Preamp

[Sean_Goddard]

Just an idea, but what if we take TWO simultaneous samples, one long and one short then process them (probably by subtraction) that would give us N(oise) + s(ignal) from which we could then average one to get S(ignal) then maybe subtract (invert one signal and add)?

Just a suggestion. I have yet to work out the semantics of how this could be best done.

Dave, I would like to use the 9ct gold ring as the target reference. if we can get a good response on that then we should get good results overall. I'm aiming to 12" in wetsand on that target. I think we can get that if we solve the stable target sample issue as the rest is just a matter of gain in the later stages IMHO.

I have also been looking at this problem from a power supply point of view. Has anyone else monitored the supply lines of the front end amp to see if there is "bounce" on them? This would imply we have to use a front end with a high PSRR.

One final thought, eveyone else samples VOLTAGE, why not sample TIME? What I mean is that setting a threshold and measuring the time taken to decay back to that (simple comparator). I wonder if there is any mileage in that as a discrimination method as we know dv/dt curves are a function of conductivity . Again, just a thought.

[Monolith]

Just an idea, but what if we take TWO simultaneous samples, one long and one short then process them (probably by subtraction) that would give us N(oise) + s(ignal) from which we could then average one to get S(ignal) then maybe subtract (invert one signal and add)?

Just a suggestion. I have yet to work out the semantics of how this could be best done.

Dave, I would like to use the 9ct gold ring as the target reference. if we can get a good response on that then we should get good results overall. I'm aiming to 12" in wetsand on that target. I think we can get that if we solve the stable target sample issue as the rest is just a matter of gain in the later stages IMHO.

I have also been looking at this problem from a power supply point of view. Has anyone else monitored the supply lines of the front end amp to see if there is "bounce" on them? This would imply we have to use a front end with a high PSRR.

One final thought, eveyone else samples VOLTAGE, why not sample TIME? What I mean is that setting a threshold and measuring the time taken to decay back to that (simple comparator). I wonder if there is any mileage in that as a discrimination method as we know dv/dt curves are a function of conductivity . Again, just a thought.

One final thought, eveyone else samples VOLTAGE, why not sample TIME? What I mean is that setting a threshold and measuring the time taken to decay back to that (simple comparator). I wonder if there is any mileage in that as a discrimination method as we know dv/dt curves are a function of conductivity . Again, just a thought.
The way to do that is by using the time to define the TC of the target.
Once we use the TC of the target instead of the amplitude, we have a GROUND BALANCE.

Explaining: When we use the amplitude of the target response, changing the distance of the ground or/and target, changes the amplitude of the response.
When we define the target as to it's TIME CONSTANT or TC, the distance does not change the signal response. Therefore, we have a fundamental GROUND BALANCE.

After all, we keep saying our PI technology is in the time domain. So why not use the time to define our target?

WARNING:We need to discuss this GROUND BALANCE METHOD EXTENSIVELY, so that everybody knows that it is in the public domain and can therefore not been patented.

[Teleno]

My proposal for ground balance: http://www.geotech1.com/forums/showt...718#post208718

[Sean_Goddard]

Yes Teleno, EXCELLENT and Monolith, I agree 100%. So get those ideas out there.

Now back to the noise. Have we identified where it is coming from? Has anyone put a 'scope on the preamp output to see what the signal is like from there?

My first thought is winding movement in the coil. Can someone pot a coil in resin in such a way that the winding CANNOT MOVE then we can see if that helps. After all we are dealing with microvolts here, less in some cases. I have to ask others to do these things as I work away from home and do not have access to my workshop or I would be doing the work myself and posting results on here.

I feel the better we can get the Rx signal the better depth we can get, remember, garbage in, garbage out. I have attached a "first thoughts" schematic on how to remove some noise with a short and long sample, but in the first instance we need to reduce the front end noise to a level where everything else is just polishing the corners.

Teleno, my maths is terrible but I see how you are doing the ground balance. Two samples with different contents. Since the ground is "constant" (in theory) then it should be possible to "null" it out?

What about bipolar pulsing to negate the ground? The theory being that a target will have little residual when non ferrous, but WILL have some residual when ferrous (BH curve) due to the RELUCTANCE of the target to have its magnetic polarity rapidly changed. Get the idea?

[6666]

Sean any chance you could repost a larger schematic please, when you zoom in to read values the picture pixelates

[Teleno]

Now back to the noise. Have we identified where it is coming from?

Mainly fron this:



The hgher the resistance, the higher the noise (increase follows a root square law)

[green]

Yes Teleno, EXCELLENT and Monolith, I agree 100%. So get those ideas out there.

Now back to the noise. Have we identified where it is coming from? Has anyone put a 'scope on the preamp output to see what the signal is like from there?

My first thought is winding movement in the coil. Can someone pot a coil in resin in such a way that the winding CANNOT MOVE then we can see if that helps. After all we are dealing with microvolts here, less in some cases. I have to ask others to do these things as I work away from home and do not have access to my workshop or I would be doing the work myself and posting results on here.

I feel the better we can get the Rx signal the better depth we can get, remember, garbage in, garbage out. I have attached a "first thoughts" schematic on how to remove some noise with a short and long sample, but in the first instance we need to reduce the front end noise to a level where everything else is just polishing the corners.

Teleno, my maths is terrible but I see how you are doing the ground balance. Two samples with different contents. Since the ground is "constant" (in theory) then it should be possible to "null" it out?

What about bipolar pulsing to negate the ground? The theory being that a target will have little residual when non ferrous, but WILL have some residual when ferrous (BH curve) due to the RELUCTANCE of the target to have its magnetic polarity rapidly changed. Get the idea?

Monolith had some good questions about noise in reply #39. To start leave the coil stationary(no EF signal). Target signal probably less than 10 Hz. Including a scope trace of integrator out sweeping a US nickel across the coil. The bottom width varied from 100 to 200 milliseconds sweeping the coin at speeds I thought would simulate sweeping the coil. The PI post amplifier circuits I've looked at roll of at 10Hz or less. Think we need to look at post amplifier out when looking for microvolt noise at the coil. Still trying to answer some of Monolith"s questions.

[Monolith]

Monolith had some good questions about noise in reply #39. To start leave the coil stationary(no EF signal). Target signal probably less than 10 Hz. Including a scope trace of integrator out sweeping a US nickel across the coil. The bottom width varied from 100 to 200 milliseconds sweeping the coin at speeds I thought would simulate sweeping the coil. The PI post amplifier circuits I've looked at roll of at 10Hz or less. Think we need to look at post amplifier out when looking for microvolt noise at the coil. Still trying to answer some of Monolith"s questions.

This looks like a perfect target response. No noise visible.
Now, if we could compare this signal with the signal at extreme distance from the coil, we would see where the signal disappears into the noise.
What are the frequencies included in that noise? The frequencies will give us hints of where the noise comes from.

By the way, I like to hang my target on a string from the ceiling. The target swinging like a pendulum past the coil gives the near equivalent of a steady sweep speed. A target in the hand is strongly influenced by the response signal of the hand itself.

The "standard" sweep speed for de-mining, is 1m/second. We use this as a very approximate standard as the ideal sweep speed is also related to the coil diameter.
Considering a time lag of 100ms from the peak target detection to the audible signal, would be a distance of 10cm. Spelled out, this means we hear the indication of the target when the center of the coil is 10 centimeters (4") past the target. Still OK for pin-pointing with a 30cm diameter coil.
A large time lag makes pin-pointing very difficult.

[Sean_Goddard]

I will do that this evening. Sorry about the size of the drawing, I'll have to redraw it as I didn't save the original *face-palm*. Values are largely irrelevant at this stage and will need tweaking.

Regarding the noise Teleno, can we activley damp the coil and will this help reduce it? I know another manufacturer cites avalanche noise from the FET as a problem which is why they use a snubber. Also those front end diodes look like a nice noise source too, can we eliminate those, maybe by actively switching out the high voltage portion of the Rx signal, and only switching through a voltage which will not either saturate the front end amp and is low enough to process directly.

I think the stage which can take the high voltages, like a common, base arrangement (nice low input capacitance) suggested earlier, along with active dampling and only switching the required section of the signal through to the front end amp is the way to go. Unless anyone has any better ideas, can we pursue this line of design please?

[Old cart]

This looks like a perfect target response. No noise visible.
Now, if we could compare this signal with the signal at extreme distance from the coil, we would see where the signal disappears into the noise.
What are the frequencies included in that noise? The frequencies will give us hints of where the noise comes from.

By the way, I like to hang my target on a string from the ceiling. The target swinging like a pendulum past the coil gives the near equivalent of a steady sweep speed. A target in the hand is strongly influenced by the response signal of the hand itself.

The "standard" sweep speed for de-mining, is 1m/second. We use this as a very approximate standard as the ideal sweep speed is also related to the coil diameter.
Considering a time lag of 100ms from the peak target detection to the audible signal, would be a distance of 10cm. Spelled out, this means we hear the indication of the target when the center of the coil is 10 centimeters (4") past the target. Still OK for pin-pointing with a 30cm diameter coil.
A large time lag makes pin-pointing very difficult.

Having a standard test method is a great idea. If we standardized pendulum length, swing width, and targets that would help a lot. Also a standard test coil is pretty much a necessity. Any thoughts on how we could accomplish this and what the standards should be?

[green]

Having a standard test method is a great idea. If we standardized pendulum length, swing width, and targets that would help a lot. Also a standard test coil is pretty much a necessity. Any thoughts on how we could accomplish this and what the standards should be?

Suggestion, pendulum length 2 meters, swing width .45 meters(.9 p-p), US nickel, 200mm coil. Someone to check my math. Ability to adjust target height above coil. Record(scope trace)of post amplifier out at target distance,(1xcoil radius)(2xcoil radius)(3xcoil radius)(4xcoil radius). Compare target signal with noise level. Maybe a 50mmx50mm piece of aluminum can for a target(easier to get for everyone)

[Old cart]

One final thought, eveyone else samples VOLTAGE, why not sample TIME? What I mean is that setting a threshold and measuring the time taken to decay back to that (simple comparator). I wonder if there is any mileage in that as a discrimination method as we know dv/dt curves are a function of conductivity . Again, just a thought.
The way to do that is by using the time to define the TC of the target.
Once we use the TC of the target instead of the amplitude, we have a GROUND BALANCE.

Explaining: When we use the amplitude of the target response, changing the distance of the ground or/and target, changes the amplitude of the response.
When we define the target as to it's TIME CONSTANT or TC, the distance does not change the signal response. Therefore, we have a fundamental GROUND BALANCE.

After all, we keep saying our PI technology is in the time domain. So why not use the time to define our target?

WARNING:We need to discuss this GROUND BALANCE METHOD EXTENSIVELY, so that everybody knows that it is in the public domain and can therefore not been patented.

Sampling the time where a signal crosses a voltage has been done before. That having been said if can be a better method. Whether it works better depends on two things.

1. The slope of the signal where it is being measured.
2. The resolution of the time/ voltage measuring method.

Since measuring time generally requires a counter then a micro processor would probably have to be incorporated.

One last comment, perhaps the best possible method would measure both time AND voltage. This could be accomplished by digitizing the entire waveform in high resolution, say 16-24 bits of vertical resolution at a sample rate of 1-10 Ms/S ( one sample every 100 nS). But this would require a fairly high cost A to D converter and significant signal processing capability. By doing this you could have.

1. Simple and automatic ground balance.
2. Discrimination of some form, particularly if you digitized the transmit part of the waveform.
3. Adaptive noise reduction.
4. Swing speed compensation.
5. Completely automatic operation.

And lots more.

Since most everything after the preamp and A-D converter is software most every parameter could be changed via software, even if you were in the field.
However, this is probably way beyond the scope of this project. Just some ideas to consider.

[Old cart]

Suggestion, pendulum length 2 meters, swing width .45 meters(.9 p-p), US nickel, 200mm coil. Someone to check my math. Ability to adjust target height above coil. Record(scope trace)of post amplifier out at target distance,(1xcoil radius)(2xcoil radius)(3xcoil radius)(4xcoil radius). Compare target signal with noise level.

I have not done the math but those seem like reasonable numbers. What we are interest in is SIGNAL to NOISE RATIO so what needs to be measured is the amplitude of the signal at the peak divided by the amplitude of the noise where there is no target. This should be done with a constant bandwidth. This can be tricky since the final stages of the detector have limited bandwidth,say 10 HZ and the preamp itself has high bandwidth, say 100KHz. Further the bandwidth of the detection device (oscilloscope) plays a large role in the noise level. Different model have different noise levels and that also varies as gain is changed. Ultimately it is probably not practical to accurately deal with all these effects so I suggest this.
20 MHz scope bandwidth limit
200mm coils
Pendulum as above
Scope Sensitivity to almost fill the screen vertically with the signal, say 8 divisions or so.
Care taken to avoid external noise caused by emi sources like compact fluorescent lamps, a cell phone in the room, wifi routers and other wireless devices, and local radio and tv transmitters.

This last item is a huge issue. Maybe it would be better define a "dummy" coil that everyone could use that wants to participate. Eric Foster used to use a figure 8 coil that was just a twisted mono coil that formed two equal and balanced loops that rejected external noise. This can get quite complicated!

[Monolith]

Suggestion, pendulum length 2 meters, swing width .45 meters(.9 p-p), US nickel, 200mm coil. Someone to check my math. Ability to adjust target height above coil. Record(scope trace)of post amplifier out at target distance,(1xcoil radius)(2xcoil radius)(3xcoil radius)(4xcoil radius). Compare target signal with noise level. Maybe a 50mmx50mm piece of aluminum can for a target(easier to get for everyone)

Many people swing the coil in a 3 dimension arc. Left to right but also up at the ends of sweep and down at the center. It is not the best way.
Swinging the pendulum above the coil kind of imitates that motion.
If we fix the coil in the vertical position instead of the horizontal position, the pendulum swing passes the coil in a parallel motion, maintaining an even distance.

I seem to remember that you posted charts of various surface area size aluminium can pieces, could you please point me to the thread?

The surface area of the target, versus the coil surface area is a very important relationship. Also important is to have an estimate of the actual wire coil diameter/radius. With a 200mm coil, it makes quite a difference in surface area, if the actual wire is 170mm or 180mm while the outer coil housing diameter is 200mm.

[Old cart]

Monolith, that is genius. Maybe these pendulum techniques should be reserved for depth testing and response speed testing. I am having more reservations about their validity for noise testing.