels only at certain target and EF delay times?

I looked at the sim setup and didn't notice anything wrong. Yes, a 2C integrator will cancel EFE, it is commonly found in Eric Foster's designs.

[green]

No thoughts why the 2C integrator cancels only at certain target and EF delay times? Tried disabling target sample switch or GB sample switch with 1C and 2C integrator https://www.geotech1.com/forums/atta...8&d=1599836935 1C integrator same signal opposite polarity. 2C integrator: similar to 1C when +input switch disabled, about 1/10 the signal when -input switch disabled. Maybe could be the cause?

Another try. I read for a 2C integrator R3 and R11 should be matched, same for C2 and C5. My simulation cancels at 486us GB delay. Signal is reduced at other delay settings. Thinking I must be doing something wrong if matching is required. Another member is beating his head with a 2x4 because he missed something simple. Someone was kind enough to point out his error. I would like to beat my head with a 2x4 and get it over with. Maybe someone could point out my error.

Maybe someone could do a 2C integrator spice simulation that does cancel at different delay settings.

Tried test with R11=1.05meg(5% difference)mV readings changed but weren't any higher than 30mV when R11 was 1meg.

[Carl-NC]

Given that EFE and ground/target signals are independently driven (assuming no major interplay here), from a purists perspective, I would have therefore thought that it's more straight forward to simply have a circuit that permanently makes A1(S3) - A3(S4) = 0 (with A3 being a new independent gain setting for the GB EFE sample), thereby avoiding any change to the ideal A1(S1)- A2(S2) (i.e. perfect world) equation. Otherwise, when you change A2 in A2(S2 - S4), the gain of the S4 sample will also be changed, and adverse consequences may arise.

EFE cancellation is achieved when S1-S3 = 0 (Target channel) and S2-S4 = 0 (Ground channel). Then when you apply whatever gain to each channel and subtract, EFE cancellation is maintained. That is to say, each channel is independently "EFE clean" before you ever apply gain or combine them. This is done at the integrators by simply making sure the EFE sample has the same pulse width as the main sample (S1=S3 and S2=S4).

What you are suggesting will work but it's the hard way to do it. You will need a separate integrator stage with a separate gain stage, and then you need to do a GB adjustment followed by a separate EFE adjustment. The way I suggest (which is the way all commercial non-bipolar ground balancing PI detectors do it) is simpler and automatic... EFE cancellation is always maintained throughout the full range of GB.

Separately, if one was to adopt the changing GB target sample width approach to ground balancing, wouldn't this enlarge the target "hole"?

Sorry, didn't understand this question.

[green]

No thoughts why the 2C integrator cancels only at certain target and EF delay times? Tried disabling target sample switch or GB sample switch with 1C and 2C integrator https://www.geotech1.com/forums/atta...8&d=1599836935 1C integrator same signal opposite polarity. 2C integrator: similar to 1C when +input switch disabled, about 1/10 the signal when -input switch disabled. Maybe could be the cause?

[green]

Can attach the program if anyone is interested

Yes please.

Instructions:
1_enter ground slope into D2, I find it can vary between -1 and -1.4. Long constant current Tx closest to -1. Shorter Tx or constant rate Tx causes steeper slope.
2_enter delay time(I2), target sample time(I3), delay time(I4), GB sample time(I5) and A2 multiplier(I7). Vary one of them until M3 is minimum(ground balanced)
3_change target TC(F2)until )O3 is minimum(hole bottom)
4_copy and paste I2 thru I6 under working chart. Enter ground slope under working chart. Insert text box(A2 gain= )into chart.
5_enter M3 into GEB off, enter absolute value of P3 into GEB on. (enter hole first row)
6_change F2 to other TC's and fill in the blanks.

Thicker long TC targets decay straight line log X log Y at start of decay. Can enter slope in B2 for those targets. Ball park, real target decay straight line log X log Y in the beginning then decay straight line linear X log Y. Depends when samples are taken.

Program kept changing so maybe not as easy to use as should be. Any questions or suggestions to improve?

[Elliot]

Can attach the program if anyone is interested

Yes please.

[green]

Separately, if one was to adopt the changing GB target sample width approach to ground balancing, wouldn't this enlarge the target "hole"? from reply #39

I have an Excel program that simulates a PI. First attempt(_9) I changed A2 gain, left second delay at 4us and changed ground sample width to ground balance with a -1.3 slope. Moved the hole. Second attempt(_10) I changed A2 gain, reduced ground sample time by 100us/A2 gain and changed second sample time to ground balance. Hole stayed near 20us. Hole size doesn't change much. Guessing the method with the best S/N ratio is best.

Can attach the program if anyone is interested.

[green]

What op-amp is U3 & U4?
Possibly it is the input current causing the imbalance.
How is the signal at 'b'? Is the imbalance in U3 or in U4?
Try putting on U4's non-inverting input a series resistor to ground equal the the Thevenin equivalent of the input & feed-back resistance.

Changed U3 and U4 to LTC6244HV, no change in waveform. b is 1/10 of a. Changing R11 makes small changes in amplitude. If switch delay times are the same, cancels. Changing 800us delay to 590us almost cancels. Still don't see why the simulation doesn't cancel. Are we sure the real 2C integrator cancels?

[waltr]

Anyone have a thought why my spice simulation https://www.geotech1.com/forums/atta...8&d=1599836935 for the 2C integrator doesn't cancel the sine input?

Changing the value of R2 from 10k to 12.24k causes it to cancel. (Attached Image 2C integrator_sine2)

What op-amp is U3 & U4?
Possibly it is the input current causing the imbalance.
How is the signal at 'b'? Is the imbalance in U3 or in U4?
Try putting on U4's non-inverting input a series resistor to ground equal the the Thevenin equivalent of the input & feed-back resistance.