Preamp

[green]

It didn't work. The target and EF samples are the same so they cancel, no signal. Have to try something different.

[Old cart]

another option

Make a test signal to replace coil. Advantages: known signal amplitude, no EMI from coil pickup. Use a external 9 volt battery, connect preamp in across (R5) 1 ohm resistor, short for 1 micro volt signal, open for 10 micro volt signal. Depending on circuit might have to disconnect fet switch. Monitor post amp out with a scope for peak to peak volts. Connecting coil back up should give an indication of increase in noise from EMI, avalanche, etc. I'll give it a try, thinking about .15 seconds on and 1 to 2 seconds off.

I don't think that will work. With that small of a signal noise from the 555 will bleed into the input. To measure the preamp noise you could disable the transmitter and remove the coil then short with a resistor equal to the coil resistance. Of course you should also measure system noise with a coil attached and the transmitter running. This could be done with a small coil of the same inductance or a figure 8 coil. I suppose if one is dedicated you could make a small coil with matching LCR characteristics and then mount it inside a shielded enclosure ( faraday cage).

[green]

another option

Make a test signal to replace coil. Advantages: known signal amplitude, no EMI from coil pickup. Use a external 9 volt battery, connect preamp in across (R5) 1 ohm resistor, short for 1 micro volt signal, open for 10 micro volt signal. Depending on circuit might have to disconnect fet switch. Monitor post amp out with a scope for peak to peak volts. Connecting coil back up should give an indication of increase in noise from EMI, avalanche, etc. I'll give it a try, thinking about .15 seconds on and 1 to 2 seconds off.

[Old cart]

Measuring noise

When measuring noise it is a good idea to amplify the test amp with an external low noise amp. Oscilloscopes are not designed to make measurements on small signals at frequencies this low. Scope external preamps are available, but are rather expensive. For example the Tektronix ADA400 but it only works with modern TEK scopes.
Such a preamp should have very low noise and adjustable gain and bandwidth. Adjustable offset is also nice to have. This group could of course design such a preamp or we could just use the design of the ADA400 as a basis. The schematic is available.
A simpler, cheaper design is available here

http://tangentsoft.net/elec/lnmp/

This design is well documented and even includes gerbers so no real design work would be necessary. It does have one issue though and that is that the input impedance is quite low at 100ohms. To fix this would require a high impedance low noise buffer to be added before the first op amp.
It has several component options explained and should work well with scopes and true RMS multimeters.

Lastly last night I read a post by Eric Foster who said the coil, if used in such a noise test must be horizontal as most noise is polarized. Placing the coil vertical will allow more external noise to enter. If we use a pendulum with a long line, like 2 M, the arc that the target makes wil be very slight and should not affect the results.

[Sean_Goddard]

Like Monolith says, we need to standardise the test environment so we are all working to the same rules.

Once the test set up has been defined, we can then try different circuits to see which gives the best performance. I have no doubt that there are those out there who are laughing at what we are doing and thinking, "OMG, if ONLY they knew"! The point being that YES someone else has done this before, probably spent years on it, the difference here is that we intend to SHARE the results of our labour with the rest of the world, not hide it away in some company safe.

If nothing else we will learn more about how hard the design of a truly good machine is. We may also learn that maybe the simplest solution is the best and we have been overthinking the whole thing. Only ONE way we will tell. Theorise, experiment, record, improve and repeat!

What about amplifying the front end amplifier using a high end audio amp (the hard work was been done there) to the point where the noise stands out, then characterise it (if possible). As Monolith says, until we know the exact source of the problem, we can't do anything about it.

Whilst "Amplifying or multiplying does not reduce the noise", averaging does and what we are looking for is the same as a motion detector, a rapid dv/dt of the TARGET signal. if the noise is of a much higher frequency then we can filter it out, maybe some form of switched integrators are needed or a completely new type of circuit which as yet has not been designed (fat chance).

Just thought that a PLL could be used and the dv/dt of the target used to modulate a baseband to give a target that would be relatively noise free. Thoughts on this method? Perhaps the whole approach is wrong and we need to think outside the box a little more.

I'm loathed to put a micro anywhere near this sort of system as we are talking a lot of RFI from clocks etc so unless we use a DSP and then DSP out the noise the DSP introduces we are left with good ole analogue.

[Monolith]

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.

For the noise testing, we want to have a small target signal. Then look closely at the noise waveform.
Is it random?
Is it a specific frequency?
Are there several frequencies superimposed?
White noise or Johnson noise or Thermal noise should be totally random. However, when we filter and amplify this noise, we often accidentally create a synthetic frequency.
Once the noise is in the system it is difficult to take it out again. Stacking and integrating helps. Amplifying or multiplying does not reduce the noise.

[Monolith]

http://www.geotech1.com/forums/showt...935#post208935 (time constant chart)
I like coil vertical. Ran the TC for a 50x50mm al can, 10.7usec (will add to chart when I do some more targets), charted amplitude vs target distance for a US nickel and a 50x50mm al can. Integrator out volts*1million/300, preamp gain=300, integrator gain=1, sample delay 7usec, target sample 6usec.

Thanks for the link and info.
Interesting to see the influence of the skin effect on the can sample. The alu is a much better conductor than the nickel, but the TC ends up being nearly the same.
On the pendulum: I used to attach a piece of wood to the string, to give it some weight. Then the target clamped to the wood with a rubber band, in different positions. I could then let the pendulum swing a few times while looking at the signal on the scope.

[green]

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.

http://www.geotech1.com/forums/showt...935#post208935 (time constant chart)
I like coil vertical. Ran the TC for a 50x50mm al can, 10.7usec (will add to chart when I do some more targets), charted amplitude vs target distance for a US nickel and a 50x50mm al can. Integrator out volts*1million/300, preamp gain=300, integrator gain=1, sample delay 7usec, target sample 6usec.

[6666]

pendulum techniques

They work ok, but you can also use a computer fan, mount the coil horizontal , useing double sided foam tape mount a wooden arm to the blade part of fan, use a few d cells to power fan and adjust speed of rotation, , mount coin or whatever on end of arm and arm just rotates over coil.

[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.

[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]

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!

[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.

[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]

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?