THE PERFECT GROUND BALANCE

[Tec]

shielding in a picture

I'm interested in others experience with bi-pulse.

Yes, here is my explanation of shielding (attached). I hope it is correct.

[moodz]

Very good explanation of shielding ....

Now consider a differential coil. During flyback both +400 and -400 volts are generated. The net charge to earth is zero due to field balance ...so differential coils are less susceptable to capacitive effects to ground.

The bipolar pulse scheme is good for EF cancellation however it modulates the rx signal so the processing is different unless you use a cross over switch scheme or something similiar at the demod / samplers.

moodz

[Tinkerer]

Thanks for the nice drawing, Tec.

I like it so much, that I took the liberty to transform it to png and post it below.
I used http://www.irfanview.com/index.htm to transform. This is free to download and it is very good, so I think it merits some free advertising too.

Moodz,

I use a center tapped RX coil with the IB-PI. During experimenting, I usually leave the coils unfinished, until I have decided on the final version.
When I shield the coils, I apply enough shield (graphite coating) so that I can not see any influence in response, when I approach my hand to the coil.
I found the center tapped, RX differential coils to be very insensitive to my hand without a shield.
However, the twisted cable or even a tween coax cable is very sensitive to the touch. This seems to be still a problem that need to be solved.

Tinkerer

[moodz]

Thanks for the nice drawing, Tec.

I like it so much, that I took the liberty to transform it to png and post it below.
I used http://www.irfanview.com/index.htm to transform. This is free to download and it is very good, so I think it merits some free advertising too.

Moodz,

I use a center tapped RX coil with the IB-PI. During experimenting, I usually leave the coils unfinished, until I have decided on the final version.
When I shield the coils, I apply enough shield (graphite coating) so that I can not see any influence in response, when I approach my hand to the coil.
I found the center tapped, RX differential coils to be very insensitive to my hand without a shield.
However, the twisted cable or even a tween coax cable is very sensitive to the touch. This seems to be still a problem that need to be solved.

Tinkerer

Tinkerer,

That could indeed be the case ...however as you know I use the same coil for transmit and receive ... The reason I don't use shielding is because I have not yet had a hand capacitance problem. There must be some difference between having a separate rx coil and a mono type coil ... I will get round to this but I am busy coding at the moment.
Moods

[Tinkerer]

Tinkerer,

That could indeed be the case ...however as you know I use the same coil for transmit and receive ... The reason I don't use shielding is because I have not yet had a hand capacitance problem. There must be some difference between having a separate rx coil and a mono type coil ... I will get round to this but I am busy coding at the moment.
Moods

The effects of the hand on an un-shielded mono coil, appear only after a gain of about 1000, or would that be about 10bits?

Tinkerer

[IBGold]

Hi Tinkerer,

On the subject of ground balance what about the attached methods .A. is of course the patented QED method and .B. is the subject of a possible divisional patent for the QED but I am sure Bugs would give us access to it.

Drawings_16-09-2010_SPEI-13771330.pdf

Regards, Ian.

[Qiaozhi]

Hi Tinkerer,

On the subject of ground balance what about the attached methods .A. is of course the patented QED method and .B. is the subject of a possible divisional patent for the QED but I am sure Bugs would give us access to it.

Drawings_16-09-2010_SPEI-13771330.pdf

Regards, Ian.

Your link doesn't seem to go to a PDF file, but displays a login window.

[moodz]

The effects of the hand on an un-shielded mono coil, appear only after a gain of about 1000, or would that be about 10bits?

Tinkerer

....the standard monocoil generates a large flyback pulse of 100s of volts ... this in turn creates an E field around the coil. This E field will couple capacitively (ie charge ) to anything remotely conductive ... like your hand or the ground or the metal detector casing and cables etc etc etc ... all standard physics.

The differential "mono" coil generates TWO flybacks simultaneously ( but only ONE magnetic field ) .... these two flybacks are equal and opposite in polarity so the E field at some distance from the coil ... like your hand or the ground is nearly 0 due to field cancellation ...again standard physics ( ie hundreds of times less than the standard mono coil )

This is not a new idea ...it has been used in things like moving coil cartridges for hifi and in MRI for medical purposes ...the differential coil is sometimes referred to as a self sheilding coil for exactly this reason.

moodz

PS there are no "hand effects" even at high amplification and 24 bits on a differential coil ...only if you are wearing a watch or a ring ...LOL

[Tinkerer]

The mystery of the disappearing and reappearing target signal

The mystery of the disappearing and reappearing target signal.

We take 2 samples and compare them in the differential integrator.
Sample 1 is the target response, sample 2 is the ground and earth field response.

A good spot for the GB+EF sample, is where the resistive and the reactive response are equal and the target response is minimal.
An adjustment of the time of this sample, a bit earlier or a bit later, allows to increase or attenuate the responses to find the best ground balance and to eliminate the EF response.

Initial tests worked fine. I could easily get 130cm on the beer can and of half of Al's titanium test sample.
The ferrous target discrimination worked fine too.
However, when I tried Ian’s 1gram titanium sample it simply disappeared.
Further tests showed that the response to all low TC targets, like foil or a Nickel, anything with less than 20us TC, disappeared.

I know that I had a very good response from Ian’s 1gram titanium sample even with a larger coil, so what happened?

I looked at and tried changes on the filters, but that is not where the short TC signal disappeared.

Still using the same first sample timing that gave very good results before, so why do the short TC targets disappear?

Eventually I found the missing targets again, at some impossible place, that is, comparing the target signal with itself in the integrator. Comparing a signal with the same signal should give 0 output, or, let’s say, it will give a small difference because of the not perfectly matched resistors and capacitors.
But how does the target signal get there?

So, the mystery is: How does the short TC target response disappear and then reappear at this spot?
Can anybody help solving this mystery?
I am preparing the RX circuit and scope pictures to show the sample timing, for posting.

Tinkerer

[moodz]

The mystery of the disappearing and reappearing target signal.

We take 2 samples and compare them in the differential integrator.
Sample 1 is the target response, sample 2 is the ground and earth field response.

A good spot for the GB+EF sample, is where the resistive and the reactive response are equal and the target response is minimal.
An adjustment of the time of this sample, a bit earlier or a bit later, allows to increase or attenuate the responses to find the best ground balance and to eliminate the EF response.

Initial tests worked fine. I could easily get 130cm on the beer can and of half of Al's titanium test sample.
The ferrous target discrimination worked fine too.
However, when I tried Ian’s 1gram titanium sample it simply disappeared.
Further tests showed that the response to all low TC targets, like foil or a Nickel, anything with less than 20us TC, disappeared.

I know that I had a very good response from Ian’s 1gram titanium sample even with a larger coil, so what happened?

I looked at and tried changes on the filters, but that is not where the short TC signal disappeared.

Still using the same first sample timing that gave very good results before, so why do the short TC targets disappear?

Eventually I found the missing targets again, at some impossible place, that is, comparing the target signal with itself in the integrator. Comparing a signal with the same signal should give 0 output, or, let’s say, it will give a small difference because of the not perfectly matched resistors and capacitors.
But how does the target signal get there?

So, the mystery is: How does the short TC target response disappear and then reappear at this spot?
Can anybody help solving this mystery?
I am preparing the RX circuit and scope pictures to show the sample timing, for posting.

Tinkerer

I imagine these short tc targets are low level ?? ...The DSP SAT works exactly the same way .... the best way to detect a very small change is to compare / integrate a slightly delayed signal with itself ... there is no better reference as the small signals are "riding the noise". On the MCP3903 24 bit converter this method easily detects changes of 10 nanovolts.

Nothing new here ...I go the idea from a meter design that can measure down to 1 femptoamp ... and that is small.

moodz

moodz.

[Tinkerer]

RX CIRCUIT

Attached is the RX circuit with the mystery short TC targets.

Tinkerer

[Tinkerer]

I imagine these short tc targets are low level ?? ...The DSP SAT works exactly the same way .... the best way to detect a very small change is to compare / integrate a slightly delayed signal with itself ... there is no better reference as the small signals are "riding the noise". On the MCP3903 24 bit converter this method easily detects changes of 10 nanovolts.

Nothing new here ...I go the idea from a meter design that can measure down to 1 femptoamp ... and that is small.

moodz

moodz.

Moodz, thank you for the feedback.
So that explains how the signal got to the integrator.
Now I need to find where I lost the signal and then everything will be fine again.

I admit, I panicked there, for a while.
Do not panic! Do not panic!

Tinkerer

[Tinkerer]

CRO OF SAMPLE PULSES

Attached is a scope picture of the sample pulses.
I also managed to get the Flyback on the same traces.

The setting is 50us/div.

You see the complete cycle of about 200us.

The Flyback and the target sample overlap a little, this gives strong FE discrimination.

The Ground sample is later. It can be moved to a later time, until just before the Flyback.

Changing the ground sample time allows for attenuating either the R or the X response and /or the response of a specific undesirable target.

Tinkerer

[Davor]

The Flyback and the target sample overlap a little, this gives strong FE discrimination.

And that says a lot about perfect ground balance for PI MDs. FB OM!

[IBGold]

[QUOTE=IBGold;149254]Hi Tinkerer,

On the subject of ground balance what about the attached methods .A. is of course the patented QED method and .B. is the subject of a possible divisional patent for the QED but I am sure Bugs would give us access to it. QUOTE]


I will try again and add another method which is also a lapsed patent that is of interest.

Regards, Ian.