PI noise

[Mechanic]

I have always found that coil noise is far in excess of amplifier noise and that is the limiting factor.

Eric.

Hi Eric,

I would have to disagree on this. Out in the field, sure some days/times of days the emi received is higher than the amplifier noise floor noise(listening to the output noise, not looking at the front end with a scope). However, on other days or late in the day/night time the external emi seems to quieten down to the point where you really only hear noise floor noise of the detector and front end gain can really be cranked up. If I were to be using a 5534 and 1k input resistors, I could not increase the front end gain and get a usable advantage, no matter what the external emi was doing. And this extra gain does make a big difference in target response for both small bits that would not otherwise be heard even if rubbed on the coil and for larger bits at depth.

Cheers Mick

[mickstv]

Green, what I'll suggest is instead of trying to reduce this small noise issue, build your backend audio stages and test the detector out in the field.

If you build a backend that has a thresold control and variable pitch audio, like the circuit I've posted a few times in other threads, the internally generated noise won't be an issue compared to external emi.


PS like Mechanic said get rid of the 1k input either use blocking fets, or be lazy like me and just use DD coils. Plus with ML DD coils you don't need blocking fets so you can sample sooner.

[green]

Hi Green and Mechanic,
The shield does add capacitance and you can see how much by measuring the change in resonant frequency and calculating back. However, the shield has two functions; one is to present a grounded barrier between the coil and the ground surface i.e. soil, wet sand, or a poke with a finger. This prevents spurious capacitive effects with any outside surface, and hence false signals. These are particularly noticeable when searching in wet grass or seaweed on a wet ocean beach. The other function is to act as a shield for r.f. interfering signals from about 1MHz to 100kHz. Unfortunately graphite paint is not very good at the latter, unless you put on several coats. It is best to use as high a conductivity shield as possible but stopping well short of the point where you are detecting the shield material and have to go to a longer delay. Always have a gap in the shield so as not to create a shorted turn coupled to the coil. With paint shields you don't have to bother as the conductivity is relatively low.

About the coil angle affecting pickup noise, it all depends on the source of the noise and how near it is. Vertical orientation of the coil is worst, but if you rotate the vertical coil you can find a minimum point. A horizontal coil is much better, but that position may not be quite the best, so tilting it at an angle one way or another may give you the best null. It all depends on the polarisation of the interfering signal, inside buildings it can be distorted. Outdoors is best, or in a building with little metal.

I have always found that coil noise is far in excess of amplifier noise and that is the limiting factor.

Eric.

Hi Eric

Your comment on coil noise far exceeding amplifier noise surprised me since I didn't remember seeing a difference in post amplifier out with the preamp input shorted or the figure 8 Rx coil connected. Recorded some scope traces with the preamp input shorted, connected to a 8 inch mono and connected to the figure 8 Rx coil. The coils are over 40 inches from the scope. A lot if not most of the EMI is coming from the scope. With the figure 8 coil the preamp noise did increase over input shorted but little change in post amplifier out. The preamp and post amplifier both increased with the mono coil. The mono coil had a lot of 60Hz signal at post amplifier out. Maybe the scope or my bench setup is causing a lot of the mono coil noise increase.

[Ferric Toes]

I suspect that some of the difference with emi reception with and without shield is due to the extra capacitance added to the coil from the shield, thus lowering the response to high frequencies.

An interesting experiment to try would be figure out how much capacitance the shield adds, then add that capacitance(make sure the capacitor is rated for high enough voltage) to an unshielded coil and see what the received emi signal is. Of coarse this won't fix the hand capacitance thing you see, it will just show if it is the shield material blocking the emi, or just the extra capacitance.

When you say you have been shorting the coil, I think you mean that you have disconnected the receive wire from the coil and shorted that to rx gnd and left the coil still transmitting?

Eric, that is an interesting observation you have with the coil being just off horizontal to pick up the least amount of noise. I will have to try this sometime. I always thought it was when the coil was horizontal that the least emi was picked up.

Cheers Mick

Hi Green and Mechanic,
The shield does add capacitance and you can see how much by measuring the change in resonant frequency and calculating back. However, the shield has two functions; one is to present a grounded barrier between the coil and the ground surface i.e. soil, wet sand, or a poke with a finger. This prevents spurious capacitive effects with any outside surface, and hence false signals. These are particularly noticeable when searching in wet grass or seaweed on a wet ocean beach. The other function is to act as a shield for r.f. interfering signals from about 1MHz to 100kHz. Unfortunately graphite paint is not very good at the latter, unless you put on several coats. It is best to use as high a conductivity shield as possible but stopping well short of the point where you are detecting the shield material and have to go to a longer delay. Always have a gap in the shield so as not to create a shorted turn coupled to the coil. With paint shields you don't have to bother as the conductivity is relatively low.

About the coil angle affecting pickup noise, it all depends on the source of the noise and how near it is. Vertical orientation of the coil is worst, but if you rotate the vertical coil you can find a minimum point. A horizontal coil is much better, but that position may not be quite the best, so tilting it at an angle one way or another may give you the best null. It all depends on the polarisation of the interfering signal, inside buildings it can be distorted. Outdoors is best, or in a building with little metal.

I have always found that coil noise is far in excess of amplifier noise and that is the limiting factor.

Eric.

[green]

I suspect that some of the difference with emi reception with and without shield is due to the extra capacitance added to the coil from the shield, thus lowering the response to high frequencies.

An interesting experiment to try would be figure out how much capacitance the shield adds, then add that capacitance(make sure the capacitor is rated for high enough voltage) to an unshielded coil and see what the received emi signal is. Of coarse this won't fix the hand capacitance thing you see, it will just show if it is the shield material blocking the emi, or just the extra capacitance.

When you say you have been shorting the coil, I think you mean that you have disconnected the receive wire from the coi and shorted that to rx gnd and left the coil still transmitting?

Eric, that is an interesting observation you have with the coil being just off horizontal to pick up the least amount of noise. I will have to try this sometime. I always thought it was when the coil was horizontal that the least emi was picked up.

Cheers Mick

Don't know why but the decay oscillation goes away with the added shield or if I touch the scope ground without the shield.

You are correct about shorting the coil connections without the coil but I have been disabling the command to the fet driver.

[Mechanic]

I suspect that some of the difference with emi reception with and without shield is due to the extra capacitance added to the coil from the shield, thus lowering the response to high frequencies.

An interesting experiment to try would be figure out how much capacitance the shield adds, then add that capacitance(make sure the capacitor is rated for high enough voltage) to an unshielded coil and see what the received emi signal is. Of coarse this won't fix the hand capacitance thing you see, it will just show if it is the shield material blocking the emi, or just the extra capacitance.

When you say you have been shorting the coil, I think you mean that you have disconnected the receive wire from the coi and shorted that to rx gnd and left the coil still transmitting?

Eric, that is an interesting observation you have with the coil being just off horizontal to pick up the least amount of noise. I will have to try this sometime. I always thought it was when the coil was horizontal that the least emi was picked up.

Cheers Mick

[green]

When you test with a coil, is the coil shielded? Quickly going through the thread, there is only a brief mention of coil shielding in 13. I have found that coil shielding makes a large difference in the r.f. noise pickup as seen on an analog scope. Also, there is an orientation angle for the coil where you can find a minimum in r.f. noise. Here, it is a few degrees off the horizontal.

Eric.

Thanks. I have been painting the coil with graphite paint. With no shielding I see some oscillation during decay that I don't see with the shielded coil. Shielding with the graphite paint is eliminating the change in output when I touch the coil. Didn't think it was shielding r.f. noise maybe wrong. Testing for amplifier noise with the coil shorted.

[green]

Hi Green,

After changing things back to the way they were meant to be, the 797 seems comparable to the lm394/5534 preamp.

If the preamp noise is high, and you also use 1k input impedance, the output noise from the detector will always be high/ bad signal to noise ratio. Also, you need to make sure you are using a fast opamp otherwise it takes too long(longer than what it could be) to settle after the tx, thus delaying when you can take your first sample.

I once did an experiment and replaced the front end fets(ML) with a 1k resistor and clamping diodes and the jump in output noise was amazing! I could have lowered the gain of the preamp to get the noise back to the same level, but in doing so depth and sensitivity would be compromised.

Testing for noise with a mono coil(or a DD for that matter) in suburbia will always be too difficult due to the electrical interference. The best way is to tackle the system noise and then when you are away from interference, this is the best case of how the detector will run.
The reason a DD coil will seem quieter is that it has less receive area for a mono of the same size, ie the receive of the DD is half the size of the coil.
However, there is the case where something in the tx cct is not quite right and the receive picks this up with a mono coil as the receive is directly coupled to the tx cct, whereas a DD the receive is isolated from the tx cct.

Cheers Mick

Hi Mick
Thanks. The 1k input resistor is causing a lot of my noise problem. Need to try fet switches again. I've been shorting the coil to compare noise levels for the amplifiers.

[Ferric Toes]

Hi Mick

Did you learn anything over the weekend? I tried the mono coil and amplifier again. Having a hard time lowering the noise with a mono coil. A mono coil would be a lot easer to make than the one I use with the differential amplifier.

green

When you test with a coil, is the coil shielded? Quickly going through the thread, there is only a brief mention of coil shielding in 13. I have found that coil shielding makes a large difference in the r.f. noise pickup as seen on an analog scope. Also, there is an orientation angle for the coil where you can find a minimum in r.f. noise. Here, it is a few degrees off the horizontal.

Eric.

[Mechanic]

Hi Green,

After changing things back to the way they were meant to be, the 797 seems comparable to the lm394/5534 preamp.

If the preamp noise is high, and you also use 1k input impedance, the output noise from the detector will always be high/ bad signal to noise ratio. Also, you need to make sure you are using a fast opamp otherwise it takes too long(longer than what it could be) to settle after the tx, thus delaying when you can take your first sample.

I once did an experiment and replaced the front end fets(ML) with a 1k resistor and clamping diodes and the jump in output noise was amazing! I could have lowered the gain of the preamp to get the noise back to the same level, but in doing so depth and sensitivity would be compromised.

Testing for noise with a mono coil(or a DD for that matter) in suburbia will always be too difficult due to the electrical interference. The best way is to tackle the system noise and then when you are away from interference, this is the best case of how the detector will run.
The reason a DD coil will seem quieter is that it has less receive area for a mono of the same size, ie the receive of the DD is half the size of the coil.
However, there is the case where something in the tx cct is not quite right and the receive picks this up with a mono coil as the receive is directly coupled to the tx cct, whereas a DD the receive is isolated from the tx cct.

Cheers Mick

[green]

HI Eric,


I have tried several lme9990 and found that they all take quite a bit longer to settle(think it was up to 5 or 7us) than the AD797. Though a mate of mine was able to use the lme49990 and get it to settle earlier than I could with the same input impedance and gain, go figure! Though he was using a different coil...
When I did noise tests, I could not see any notable difference between the 797 and the lme. Same goes for using an lm394/5534 preamp. This settles as fast as the 797 and there seems to be no notable output noise difference. Though one thing I am still to try, from one of my initial tests, it looks as though the 797 is not as sensitive as the lm394/5534 preamp, however some other things were set up differently. Currently changing back those differences and will test on the weekend. I will know more after the weekend..

Cheers Mick

Hi Mick

Did you learn anything over the weekend? I tried the mono coil and amplifier again. Having a hard time lowering the noise with a mono coil. A mono coil would be a lot easer to make than the one I use with the differential amplifier.

green

[green]

[ATTACH]36592[/ATTACH]

Attached is Eric's coil chart. This will give you an idea for choosing the best coil diameter for the Nickel.

Are you using the 2 RX coils in differential mode?

I had seen Eric's chart before but I don't remember reading the explanation with it. I ran a test with three different size coils, 200mm/1.5, 200mm and 200mm x 1.5 that I posted in another thread awhile back. Same current profile for all three coils. Instead of same number of turns I multiplied amplitude reading by 300uH/coil inductance for each coil. Same inductance instead of same turns. The results look similar except signal out is the same at a distance half way between their radius. 200mm coil, 300mm coil((100+150)/2=125mm))crossing.

[Monolith]

coil calculations

If I read Eric's chart correctly a 30 inch diameter coil would give the best signal for any target at 15 inches. Maybe a diameter between 20 and 30 inches for the best S/N.

The Rx coils are in differential mode.

What is the answer to the ring at 20 inches with a 10 inch coil question?

Attached is a .pdf with links to calculators that are very useful for all sorts of calculations for metal detectors. Also screenshots with some calculations for an 8inch coil and a 14inch coil

[green]

[ATTACH]36592[/ATTACH]

Attached is Eric's coil chart. This will give you an idea for choosing the best coil diameter for the Nickel.

Are you using the 2 RX coils in differential mode?

If I read Eric's chart correctly a 30 inch diameter coil would give the best signal for any target at 15 inches. Maybe a diameter between 20 and 30 inches for the best S/N.

The Rx coils are in differential mode.

What is the answer to the ring at 20 inches with a 10 inch coil question?

[Monolith]

Not sure what the 2Hz oscillation is. It's not always there, comes and goes. The butterworth filter overshoots at about that frequency with a step input. Next order I'll add a Bessel filter and try it. Sampling at 1kHz, about 10usec target and EF sample.

The noise is a lot lower with the integrator input connected to common, so I'm thinking the switches aren't causing the problem.

More scope traces comparing the different output stages in(Amplifier frequency response)thread. Look at the enclosed schematic. Integrator cutoff 16Hz low pass, two more stages with a 16Hz low pass cutoff. More on the added filtering in(Amplifier frequency response). I don't know what the best frequency cutoff is. A scope trace of target signal and noise should give a good indication.

I use a bottle of water for the pendulum. I caught it on the return swing so at least half of the trace was noise.

When I first joined this site I read a reply in a thread that stated if you can't detect a US nickel at 15 inches keep looking. From my testing a mono coil gives the best signal but so far I haven't been able to get the best signal to noise using a mono coil. Could probably get there using what I have by increasing coil diameter and peak current. Or by lowering the noise.

Stated the Rx coils were 8 inch. They are spider web(flat basket)wound, 8 inch I.D. so the mean diameter is about 8.3 inches

Eric's coil chart.pdf

Attached is Eric's coil chart. This will give you an idea for choosing the best coil diameter for the Nickel.

Are you using the 2 RX coils in differential mode?