PI noise

[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.

[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.

[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.

[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]

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.

[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]

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.

[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.

[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

[Qiaozhi]

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

What Eric actually stated was: "I have always found that coil noise is far in excess of amplifier noise and that is the limiting factor."
He never mentioned EMI.

[Mechanic]

What Eric actually stated was: "I have always found that coil noise is far in excess of amplifier noise and that is the limiting factor."
He never mentioned EMI.

Hmmmm, coil noise, do we have a definition for that one? I translated that as the noise that the coil picks up is greater than the amplifier noise.

Cheers mick

[Qiaozhi]

Hmmmm, coil noise, do we have a definition for that one? I translated that as the noise that the coil picks up is greater than the amplifier noise.

Cheers mick

I read Eric's statement to mean that any noise generated by the amplifier would be low compared to when the coil is connected (in an EMI-free environment).
Eric will no doubt enlighten us further.

[moodz]

Not sure what the debate here is ... Coil noise is way bigger than resistor noise in all cases except bad design :-).

EMI is everywhere so the no-EMI measurement is not relevant.

PIX shows the comparison of shielded coil ( yellow ) vs 1K resistor ( blue ).

[green]

Thanks for the replies.

I have tested the detector in the yard. The ground signal is a lot higher than amplifier noise. The detector isn't usable without GEB. Noise goes up when I add GEB so I'm trying to minimize the noise.

I wonder why some detectors add switches to eliminate the 1k input resistor if coil noise is always higher than 1k resistor noise.

I'm learning so what I'm thinking might not be correct. Target signal is less than 10Hz, ground and EF maybe closer to 1Hz so maybe other than line frequency(50 or 60Hz)most other frequencies are above the sampling rate, if not they would be filtered anyway. The maximum frequency after sampling is 1/2 the sampling frequency. If we filtered all frequencies above 10Hz the only higher frequencies that would matter are 10Hz and below and within 10Hz of multiples of the sampling frequency. Sample rate 1kHz(1010,2010,3010,3990 etc.) 5kHz(4990,10010,19990,30010 etc.) maybe one of the reasons higher sampling frequency might have less noise, fewer frequencies to alias on.

[Ferric Toes]

Eeek! Maybe I was too economical with words again. Coil noise is EMI. EMI includes power line noise, radio stations, mobile phones, lightning, static generated by hot windblown sand, waves crashing on a seashore, micropulsations in the earth's field; in fact any signal that blows into the detector via the coil when the sampling gates are open. Except of course, the coherent, response from a metal target. You might question the waves thing, but, you have rotating conductive water in the earth's magnetic field. Yes, EMI is generated. A coil of even a few ohms resistance generates little noise of itself compared to the preamp input resistor.

A mono coil picks up a large amount of emi even if well shielded. I use one of my 11in shielded mono coils as an antenna on my workshop radio. Noticeable attenuation only comes in at upward of 7MHz. A figure 8 coil is good for workshop testing but even that becomes unbalanced upwards of 50kHz unless you are very careful about each half being an exact mirror image of the other. This can be overcome with printed circuit coils as I did for an industrial application a while back. This was a four layer board with electrostatic comb shielding as top and bottom layers. Not something for hobby experimentation though.

Another technique is to use a mono coil as TX/RX1 and an identical mono coil mounted rigidly above it as an RX2. TX/RX1 is fed to one preamp input, say the non-inverting, and RX2 fed to the inverting. EMI cancels nicely right at the front end. You have to have sufficient spacing between coil so as not to cancel the target signal and the coil array is a bit unwieldy. The front end can be tweeked to give best common mode noise rejection but to every benefit there is a trade-off. Here, it is two coils and two man operation.


Try it with a couple of small coils, say 8in.

I would say that here in the UK we suffer more from man made EMI than you would in Australia. Out in the bush in Victoria I could dispense with the dual coil in the picture, as a single mono was just as quiet and much easier on the arms.

Eric.