RX Flyback Blocking for Best Op-Amp Performance

[jon-ecm]

Been experimenting a bit with flyback blocking and wondering what other people have tried, and with what success.

Seems the standard method to put a resistor with bi directional diodes is a source of noise and also over-saturates the op amp.

Thanks!
Jon

http://www.miymd.com

[mikebg]

Been experimenting a bit with flyback blocking and wondering what other people have tried, and with what success.

Seems the standard method to put a resistor with bi directional diodes is a source of noise and also over-saturates the op amp.

Jon, I think the title of your WEB site should be changed:
"Design yourself a high-end metal detector" instead "Make It Yourself Metal Detector".
You are no more a builder. When you are
"wondering what other people have tried, and with what success", that means you started to become a designer, because the first step of design process is;
COPY THE BEST DESIGN SOLUTION.

As second step, the designer is asking:
HOW TO DISCERN THE BEST DESIGN SOLUTION?
Then the designer starts to learn theory (if he wish to discover the answer:-).
Unfortunately most beginners and MD builders prefer to try and experiment instead to think and learn theory.

The problem with flyback voltage arises when you use MONOCOIL - the worst type sensing head. When the receiver is connected to TX coil, that means maximal possible AIR signal, ie the MONOCOIL generates target signal as extremely low modulation index of AIR signal. The "other people have tried" the induction balance. Even the first pulse induction metal detectors, created in 19th century, use induction balance. The placed in balance RX coil decreases several hundert times the flyback voltage (AIR signal), ie it increases the modulation index of target signal.

No need to use low noise preamp, no need to avoid diodes in input, because synchronous demodulators can suppress thermal noise. The sensitivity of metal detectors is not limited by thermal noise, but by large AIR & GND signal existing in input of preamp. It decreases modulation index and limits the usable gain of preamp. A high-end metal detector should suppress the AIR & GND signal in input of preamp.

[moodz]

good work and some comments.

Firstly Jon ... good work on your website and first project.

I disagree with some comments on monocoil.

I reckon the monocoil is one of the best coils ....

1. Easy to make ( less effort )

2. Cheap to make ( less money )

3. Simplicity ( less connections = less complex = less brainwork )

4. Good performance ( taking three factors above ... the performance to cost/effort/complexity benefit is better. )

5. Best pulse induction research platform ( for all the reasons above + there are things that have not been discovered yet about monocoil ) Yes the modulation index is low ... but if you look at the wrong measurement you will get the wrong information.

Best regards to all ....

moodz.

[mikebg]

Moodz, there is a sixth advantage:
THE MONOCOIL HAS NO BACKREADING ZONE.
Each induction balanced loop configuration has a zone where a nonferrous target appears as if is made of ferrous metal.
However the MONOCOIL is not suitable for high-end metal detector. Even competent designed BFO metal detectors use induction balanced loop configuration in search oscillator.

[Aziz]

Hi all,

I don't agree with MONO COIL lovers.
I admit, that MONO COIL's being easy to build & very cheap.

But from the point of signal response, it's the worst coil configuration.

My 3.1415 cents.
Aziz

[WM6]

But from the point of signal response, it's the worst coil configuration.

It depend of what signal and what response do you thinking about.

"The worst" is not always the worst.

There are ways, how to out of seemingly negative, we get a positive results.

You only have to thinking outside of lectured box and simulators.

As well in present reality, if we are talking about detecting depth in PI detectors, mono is still the best, and not the worst, coil. And detecting depth is our main problem - isn it?

[Monolith]

Hi everybody,

a lot of good comments.

John, look at the Kingfisher project. He uses Flyback blocking fets. So do ML detectors.

Mikebg, how do you suppress the Johnson noise?

moodz, I would not put your differential coil into the same bucket as a mono coil. It is a great invention and should be recognized as such.

Aziz, I agree.

WM6, wrong, IB coils can give much greater depth.
Depth is not the most important feature anyway. Once you make a detector that can go deeper than 1 meter, the most important thing is to know what kind of target is down there, before you dig.
Digging to 20cm is quick and cheap.
Digging to 1 meter will be a day's work.

Monolith

[WM6]

WM6, wrong, IB coils can give much greater depth.
Depth is not the most important feature anyway. Once you make a detector that can go deeper than 1 meter, the most important thing is to know what kind of target is down there, before you dig.
Digging to 20cm is quick and cheap.
Digging to 1 meter will be a day's work.

Only in your theoretical world, dear Monolith, not in mine. My world is in reality. In reality even famous bulgarian REMI group (born in simulator), use for bigger depth exclusively mono coils, no DD, no OO, no OOO etc. - mono solely.

And totally wrong logical inference.

What good is your power of discrimination, if target can not be even detected. First you have to detect the target and then dispute about diferentiation.

So depth is always first, without depth nothing else exist.

[Aziz]

Bull$hit, bull$hit, bull$hit.

IB coil system allows the lowest noise designs.
IB coil system allows the highest power efficiency.
IB coil system allows the highest power output.
IB coil system allows the best ground balancing possibility.
IB coil system allows the best discrimination possibility.
IB coil system allows the best overall depth performance.


Just think outside of the box!

Aziz

[mikebg]

Mikebg, how do you suppress the Johnson noise?

Monolith, please make an experiment.
Connect a resistor R instead RX coil as shown below. Then see with oscope the point A (output of preamp) and the point B (output of filter after SD - synchronous demodulator). After filtration of demodulated signal, there are only 1/f noise. The SD can not see the Johnson noise. However you can not see also Johson noise in output of preamp, because its gain is limited by large AIR & GND signal in input.

[simonbaker]

Jon, I think the title of your WEB site should be changed:
"Design yourself a high-end metal detector" instead "Make It Yourself Metal Detector".
You are no more a builder. When you are
"wondering what other people have tried, and with what success", that means you started to become a designer, because the first step of design process is;
COPY THE BEST DESIGN SOLUTION.

As second step, the designer is asking:
HOW TO DISCERN THE BEST DESIGN SOLUTION?
Then the designer starts to learn theory (if he wish to discover the answer:-).
Unfortunately most beginners and MD builders prefer to try and experiment instead to think and learn theory.

The problem with flyback voltage arises when you use MONOCOIL - the worst type sensing head. When the receiver is connected to TX coil, that means maximal possible AIR signal, ie the MONOCOIL generates target signal as extremely low modulation index of AIR signal. The "other people have tried" the induction balance. Even the first pulse induction metal detectors, created in 19th century, use induction balance. The placed in balance RX coil decreases several hundert times the flyback voltage (AIR signal), ie it increases the modulation index of target signal.

No need to use low noise preamp, no need to avoid diodes in input, because synchronous demodulators can suppress thermal noise. The sensitivity of metal detectors is not limited by thermal noise, but by large AIR & GND signal existing in input of preamp. It decreases modulation index and limits the usable gain of preamp. A high-end metal detector should suppress the AIR & GND signal in input of preamp.

Hi mikebg:

Can you define exactly what you mean by "air signal"? I think you defined these terms somewhere but I don't remember.

I keep hoping there is a way to make a great mono coil detector because so much easier to build mono coils. Or at least unbalanced two coil detectors.

What about electrically cancelling the air signal?

Regards,

-SB

[Monolith]

Bull$hit, bull$hit, bull$hit.

IB coil system allows the lowest noise designs.
IB coil system allows the highest power efficiency.
IB coil system allows the highest power output.
IB coil system allows the best ground balancing possibility.
IB coil system allows the best discrimination possibility.
IB coil system allows the best overall depth performance.


Just think outside of the box!

Aziz

I agree.
And I specially appreciate that your comments come from insights earned through a lot of hard work. I still remember the time when you hated IB coils and swore on mono coils.

all the best

Monolith

[Monolith]

Monolith, please make an experiment.
Connect a resistor R instead RX coil as shown below. Then see with oscope the point A (output of preamp) and the point B (output of filter after SD - synchronous demodulator). After filtration of demodulated signal, there are only 1/f noise. The SD can not see the Johnson noise. However you can not see also Johson noise in output of preamp, because its gain is limited by large AIR & GND signal in input.

Hi Mikebg,

thanks for the answer.
We filter and filter and filter, but in the end the low frequency noise is still there, because this is the frequency that the pure target signal is. But let me take advantage to ask the professor about the Johnson noise. I had the understanding that this is of very wide spectrum and includes the 1/f noise. Could you dispel my doubts and explain in more detail?

For very small targets I need up to 300kHz bandwidth in the preamp. For large targets, 50kHz is good enough. I use IB, by the way.
But where does the sweep speed come in? with perfect Ground Balance the detector only "sees" the difference between the Ground and the Target and this difference happens during the time the coil sweeps over the target.

I much appreciate your scientific explanations and your effort to make it understandable to us common mortals.

All the best

Monolith

[Monolith]

Only in your theoretical world, dear Monolith, not in mine. My world is in reality. In reality even famous bulgarian REMI group (born in simulator), use for bigger depth exclusively mono coils, no DD, no OO, no OOO etc. - mono solely.

And totally wrong logical inference.

What good is your power of discrimination, if target can not be even detected. First you have to detect the target and then dispute about diferentiation.

So depth is always first, without depth nothing else exist.

WM6,
I agree with this one:depth is always first, without depth nothing else exists.
But I ask you: how many 1 meter holes have you dug?
Old fashioned mono coil detectors detect only large targets at that depth.
Out of the box modern IB detectors are capable of detecting even small targets at the same depth. It then becomes imperative that you can select only the higher probability targets for digging, because you may dig only one target per day.

But of course mono-lovers can not accept that, because it is not possible to discriminate with mono coils. So the mono coil is "THE BOX".

Theoretical world? No, practical world. My arms just can not cope with that much digging.

All the best

Monolith

[simonbaker]

Hi Mikebg,

thanks for the answer.
We filter and filter and filter, but in the end the low frequency noise is still there, because this is the frequency that the pure target signal is. But let me take advantage to ask the professor about the Johnson noise. I had the understanding that this is of very wide spectrum and includes the 1/f noise. Could you dispel my doubts and explain in more detail?

For very small targets I need up to 300kHz bandwidth in the preamp. For large targets, 50kHz is good enough. I use IB, by the way.
But where does the sweep speed come in? with perfect Ground Balance the detector only "sees" the difference between the Ground and the Target and this difference happens during the time the coil sweeps over the target.

I much appreciate your scientific explanations and your effort to make it understandable to us common mortals.

All the best

Monolith

Is that an IB coil on a PI detector, or VLF detector?

-SB