northwest anti interference coils: are they the way to go?
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australian electronic gold prospecting forum.com  |  Detector Technology and Electronics and new detectors  |  Detector Coils (Moderator: Goldman)  |  Topic: anti interference coils: are they the way to go? 0 Members and 1 Guest are viewing this topic. « previous next »
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Aziz
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« Reply #60 on: Sunday November 21 2010 16:23:33 AEDT PM »

Hi Doug,

it seems, that the ML ground balancing is not working perfect.
 
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Mechanic
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« Reply #61 on: Sunday November 21 2010 18:59:19 AEDT PM »

The DD coil run in cancel is not the best method to cancel emi. The reason being that the coils overlap down the centre of the coil and the windings are too close. This would also cancel some target signal. With the figure 8 arrangement there is a lot less overlap/ closeness between the windings therefore a lot less signal would be cancelled. Hmmm I hope that makes sense!

Cheers Mick
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Aziz
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« Reply #62 on: Sunday November 21 2010 22:11:49 AEDT PM »

  
it seems, that the ML ground balancing is not working perfect.
 

Did someone understand the prose?

Some AI configurations can become a part of the GB system. This is due to the coil coupling coefficient change, which mineral grounds cause. The higher the coil coupling coefficient, the higher the participation of to the GB system.

Indeed, the AI configuration can become a non-trivial problem. So one need really a good working GB system.
 
Aziz
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« Reply #63 on: Sunday November 21 2010 23:04:40 AEDT PM »

  
  
it seems, that the ML ground balancing is not working perfect.
 

Did someone understand the prose?

Some AI configurations can become a part of the GB system. This is due to the coil coupling coefficient change, which mineral grounds cause. The higher the coil coupling coefficient, the higher the participation of to the GB system.

Indeed, the AI configuration can become a non-trivial problem. So one need really a good working GB system.
 
Aziz

A1 coils do not seem to handle highly variable, mineralized ground as well as DD's or maybe even mono's on ML detectors.This is in my opinion another or their downsides.
doug happy face
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« Reply #64 on: Sunday November 21 2010 23:28:15 AEDT PM »

Hi Doug,

I don't see any downside of the AI coil anymore. The opposite happens.
They have the potential for technology breakthrough in detector performance. Provided that, a good working GB system is part of the AI coil system.

Aziz

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« Reply #65 on: Sunday November 21 2010 23:38:20 AEDT PM »

  
Hi Doug,

I don't see any downside of the AI coil anymore. The opposite happens.
They have the potential for technology breakthrough in detector performance. Provided that, a good working GB system is part of the AI coil system.

Aziz




Well we then need a new GB system if we are going to use them with ML detectors over very bad ground! ::54::
What would be interesting is to try an  A1 coil with the TDI or the gs5b which just uses a single Tx pulse length and does not have to use MPS for GB.
How they would go I just don't know and what sample delay one would have to use I also don't know.
Maybe Eric or Reg could answer this?
Also would like to see how an A1 coil would go on the QED!
doug happy face
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« Reply #66 on: Monday November 22 2010 00:04:09 AEDT AM »

Hi Doug,

I have noted that there has been comment about MPS and that MPS is required for ground balance in a Minelab detector.   MPS is purely about two time constant channels. A short time constant channel and a long time constant channel. Its about trying to bridge the gap between a small target and a large target as sometimes the ground response looks very much like target response and visa versa at one frequency. 

A  GB balance is derived for each channel and uses the long time constant channel as  reference input to the calculation.  I can operate the SD2000 as a  single frequency  machine either short or long time constant channel at the flick of a switch.  Whether the GB is effective is another matter. Whether Minelab has got it wrong or right is again another matter.

So guy's MPS is not GB.

Regards,

Stefan
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« Reply #67 on: Monday November 22 2010 00:10:06 AEDT AM »

  
Hi Doug,

I have noted that there has been comment about MPS and that MPS is required for ground balance in a Minelab detector.   MPS is purely about two time constant channels. A short time constant channel and a long time constant channel. Its about trying to bridge the gap between a small target and a large target as sometimes the ground response looks very much like target response and visa versa at one frequency. 

A  GB balance is derived for each channel and uses the long time constant channel as  reference input to the calculation.  I can operate the SD2000 as a  single frequency  machine either short or long time constant channel at the flick of a switch.  Whether the GB is effective is another matter. Whether Minelab has got it wrong or right is again another matter.

So guy's MPS is not GB.

Regards,

Stefan

But don't  ML in MPS use linear combination's of the short and long pulses to null out the ferrite signal at least in the later detectors?
doug  happy face
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« Reply #68 on: Monday November 22 2010 00:52:00 AEDT AM »

Hi Doug,

I know in the SD2000 in either the short or long time constant channel, a sample is taken after an appropriate delay after the flyback and then a second sample is taken some time later and subtracted from the earlier sample. This then is supposed to take into account the ground response.  These samples are kept seperate and not used in the other channel.   A GB sample is taken from the long time constant channel and a period in between the pulse streams and is fed as a further GB input to each channel.

Using this method and if you only had one frequency pulse stream you could still get a GB channel without the need for a short channel response.

Maybe some of the guys who have delved into the depths of the 3500 or in the 4500 can provide some clarity on how the GB is implemented in the latest units.

Regards,

Stefan
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Aziz
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« Reply #69 on: Wednesday November 24 2010 00:51:44 AEDT AM »

Hi guys,

I have made some measurements and it's looking quite good. Potential signal-to-noise increase of 20 dB is possible. Coil design is also getting quite easy. There are many possibilities to achieve this depending on bypassing patent issues (if any).  ::38::

Unfortunately, a new front-end receiver is necessary. The common MD's can not be upgraded with the new AI coils.

Aziz
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« Reply #70 on: Wednesday November 24 2010 08:44:33 AEDT AM »

Hi all,

you will get surprized now:
Everything can be made quite simple:
- the coil (quasi mono coil -> no seperate RX coil, simple round coil design, hot spot at the centre of the coil configuration)
- the front-end receiver (simple single adder-amplifier stage)

A simple EMI adjustment (for exact EMI cancelling) can be made with potentiometer. After cancelling the EMI, you need more gain to compensate the response losses (in the range of 9-10 dB). If you can achieve a 30 dB EMI cancelling, you have 20 dB more room for gain to get more depth performance).

Remember: amplifier noise is less than EMI noise!
 ::38::
Aziz
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« Reply #71 on: Wednesday November 24 2010 09:27:53 AEDT AM »

  
Hi guys,

I have made some measurements and it's looking quite good. Potential signal-to-noise increase of 20 dB is possible. Coil design is also getting quite easy. There are many possibilities to achieve this depending on bypassing patent issues (if any).  ::38::

Unfortunately, a new front-end receiver is necessary. The common MD's can not be upgraded with the new AI coils.

Aziz

Hi Aziz .. is this because of special filtering required..
agree this may work with your AI coils.. as I dont think they can work alone..
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« Reply #72 on: Wednesday November 24 2010 09:41:00 AEDT AM »

  
Hi Aziz .. is this because of special filtering required..
agree this may work with your AI coils.. as I dont think they can work alone..

No special filtering required. Just doing some analog math with two resistors (addition) in an adder amplifier.
Aziz
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« Reply #73 on: Wednesday November 24 2010 10:39:04 AEDT AM »

  
  
Hi Aziz .. is this because of special filtering required..
agree this may work with your AI coils.. as I dont think they can work alone..

No special filtering required. Just doing some analog math with two resistors (addition) in an adder amplifier.
Aziz


I think that before we do any signal processing to reduce EMI we should have a digital bandpass filter between the coil and the coil preamp. This bandpass filer would have user adjustable corner frequencies. I would suggest a band pass from 2hz-100khz with steep roll offs ,the upper frequency  limit would have to  determined  by testing because if it this is too low then the  sensitivity and depth on some targets  may be dramatically reduced. Then we need DSP on the Rx signal so that very fast, very narrow high amplitude EMI noise spikes are rejected as noise.
I believe that this would go a long way to reducing some  EMI and then perhaps other methods like Aziz's could  be used as well?
doug happy face
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« Reply #74 on: Wednesday November 24 2010 11:05:44 AEDT AM »

Good thinking Dougy..
"Then we need DSP on the Rx signal so that very fast, very narrow high amplitude EMI noise spikes are rejected as noise.
I believe that this would go a long way to reducing some  EMI and then perhaps other methods like Aziz's could  be used as well? ""
thou dont know about doing it between coil and coil preamp.. can be done in later stages
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« Reply #75 on: Wednesday November 24 2010 11:29:51 AEDT AM »

  
Good thinking Dougy..
"Then we need DSP on the Rx signal so that very fast, very narrow high amplitude EMI noise spikes are rejected as noise.
I believe that this would go a long way to reducing some  EMI and then perhaps other methods like Aziz's could  be used as well? ""
thou dont know about doing it between coil and coil preamp.. can be done in later stages

 Maybe you could use something like these? But they are incredibly expensive! ::54:: ::54::
  
doug happy face
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« Reply #76 on: Wednesday November 24 2010 11:31:28 AEDT AM »

No, No, Hell No!

You guys are thinking very complex.

My idea is the simplest one. Very cheap, very simple, very easy, very tricky, very funny, very genius, ...
It's so much simple, that every engineer won't think of it.

Here is the underlying very very complex math:
(2*EMI+(-EMI)) + (-EMI) = 0
or simplified
2*EMI - EMI - EMI = 0 (no EMI)
 rolleye 1
Aziz
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« Reply #77 on: Wednesday November 24 2010 11:34:33 AEDT AM »

yep looks great in theory  ::53::
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« Reply #78 on: Wednesday November 24 2010 21:43:57 AEDT PM »

 Another idea for an anti interference coil?
Transmit on the center mono coil and receive anti phase on the 2 outside coils?
doug happy face


* dougs coil 1.jpg (37.32 KB, 846x351 - viewed 160 times.)
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« Reply #79 on: Wednesday November 24 2010 23:20:38 AEDT PM »

 Another thought maybe crazy, ::53:: or not feasible, but what the  heck its about putting ideas and thoughts forward. Transmit on all 3 windings but receive anti phase on the 2 outside coils?
doug happy face
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