TESORO GOLDEN SABRE

[simonbaker]

I have played with multi frequency detection for a couple decades, as a side hobby building my own toys. What I do not see is much discussion over this subject, as well as DSP, and comb filtering. On a side note I am wondering. I have found a super deal on 100 TS904IN Quad OpAmps, see pdf for specs. What I am curious about is if anyone sees that using the two standby pins for synchronous switching (HI-Z output states) would in effect do the job of choppers instead of adding a 4066, discrete fets, or similar parts? I could be way off the mark since as is typical the data sheet is poorly written and no circuit examples are given of using the standby inputs.

I should add does anyone else remember when data sheets were more in depth and contained many example circuits?

It's a cool idea if you can reduce part count that way. Sounds worth experimenting with.

-SB

[dfbowers]

I got around to trying a crude approximation of that filter thing. There is some EMI that suddenly appeared in my workshop, so I tried wrapping the MD cable around a big piece of slotted ferrite a few times. It did not seem to affect the particular EMI I'm seeing; of course I don't know if my technique is same as that power brick thing, or if the EMI is the wrong type, or entering someplace else. I'll keep an eye on that technique though, it should block common mode EMI I would think.

Note: the cable config was ungrounded RX leads (wet grass).

Regards,

-SB

In reading a little about these ferrite beads, it appears as though they are designed to attenuate common mode EMI in the MHz ranges. They have their uses. I see them used quite a bit in power supples and used on single ended SCSI busses inside larger computer systems.

[Dr Vel]

I dont know if we are allowed to talk of ML architecture on here. Im interested in the multi frequeny Tx.

Whether the coils are non resonant - to cover 100kHz of BW.

And I like to know if each Sub Tx frequency is listened to by a series of matching Rx 'channels'

e.g. 20 Tx ch with 20 corresponding Rx channels.

As these puppie do 14" in soil vs std singl tone IB VLF at ~6"

S

I have played with multi frequency detection for a couple decades, as a side hobby building my own toys. What I do not see is much discussion over this subject, as well as DSP, and comb filtering. On a side note I am wondering. I have found a super deal on 100 TS904IN Quad OpAmps, see pdf for specs. What I am curious about is if anyone sees that using the two standby pins for synchronous switching (HI-Z output states) would in effect do the job of choppers instead of adding a 4066, discrete fets, or similar parts? I could be way off the mark since as is typical the data sheet is poorly written and no circuit examples are given of using the standby inputs.

I should add does anyone else remember when data sheets were more in depth and contained many example circuits?

[Farside]

Sorry, mis-posted and can't delete the message. I've found the correct topic for this now.

[Farside]

Hi all,
My apologies if this has been answered before...

Everyone seems to be using 0.2 - 0.25mm dia wire for their coils. Is there a reason for this?

I'm thinking of using a different guage based on some research in a totally different topic which has some uncanny similarities.

Thanks in advance

[simonbaker]

In one of the other posts entitled "Metal Detector -Upgrade for Greater Depth- 25% to 30% " I decided to dig a little deeper on what this guy was selling, and here is the only thing I could figure:http://metaldetectorsnow.co.uk/4-99-...garrett-etc-2/

Certainly not my idea .. but with the way that the TGSL reacts to EMI - robbing detection range, why not try a quick and dirty experiiment.

I clipped this off of an old PC power brick and Voila! Pretty early to tell, but it believe it DOES tend to improve detection range. I have to get out of subjective mode now and take some hard measurements. Noise measurements to follow.. Simon, you HAVE to try this..

Don

I got around to trying a crude approximation of that filter thing. There is some EMI that suddenly appeared in my workshop, so I tried wrapping the MD cable around a big piece of slotted ferrite a few times. It did not seem to affect the particular EMI I'm seeing; of course I don't know if my technique is same as that power brick thing, or if the EMI is the wrong type, or entering someplace else. I'll keep an eye on that technique though, it should block common mode EMI I would think.

Note: the cable config was ungrounded RX leads (wet grass).

Regards,

-SB

[golfnut]

I dont know if we are allowed to talk of ML architecture on here. Im interested in the multi frequeny Tx.

Whether the coils are non resonant - to cover 100kHz of BW.

And I like to know if each Sub Tx frequency is listened to by a series of matching Rx 'channels'

e.g. 20 Tx ch with 20 corresponding Rx channels.

As these puppie do 14" in soil vs std singl tone IB VLF at ~6"

S

[der_fisherman]

I think there are a lot of assumptions built into that statement you have to be aware of to interpret it correctly. But it may be true for a well designed MD that operates at the "fringe" of its magnetic field. A lousy MD may operate closer in to the coil and might be easier to double the distance with less power (maybe???).

Coil magnetic fields do not follow the inverse square law of radiation I think. But assuming they did follow inverse square law, you have to factor in that the target signal also falls off like inverse square law -- so the round-trip signal is falling off by at least the fourth power. I think someone calculated that with a loop coil, the end result is a sixth power fall off. So 2^6 gives 64, hence 64 times the power to double the distance.

I think it is not as pessimistic as it sounds though. You calculated that 5 times the power would only buy you 7.8 %. However, my calculations indicate it would buy you 30%. Not sure I computed it correctly, but I calculate:

Distance Factor = (Power Increase) ^ 1/6

So DF = 5 ^ (1/6) == 1.31 or about 31% increase.

As for shielding --- that's a crazy subject I hope to examine, but I want to study some physics first and then probably need to do some computer simulations also.

As always, your questions and insights are interesting.

Regards,

-SB

Not as interesting as your (and others!) answers!!!!

Thanks

Andy

[simonbaker]

I read it too, who originally said that?
Originally Posted by kliner
I think it was said that to double the detection depth a power increase of 64 times would be required
Originally Posted by kliner
I think it was said that to double the detection depth a power increase of 64 times would be required

Andy

I think there are a lot of assumptions built into that statement you have to be aware of to interpret it correctly. But it may be true for a well designed MD that operates at the "fringe" of its magnetic field. A lousy MD may operate closer in to the coil and might be easier to double the distance with less power (maybe???).

Coil magnetic fields do not follow the inverse square law of radiation I think. But assuming they did follow inverse square law, you have to factor in that the target signal also falls off like inverse square law -- so the round-trip signal is falling off by at least the fourth power. I think someone calculated that with a loop coil, the end result is a sixth power fall off. So 2^6 gives 64, hence 64 times the power to double the distance.

I think it is not as pessimistic as it sounds though. You calculated that 5 times the power would only buy you 7.8 %. However, my calculations indicate it would buy you 30%. Not sure I computed it correctly, but I calculate:

Distance Factor = (Power Increase) ^ 1/6

So DF = 5 ^ (1/6) == 1.31 or about 31% increase.

As for shielding --- that's a crazy subject I hope to examine, but I want to study some physics first and then probably need to do some computer simulations also.

As always, your questions and insights are interesting.

Regards,

-SB

P.S. I will add that using a shorter time interval does not necessarily save power. By using a continuous signal, we can essentially integrate out noise more effectively, and gain signal to noise ratio. You can't get something for nothing. That doesn't mean we're using power as well as we can though, so design changes may be practical, including shorter time interval if in fact we're not utilizing our long interval to the fullest. Another idea is more resonant TX coils to increase current without using more power; this may have other disadvantages, but something I'm trying to explore.

[der_fisherman]

I think it was said that to double the detection depth a power increase of 64 times would be required

I read it too, who originally said that?

Andy

[der_fisherman]

Further discussions

2 Pi radians is a measure of phase between cycles, or in effect the measure of one cycle or 360 degrees in terms of angular frequency. Or rad = degrees times Pi/180. I probably should have just mentioned phase in terms of degrees. If the difference is greater than one cycle we are talking about a frequency difference not phase unless the RX signal is a non integer difference, said difference being measured in electrical degrees between cycles. Which puts us in line with the words of simonbaker and golfnut. This is how I see it for this method of detection.

I do doubt the need for a 64 fold increase in TX power being needed to produce a greater depth of detection. This is like saying if I can put one watt of energy into my search coil and detect a coin at say 15 inches that I would need to step up to 64 watts to go any deeper. While yes it would this does not mean I will not see improvement dissipating say 5 watts in the TX coil. To go to extreme power levels you do realize the size of the battery needed to operate the detector. Of course waveshape needs to be considered. If the TX signal is a fast pulse of high peak magnitude we can achieve greater depth while using less average energy from the supply.

In other words a PI detector. Yet the lack of phase information becomes the issue meaning inability to discriminate. Probably off topic to bring it up here but it seemed to fit the conversation. I would say proper TX waveshape, coil design, and fast processing will overcome this discrimination problem and to be honest reading what Carl and moodz are doing is to me the right track. I am not as versed in metal detector theory as many here having spent 50 years designing electronics in the field of radio but I am working on it.

On the subject of various materials for the coil shielding what I was trying to consider was the magnetic properties of the shield metal and whether diamagnetic properties could make a difference.

Firstly I must thank you for your further comments and ideas, you are a much needed person here to provoke good discussions in this area and I am sure that everyone here will welcome your radio knowledge as being far more positive than negative. I hope others will join me in thanking you and our "other" experts, of which there are many, thank God.

I personally am a "Digital Electronics" person and I flounder dreadfully in analog areas, I can follow a good explanation, but I am loath to say too much, though I try and find bits of info, like the Gold and Silver leaf idea that can be tested by other members and discussed by the experts.....

I was not the expert that said that a 64 fold increase in power was needed to double the Tx range. I always worked with the idea of a 4:1 increase for some ancient reason (that I have completely forgotten, Radio transmissions??), but I am not clever/knowledgeable enough to know which is correct!! If it actually follows the rules of Radio transmission, then power increases will be more effective (I think!), so can you add something to the discussion please on that point and correct my thinking?

I do like your thoughts of NOT have a sine wave, but having a fast pulse with a fairly large gap to save on battery power, similar to a PI, but in the VLF (15 KHz) range but the return signal being examined in a more VLF/TR manner, not PI, it does sound as being possible.....what does everyone think? Battery power and transmitting power being both very important.

If the factor of 64 is truly correct (I have no idea!), then going from 1 watt to 5 watt will bring only extra detection of the order of 7.8 % !! With a small object being say detected at 5" with 1 watt, it will now be detected at around 0.39" deeper. The variation on height when sweeping may even just lose this mini "gain" completely. As I previously mentioned, simply its not worth to increase Tx power so greatly for such a mini win.....Not even 0.5"!!! For larger deeper objects maybe it will help better....

Please comment further with more detail with regard to gold and silver leaf shielding....If I understand correctly what others have posted, is that not only is the type of shielding material important (what is diamagnetic etc..), but the thinner the better (more important than material type is my impression from other posts) and what is thinner than gold and silver leaf? I would guess that even the graphite sprays some use are still thicker than such leaf.......please anyone comment......Simon, Eduardo, Golfnut and Don.B etc....all of you.....we need to get some testing done before the weather improves......

I did some further research on Gold only here in Germany and the price for 10 sheets of gold leaf on a plastic carrier between 4 and 5 microns thick is €1.50, or less than US$2.00
Each leaf is 4.5 cm square. It should be "stronger" as its on a plastic sheet and easier to apply because of that.

24 sheets would be around US$4.00....and be more than enough for even two coils in a search head of 8 - 10" diameter......though as with the emergency blankets, you have to watch which side you use to contact the ground wire! Check with an Ohmeter meter first.....
Please discuss all points further....

Have a great day

Andy

[simonbaker]

2 Pi radians is a measure of phase between cycles, or in effect the measure of one cycle or 360 degrees in terms of angular frequency. Or rad = degrees times Pi/180. I probably should have just mentioned phase in terms of degrees. If the difference is greater than one cycle we are talking about a frequency difference not phase unless the RX signal is a non integer difference, said difference being measured in electrical degrees between cycles. Which puts us in line with the words of simonbaker and golfnut. This is how I see it for this method of detection.

I think it just boils down to phase modulation, which does not change the center frequency regardless of the amplitude of the phase change. You can modulate .1 radians or 20 radians. However, any phase modulation, rate or amplitude, will spread the spectrum of the carrier signal proportionally.

On the subject of various materials for the coil shielding what I was trying to consider was the magnetic properties of the shield metal and whether diamagnetic properties could make a difference.

I'm interested in those physics questions too. Probably a physics dude could simulate our coils and get some answers that would be useful to know.

Regards,

-SB

[kliner]

I think it was said that to double the detection depth a power increase of 64 times would be required

[Dr Vel]

2 Pi radians is a measure of phase between cycles, or in effect the measure of one cycle or 360 degrees in terms of angular frequency. Or rad = degrees times Pi/180. I probably should have just mentioned phase in terms of degrees. If the difference is greater than one cycle we are talking about a frequency difference not phase unless the RX signal is a non integer difference, said difference being measured in electrical degrees between cycles. Which puts us in line with the words of simonbaker and golfnut. This is how I see it for this method of detection.

I do doubt the need for a 64 fold increase in TX power being needed to produce a greater depth of detection. This is like saying if I can put one watt of energy into my search coil and detect a coin at say 15 inches that I would need to step up to 64 watts to go any deeper. While yes it would this does not mean I will not see improvement dissipating say 5 watts in the TX coil. To go to extreme power levels you do realize the size of the battery needed to operate the detector. Of course waveshape needs to be considered. If the TX signal is a fast pulse of high peak magnitude we can achieve greater depth while using less average energy from the supply.

In other words a PI detector. Yet the lack of phase information becomes the issue meaning inability to discriminate. Probably off topic to bring it up here but it seemed to fit the conversation. I would say proper TX waveshape, coil design, and fast processing will overcome this discrimination problem and to be honest reading what Carl and moodz are doing is to me the right track. I am not as versed in metal detector theory as many here having spent 50 years designing electronics in the field of radio but I am working on it.

On the subject of various materials for the coil shielding what I was trying to consider was the magnetic properties of the shield metal and whether diamagnetic properties could make a difference.

[simonbaker]

This is a homodyne radar in effect, or a zero IF RX, the image lands at DC.

Its my view that the Tx signal phase is what is pulled by the metal target. Rather than listening to a true return from a target (A PI does clearly)

A signal (phase pull) is generated from the motion over a target, slow sweep = low phase component return.

Steve

I don't think of it as Tx phase pull, because the synchronous detector is synced to the TX signal and would not notice any phase modulation of the TX signal.

Rather, I think the target eddy currents are transmitting a different phase signal to the RX coil at the same frequency. We detect that tiny signal, buried in the null signal, as a differential during the target sweep, since the null signal just results in a DC voltage.

Regards,

-SB