Amplifier frequency response

[Monolith]

Test set-up

I like your ideas for the tester. I would like to see something anyone could make for the test setup. Been doing some testing with 6x6mm pieces cut from the side and bottom of an aluminum beverage can. Might consider them as targets. Targets glued to a piece of poster board 25x25mm. A picture would allow easy target size check. Was playing in the workshop trying a couple ideas. Maybe a wooden dowel or yard stick with a specified length suspended from a shaft with enough space below to raise and lower the coil to adjust target distance. Start with the dowel at some angle, maybe 90 degrees. Just some thoughts. Looking forward to your schematics and some way to compare the signal to noise I get with someone else. Thanks. Another thought was using the glass or plastic bottle. Some hot ground could be put in the bottle as it swung to check for GEB at the same time. Don't know how to get everyone using the same ground. Coil inductance, diameter, Tx waveform and peak current are a few things that would be nice to know.

Green, your tests have been very interesting and educational. One thing that seems to have evolved from them, is a "standard target" of a 1/4x1/4 inch can side, aluminum target. Maybe we should start with that. It is a target that anyone can easily make at no cost. Actually it took me quite a few tries to get the size reasonably close, so we probably need to allow for some tolerance. Inches? Millimeters? Mabe a set of targets, let's say 5x5mm, 10x10mm, 20x20mm
30x30mm?
The swinging target pendulum:
A glass bottle seems to work OK. A longish shaft tends to wobble more. Maybe a glass bottle filled with different kinds of sand, resistive, permeable etc. might be a good idea. The target stuck on the bottom with some double sided tape. The more the weight is concentrated on the end that is closest to the coil, the more stable it is. The longer the string that suspends it, the flatter the curve, but we are limited by the height of the ceiling.

The coil, used in the test above, is induction balanced. About 700uH inductance, 24cm diameter, wound with enamelled magnet wire AWG 19 or 20. The target was 35mm above the center of the coil housing, somewhere near 40mm from the coil wires, at the closest part of the swing. The coil sits on a plastic box about 44cm above the floor. All sorts of re-bars and copper water pipes in the reinforced concrete floor. I have not measured the coil current, as the PSU and Scope are non-isolated and make a short. Will setup a measuring scheme today. Also will work on the schematic today.

KingJL seems to have made some test circuit PCB's, I have not had the time to look at them yet.

[green]

Green, your tests have been very interesting and educational. One thing that seems to have evolved from them, is a "standard target" of a 1/4x1/4 inch can side, aluminum target. Maybe we should start with that. It is a target that anyone can easily make at no cost. Actually it took me quite a few tries to get the size reasonably close, so we probably need to allow for some tolerance. Inches? Millimeters? Mabe a set of targets, let's say 5x5mm, 10x10mm, 20x20mm
30x30mm?
The swinging target pendulum:
A glass bottle seems to work OK. A longish shaft tends to wobble more. Maybe a glass bottle filled with different kinds of sand, resistive, permeable etc. might be a good idea. The target stuck on the bottom with some double sided tape. The more the weight is concentrated on the end that is closest to the coil, the more stable it is. The longer the string that suspends it, the flatter the curve, but we are limited by the height of the ceiling.

The coil, used in the test above, is induction balanced. About 700uH inductance, 24cm diameter, wound with enamelled magnet wire AWG 19 or 20. The target was 35mm above the center of the coil housing, somewhere near 40mm from the coil wires, at the closest part of the swing. The coil sits on a plastic box about 44cm above the floor. All sorts of re-bars and copper water pipes in the reinforced concrete floor. I have not measured the coil current, as the PSU and Scope are non-isolated and make a short. Will setup a measuring scheme today. Also will work on the schematic today.

KingJL seems to have made some test circuit PCB's, I have not had the time to look at them yet.

A chart of some targets I posted awhile back. Didn't get the same on two 10x10mm targets cut from the side of the can. Maybe because I didn't cut them exactly the same, maybe one cut closer to the end than the other, one was a coke can the other something different. When we decide on size I could try to find out why. I like the 5x5, 10x10, 20x20 and 40x40mm targets. Gives some different TC's. The larger sizes are easier to cut to size. Target distance can make up for the smaller size. Do we need a way to determine EMI pickup vs circuit noise? Looking at your test in reply#41. I assumed 200msec/div and the bottle crossed the coil twice. Calculated pendulum length. Got about 43cm different than what you had. What am I doing wrong?

[Monolith]

You are correct, the target crossed twice. I think it is important to record the weight of the target and bottle, as well as the degrees of arc of the swing, if we want to calculate the speed at coil crossing.
To know the speed is important, as reference to the sweep speed of the coil in the field, as we use a band-pass filter related to the sweep speed. Official standard sweep speed is 1 meter/second.
The distance between the attachment point on the ceiling, to the target, is about 211cm.
Testing inside an inhabited building could be considered as worst case EMI environment.

I like to hunt down every individual noise source and try to eliminate it.

Let's settle on the 5,10,20,40mm sizes for alu can.

[green]

You are correct, the target crossed twice. I think it is important to record the weight of the target and bottle, as well as the degrees of arc of the swing, if we want to calculate the speed at coil crossing.
To know the speed is important, as reference to the sweep speed of the coil in the field, as we use a band-pass filter related to the sweep speed. Official standard sweep speed is 1 meter/second.
The distance between the attachment point on the ceiling, to the target, is about 211cm.
Testing inside an inhabited building could be considered as worst case EMI environment.

I like to hunt down every individual noise source and try to eliminate it.

Let's settle on the 5,10,20,40mm sizes for alu can.

I calculate 2.92 seconds for the pendulum period. Can you explain where sec/div is on your scope picture and what the numbers in the lower left screen mean. Thanks The test is showing signal to noise. Everyone is going to have different EMI, I think it would be nice to show noise level with and without EMI. Maybe a resistor across the amplifier input instead of the coil for circuit noise?

[Old cart]

Yes or possibly use a noise cancelling coil, like a figure 8. As long as the target is small it should really only affect 1/2 of the coil.

[Monolith]

Yes or possibly use a noise cancelling coil, like a figure 8. As long as the target is small it should really only affect 1/2 of the coil.

Thanks for the feedback.
Are you using a figure 8 coil in the field?

For a lab circuit noise test it might be a good idea.

[Monolith]

I calculate 2.92 seconds for the pendulum period. Can you explain where sec/div is on your scope picture and what the numbers in the lower left screen mean. Thanks The test is showing signal to noise. Everyone is going to have different EMI, I think it would be nice to show noise level with and without EMI. Maybe a resistor across the amplifier input instead of the coil for circuit noise?

Thanks for the feedback.

The numbers on the left: The positions of the cursor lines and channel 2 are of no significance on the picture. The red trace is the static signal, at 100mV/div, the blue line is the same red trace signal, saved, when the target is swinging above the coil.
Time is 200ms/div, (M:200ms) acquire setting on sample, no averaging.

I think the actual speed of the target crossing the coil is the significant part. The weight of the target and bottle and the degrees of arc of the swing play a role in that speed. Need to look for the formula to calculate that.

[Old cart]

Thanks for the feedback.
Are you using a figure 8 coil in the field?

For a lab circuit noise test it might be a good idea.

I do not use a figure 8 in the field. I thought this would be good for testing.

[Old cart]

Thanks for the feedback.

The numbers on the left: The positions of the cursor lines and channel 2 are of no significance on the picture. The red trace is the static signal, at 100mV/div, the blue line is the same red trace signal, saved, when the target is swinging above the coil.
Time is 200ms/div, (M:200ms) acquire setting on sample, no averaging.

I think the actual speed of the target crossing the coil is the significant part. The weight of the target and bottle and the degrees of arc of the swing play a role in that speed. Need to look for the formula to calculate that.

Good idea.
Those formulas are here:
http://www.school-for-champions.com/...m#.VzntCtQ8KrU

[bbsailor]

For those who are analyzing amplifiers that integrate many RX pulses that are synchronized with the TX pulses, as all Pulse induction (PI) detectors do, are utilizing a technology that is used to extract very weak signals in the presence of noise called lock-in amplifiers: https://en.wikipedia.org/wiki/Lock-in_amplifier. How long the target stays within the coil sensitive area while the coil is being swept across the ground will dictate how many RX signals are being integrated and how sensitive the target response will be. Eric Foster was one of the early adopters of this technology in his low power PI machines. He designed PI machines rated in the 3K Pulse Per Second region and calculated that with a slow sweep speed that he would integrate between 1000 to 1500 pulses depending on the actual user sweep speed and coil size. See the above web link to see how lock-in amplifiers and integrated Pulse Induction metal detector RX signals are very similar and affects the sensitivity to a variety of targets.

With modern microprocessor controlled PI machines the TX rate, delay, as well as the RX window width can be adjusted and optimized for the electrical characteristics of the primary targets sought. However, the ground conditions may not allow the optimum TX and RX parameters to be used so the ground needs to be analyzed first.

Joseph J. Rogowski

[green]

An attempt at calculating offset. Pendulum length=211cm, period=2.92sec, offset for 1meter/sec=46cm. Is 211cm a good length for everyone? Has anyone else done the calculations?

[Monolith]

Good idea.
Those formulas are here:
http://www.school-for-champions.com/...m#.VzntCtQ8KrU

Thanks for the link. So I was wrong, when saying that the weight of the pendulum plays a role.

[Monolith]

The blue trace is the 5x5mm alu can target swinging over a 25cm coil. Distance to coil TX wire, about 8mm. To the coil RX wire about 11mm. The target is only visible when passing the coil wires.

The red trace is the no target signal. Very noisy.

Next thing is to clean up some noise.

[green]

Thanks for the link. So I was wrong, when saying that the weight of the pendulum plays a role.

Hi Monolith, Got my pendulum hung. I get about the same as you for time between pulses. Had a brain cramp and was reading time between peaks when I should have read time between every other peak because of the capacitor coupling.

[Chet]

I think the 5, 10, 20, 40mm sizes for aluminum can and pendulum arangement are good standards. Can you recommend an output level to use as a standard for an acceptable detection? This could be something like a signal to noise ratio at the final audio output jack across a standard resistor value. This would allow comparisons between commercially built detectors that have no access or difficult access to the internal circuity.
For do-it-yourself detectors a similar signal to noise ratio level could be established for the output of the receiver and/or the integrator.


Have a good day,
Chet