DEEPER PI DETECTION DEPTH

[Aziz]

The noise in the out of band correlates with the region of interest band.

- I don't think that is correct for gaussian noise. Maybe some other kind of noise.

An example?
E-Motor switching on (just imagine, you are detecting in an industrial area). You don't know, when the E-motor is switched on (random/gaussian behavior). But if switched on, it's response characteristics is more or less definite (depends on the type of noise source). You can predict it's behaviour if you can detect such an event.

More examples?
Other inductive electrical devices during switch on/off. Switching lights on/off. ....

There are more types of noise sources of this behaviour. And they give you the chance to cancel them.

Aziz

PS:
A good example now:
Swinging the coil over a strong magnet! The detector doesn't know, when you do it. The magnet could be hidden in the ground. But it generates a wide band response, which can't be handled by simple filters!!!

[simonbaker]

An example?
E-Motor switching on (just imagine, you are detecting in an industrial area). You don't know, when the E-motor is switched on (random/gaussian behavior). But if switched on, it's response characteristics is more or less definite (depends on the type of noise source). You can predict it's behaviour if you can detect such an event.

More examples?
Other inductive electrical devices during switch on/off. Switching lights on/off. ....

There are more types of noise sources of this behaviour. And they give you the chance to cancel them.

Aziz

PS:
A good example now:
Swinging the coil over a strong magnet! The detector doesn't know, when you do it. The magnet could be hidden in the ground. But it generates a wide band response, which can't be handled by simple filters!!!

I probably should have said "white noise" or "pink noise" rather than just call it gaussian, which really refers to the amplitude statistics I think.

I agree, if you have good "a priori" knowledge of a noise signal "profile", then all you have to do is detect and estimate it as it occurs and apply the profile you know from experience. And you should be able to detect/estimate a known signal with a few points outside a certain band, I would think. There will probably be a new type of "false positive" error, where you think you detect this signal but are wrong, and you apply the profile, creating a greater noise signal.

I'm not sure how swinging a coil over a strong magnet is very wide band unless you do it super fast, like a bullet. But I see what you are getting at; and I believe it does require a lot of historical information about this signal that may arise. If the noise is a totally unknown signal, you wouldn't be able to predict the content in one band based on another without more information, I'm pretty sure.

Regards,

-SB

[golfnut]

I guess the 4kTR also has another factor B - bandwidth.


If the Input of the Rx is not restricted to the Usable Rx BW of the system then you let extra noise in. The noise floor goes up as it gets hit with noise from a wider frequency span. S/N suffers = you would see less targets as they are in the noise.

Restricting the Rx input bandwidth with an appropriate filter will limit noise from mains tv cell too.

Separating the Rx from the Tx is a good move...
1) You could run the Rx amp common mode to cancel a load of picked up noise
2) The Rx signals are not being burned away as heat by the De Q resistor on the Tx coil
3) You would not need the inverse pair of diode limiters -as your not Txing up your Rx
Always a bad move

S

[Tinkerer]

I guess the 4kTR also has another factor B - bandwidth.


If the Input of the Rx is not restricted to the Usable Rx BW of the system then you let extra noise in. The noise floor goes up as it gets hit with noise from a wider frequency span. S/N suffers = you would see less targets as they are in the noise.

Restricting the Rx input bandwidth with an appropriate filter will limit noise from mains tv cell too.

Separating the Rx from the Tx is a good move...
1) You could run the Rx amp common mode to cancel a load of picked up noise
2) The Rx signals are not being burned away as heat by the De Q resistor on the Tx coil
3) You would not need the inverse pair of diode limiters -as your not Txing up your Rx
Always a bad move

S

The signal of a small target has a TC of less than 5us. To capture such signals, the bandwidth of the coil and preamp should be above 100kHz.
When people used the NE5534 preamp with a gain of 1000, this automatically reduced the bandwidth. However, using better opamps with more bandwidth, a low pass filter before the preamp if the configuration allows, and /or bandwidth limiting the preamp reduces the noise significantly.

I like to use a differential input preamp. Now how does a lowpass filter in front of the preamp influence the common mode rejection?

Tinkerer

[golfnut]

The filter would be duplicated, one per input. The common center terminal of the filter containing the filtered signals goes to gnd.

If the filter has symmetry there would be no penalty to the common mode rejection.

steve

[Tinkerer]

The filter would be duplicated, one per input. The common center terminal of the filter containing the filtered signals goes to gnd.

If the filter has symmetry there would be no penalty to the common mode rejection.

steve

Hi Steve,

would you like to help me improve this RX schematic of mine?

It works, but I am sure it could be improved a lot.

I would much appreciate.

Tinkerer

[golfnut]

HI, with the cct you posted - If LP filters were what U required I would fit them here.

Having them close to the amp gives the best chance of catching noise.

If U wanted this topology I could get U some values for a roll-off corner of your choice.

Steve

[Tinkerer]

HI, with the cct you posted - If LP filters were what U required I would fit them here.

Having them close to the amp gives the best chance of catching noise.

If U wanted this topology I could get U some values for a roll-off corner of your choice.

Steve

Thanks for the feedback.

I would like to try 2 different frequencies. About 100kHz and about 150kHz to see what the influence on the small and very small target response is.

The RX coil is center tapped, about 300uH, but I still want to try different inductance.

Thanks for the help

Tinkerer

[golfnut]

I would remove the RC filter R7,9 C8 - The RC filters loose signal power in the resistive elements - the LC ones dont to speak of.

The order of this filter is a little more than I sketched, The input impedance I chose 500R and the output z to op amp came out as 560R

S

[Tinkerer]

I would remove the RC filter R7,9 C8 - The RC filters loose signal power in the resistive elements - the LC ones dont to speak of.

The order of this filter is a little more than I sketched, The input impedance I chose 500R and the output z to op amp came out as 560R

S

Thank you for the filter.
What are the specs for the inductors?
A question about the impedance:
I have a variable voltage divider between R9, U7, R13. I use this to adjust both legs of the RX coil to the same signal amplitude.
The diodes should not conduct, they are there as a precaution only for when there is a problem with the induction balance.
R34, is to adjust the offset caused by the shield of some types of cables.

Increasing the impedance, reduces the amplitude of the signal, which allows for more amplification in the preamp, at the cost of more noise.

Eliminating more noise with the filters, before the input of the preamp is going to help.

I feel there is a sweet spot, where the S/N is best.
How do I determine that spot?

Tinkerer

[golfnut]

whoa, breathe dude..

What are the specs for the inductors?
A question about the impedance:
I have a variable voltage divider between R9, U7, R13. I use this to adjust both legs of the RX coil to the same signal amplitude.
The diodes should not conduct, they are there as a precaution only for when there is a problem with the induction balance.
R34, is to adjust the offset caused by the shield of some types of cables.

Increasing the impedance, reduces the amplitude of the signal, which allows for more amplification in the preamp, at the cost of more noise.

Eliminating more noise with the filters, before the input of the preamp is going to help.

I feel there is a sweet spot, where the S/N is best.
How do I determine that spot?

What are the specs for the inductors?

No constraints really, if the value is good, The R value will only be an ohm or so.
Layout in more important, dont have th coils close together as they will couple signal rather than conduct signal.

A question about the impedance:
I have a variable voltage divider between R9, U7, R13. I use this to adjust both legs of the RX coil to the same signal amplitude.

ok


The diodes should not conduct, they are there as a precaution only for when there is a problem with the induction balance.

yes

R34, is to adjust the offset caused by the shield of some types of cables.

hmm


Increasing the impedance, reduces the amplitude of the signal,

aha


which allows for more amplification in the preamp,

which would then require more amplification in the preamp to get the same signal level back.


at the cost of more noise.
smaller signals in - need to crank the gain, more noise.. S/N suffers.


Eliminating more noise with the filters, before the input of the preamp is going to help.

It will also reduce falses from pick up of any signals above say 0.5MHz as the attenuation here is getting real high, plus the usable gain window for wanted signals should be better as less blocking due to removal of the myriad of high power signals being filtered away. Keeps the dynamic range.


I feel there is a sweet spot, where the S/N is best.

That is an engineering question and a good one. All radio links have a system plan/design and a system budget.
You look at the signals and their amplitudes max and min and the noise powers and you look at the Reciever noise contribution (in the appropriate system BW)

This lot works if you have all of the puzzle pieces - We dont have this luxury.

All we know is we want to detect real deep and small gold etc.

For us this amounts to the smallest S/N we can get a reliable threshold detect.

How do I determine that spot?

As I eluded, there isnt a spot which is optimal the nature of the game is to have the highest S/N you can - to get the smallest and deepest you will have nothing spare in S/N.


A large Tx signal
A quiet Receiver
A detection threshold minutely above the ambient noise out of the Rx - in order to see small signals is the best u can hope for out of a simple machine like these.

If you picked a spot S/N - the axes of freedom are then Tx power, Rx NF and gain, depth, soil type and target size and material.

You would reliably be able to say then at this spot S/N for my system - If I use this Tx power, Rx NF and gain, depth, soil type and target size and material - I will get a target detected.

I can guess ur next question and I dont have the answer to hand

Steve

[Tinkerer]

Many thanks Steve,

yes, I have about 1000 questions left but I guess I must not overdo it. On the other side, there are probably many other members of the forum who dare not ask themselves but are glad to learn from your answers.

So please, just one more...
R34, is to adjust the offset caused by the shield of some types of cables.
hmm

I don't know why this happens. The pot solves the problem, but I would prefer to avoid the problem all together. The cables are also very sensitive to touching with the hand and moving /vibration.

What can be the cause for that?

We, the people, or forum members, greatly appreciate your help.

Ah, by the way, I use the TEM TX method above, where I recycle most of the power. I like a powerful TX that gives a high amplitude RX signal that is way above a lot of the pesky noise.

Tinkerer

[golfnut]

I would be speculating if I thrust an answer at you..

1)Having a Star ground point can have advantages. ie a single elected gnd/


It should be Low R and low L and then sheilds, bonds, battery -ve, pcb gnds all go to it.


2) If you are running a fair Tx signal the wrapped metalised polyester film cable shield may not be low enough L or R and will have signals resident on it.



Often if U have signals on braid it can be a sign that things are mismatched. I suspect this is partly the case. The characteristic impedance of the cable wont be the same as the impedance of the loop antenna system.

If U have a small amount of signal on Ur braid - which I guess U have - You burn these away in your pot. If U can live with it its quite an elegant solution.

U could try to work out the impedance of your loop to The Tx signals then either do a match cct so all of ur signal goes in, or pick a cable with a characteristic Z closer to you Tx loop at Tx frequencies.

Or fit a sleeve balun at the head end to trap braid currents

Or Use a ferrite clip on at the head end to absorb braid currents (ironically) due to eddy current losses in the ferrite.

S

[Carl-NC]

...then all you have to do is...

This is a running joke at work, especially with the software engineers... "all you have to do is..."

Eng. manager: "I want a PI that discriminates iron."
SW engineer: "How do I do that?"
Eng. manager: "All you have to do is write software to identify iron and eliminate it."
SW engineer: "Uh... OK."

[simonbaker]

This is a running joke at work, especially with the software engineers... "all you have to do is..."

Eng. manager: "I want a PI that discriminates iron."
SW engineer: "How do I do that?"
Eng. manager: "All you have to do is write software to identify iron and eliminate it."
SW engineer: "Uh... OK."

Feels familiar...

Seriously - detecting and estimating that signature is probably more work load than it is worth -- and would require digital processing for sure. In fact, really a "smoothing" problem, easier done after the fact. On top of that, the signatures Aziz refers to probably would be characterized by an average shape with a large stochastic variation, which you'd be left with anyway after "subtracting out". So a lot of work for one blip, even if you knew all the theory and had all the data to do it.

Probably easier to just dig.

-SB