Something has been bothering me for some some time, and what it consists of is claims that a number of what I'd consider VLF detectors are really PI. These are the multifrequency detectors (EX II, Sov., DFX, CZ), and I've been hearing jargon being slung left right & centre; frequency domain, time domain, etc. Now, according to what I've picked up reading through the various forums regarding how a PI detector works, this is just so much more B.S. being slung around. Now, maybe I'm wrong and one or all of the aformentioned detectors might really be PI units, so therefor I figured that a post on this forum should clear the air. So whaddya say? ...Willy.
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Pulse induction or pure B.S.?
- Thread starter willy
- Start date
Think of a VLF metal detector in terms of the search head emitting a continuous waveform through a set of nulled coils that function like a loosely coupled transformer where the proximity of a metal target upsets the null. Further circuit analysis measures the phase difference of the null upset and displays the result as a phase angle reading either directly or according to a table of common targets.
Now, think of a PI detector sort of like a radar antenna where a high voltage pulse is emitted from the transmit coil then after the pulse and ringing dies down, the same coil (or another coil as in a DD coil) becomes the receive coil and senses the induced residual currents flowing in a target near the search coil. The processing is done in terms of time, in particular, time displacement of TX versus RX signals. As RX sampling is brought closer to the TX pulse (10 to 12uS and under), currents in poor conductors, like gold, can be detected better and ground responses are largely ignored. Residual currents in coins and iron last much longer so some crude types of discrimination can be done by comparing the early versus the later responses to unknown targets.
I hope this makes the technology a little more understandable.
bbsailor
Now, think of a PI detector sort of like a radar antenna where a high voltage pulse is emitted from the transmit coil then after the pulse and ringing dies down, the same coil (or another coil as in a DD coil) becomes the receive coil and senses the induced residual currents flowing in a target near the search coil. The processing is done in terms of time, in particular, time displacement of TX versus RX signals. As RX sampling is brought closer to the TX pulse (10 to 12uS and under), currents in poor conductors, like gold, can be detected better and ground responses are largely ignored. Residual currents in coins and iron last much longer so some crude types of discrimination can be done by comparing the early versus the later responses to unknown targets.
I hope this makes the technology a little more understandable.
bbsailor
A
Anonymous
Guest
You also have the off resonance discriminator which uses one loop winding and is not a Pulse, or a straight TR, or a BFO. When a target is hit by the loop's field a frequency shift takes place and that is what is measured-with this system you do not have a depth loss as from a TR disc/motion circuit when turning up the disc. level.
Besides, you can have PIs with multiple TX and Receive windings-I have one.
Besides, you can have PIs with multiple TX and Receive windings-I have one.
What about the claims being made that the BBS detectors and a couple of other multifrequency detectors are really PI? I'm basically familiar with the difference between VLF & PI. What is really bothering me is that, unless the aforementioned detectors really are PI, someone is either unintentionally spreading misinformation.. or is deliberately lying. I'm not really affected, except at an intellectual level, since I'll research stuff pretty extensively, but what about the poor SOB who, not knowing any better plunks down $1000 of hard earned cash for something totally inappropriate? ...Willy.
Sorry, I can't speak for marketing claims. I have a hard enough time getting the technical stuff straight. Send your questions to the companies that make the devices that you question, then post their answers so we can all be educated.
Remember that marketing folks take a lot of liberties with the functionality of how things work and rely on words like "virtually" as an escape clause. Once you really know how something works, then treat anything to the contrary as marketing hype. We seem to have lost "truth in advertising" as the lawyers have really twisted it for a marketing advantage. That is why it pays to be an informed consumer and forums like this are valuable for keeping us informed.
bbsailor
Remember that marketing folks take a lot of liberties with the functionality of how things work and rely on words like "virtually" as an escape clause. Once you really know how something works, then treat anything to the contrary as marketing hype. We seem to have lost "truth in advertising" as the lawyers have really twisted it for a marketing advantage. That is why it pays to be an informed consumer and forums like this are valuable for keeping us informed.
bbsailor
I have seen something in the past on this question. Someone who had looked at the patent, described it as a hybrid, there is elements of both in it the way the signal is handled and processed. Basicly, it is it's on type, something different, even though I think most people consider it a VLF type. Don
What I'm talking about has no bearing on the company claims (though they warrant a bit of skepticism) but what certain individuals are saying on various forums. here's an example: http://www.findmall.com/read.php?55,264133,264155#msg-264155 Now here is a person stating that certain detectors ARE PI. ...Willy.
Hi Willy,
First, I think people are trying to define these multi frequency units based upon the present understanding and typical design criteria that has been used to define the typical VLF and the PI. As such, the multi freq units such as the explorer doesn't really fit either.
A true pulse induction would be defined as a pulse transmit and some time later, a sample taken, once the signal has decayed to zero volts.
That isn't happening on the Explorer and the sampling is more along the lines of what is done with a VLF.
Is the signal output of these machines a form of a pulse? Yep since a rectangular wave is a form of a pulse, but it is not like those found in a typical PI. The current is continuous so it doesn't have a flyback pulse type signal. As such, the receive signal is analyzed more like the VLF.
I think Don best described the multi freq units when he indicated them to be hybrids. They appear to have some features of both.
Reg
First, I think people are trying to define these multi frequency units based upon the present understanding and typical design criteria that has been used to define the typical VLF and the PI. As such, the multi freq units such as the explorer doesn't really fit either.
A true pulse induction would be defined as a pulse transmit and some time later, a sample taken, once the signal has decayed to zero volts.
That isn't happening on the Explorer and the sampling is more along the lines of what is done with a VLF.
Is the signal output of these machines a form of a pulse? Yep since a rectangular wave is a form of a pulse, but it is not like those found in a typical PI. The current is continuous so it doesn't have a flyback pulse type signal. As such, the receive signal is analyzed more like the VLF.
I think Don best described the multi freq units when he indicated them to be hybrids. They appear to have some features of both.
Reg
A
Anonymous
Guest
The bottom line is that a Pulse detector has no phase angle-and that is why they cannot discriminate like a VLF, and use pulse delay decay rate to help identify. The multi-frequency vlfs are exactly that VLF-not pulse.
Minelab clearly identifys their detectors as pulse, or vlf.
Minelab clearly identifys their detectors as pulse, or vlf.
Hi all,Many people are fooled by the difference between a square wave pulsed VLF and a PI detector. The truth is that an on-off square wave is just as usable in a VLF detector as is a sinusoidal transmit signal.
A square pulse contains energy at it's fundamental frequency as well as at harmonic frequencies. This is used to advantage in multi-frequency detectors.
Some advanced pulsed VLF detectors use a series of long and short pulses to provide the equivalent of transmitting a plurality of frequencies. All of these VLF detectors sample at least twice during each transmit cycle A first sample is taken from the receive signal at close to zero degrees and a second at ninety degrees relative to the phase of the transmit signal. Most multi-frequency detectors sample the receive signal twice for each frequency.
Each type of target provides an output signal from the demodulators which changes with different transmit frequencies. Multi-frequency detectors can thus make a finger print so to speak for each type of target as they collect more target information than single frequency detectors.
A single frequency VLF measures the ratio of the two demodulated signals. The ratio of these signals is often output as a number and is displayed as a target ID. Measuring the ratio of the two demodulated signals also provides the detector with it's discrimination ability.
A Pulse Induction detector is different from a VLF in as much as it transmits a signal by switching the battery voltage directly across the transmit or transmit - receive coil depending if the detector is using a balanced coil such as dual D or a simple mono coil.
Switching the coil across the power supply circuit causes a very heavy current to flow through it. the current causes a magnetic field to form around the coil. After a specific time period the coil is switched out of the circuit. A further short delay is provided so the circuit can settle down before one or more samples of the receive signal are taken. These samples are taken after the PI's transmit period ends unlike the VLF which samples during it's transmit cycles or pulses.
OK, back to the VLF. A VLF measures two components of the receive signal. The first component is in phase with the transmit signal and is due to the reactive component of the signal. This is referred to as "X" The second component is 90 degrees from the transmit signal and is due to the resistive component of the signal. This is referred to as "R". The mix or ratio of the "X" to the "R" component is what a VLF measures in order to discriminate. As the ground signal is almost a pure reactive signal then one can use the resistive component for a ground cancelling mode. A control or automatic circuit sets the "R" demodulator for a precise zero ground content. The clean "R" signal is an all metal signal.
The reactive or "X" component is required if we are to discriminate and or identify the target with a TID. The problem is that the ground signal adds and subtracts from the reactive signal as the coil moves across the ground which makes measurements useless. This problem has been solved to a certain degree in motion detectors by filtering both the reactive and the resistive signals with filters which are designed not to pass the slowly changing ground signal. Such filtering is employed prior to measuring the ratio of the signals. Both channels are filtered even though most if not all of the ground signal resides in the "X" component. This keeps the two signals together in their timing prior to their ratiometric comparison.
It is possible to make a hybrid VLF-PI. Such detectors are referred to as PIB's or Pulse Induction Balance detectors. The sample taken after the transmit pulse ends provides the "R" sample. a sample is also taken during the time that the transmitter coil is energized. This sample provides the "X" sample. A balanced coil is required for a PIB to discriminate. The coil can be either a dual D or a concentric coplanar. A balanced coil is required as the "X" sample is taken during the period when the transmit coil is energized.
In air any target can easily be discriminated using the "X" and "R" samples. Filtering is possible as with a VLF motion detector although depth is easily lost as the "X" sample is a little different from that which is obtained by a VLF. New methods and circuitry have recently overcome this problem 100%.
There is a bright future for PIB's. The marriage of the PI's depth capability and it's ability to ignore black sand etc. with the full range discrimination abilities of a VLF will provide super depth detectors with disc, notch, TID and all the features once thought reserved for the VLF detector with it's inferior depth capability.
I hope that this helps clear things up, Dave. * * *
Captain Kirk
New member
Willy,
Cody and I have a basic disagreement on whether or not the Explorer II is a PI machine. Even though both technologies use time domain analysis of the received signal, this is not what characterizes a PI detector.
I think bbsailor's comments about a radar pulse are quite good. That is the same analogy that I use. I think the basic PI concept is that there are two separate and distinct operations.
#1 The target is stimulated and then the transmitter pulse is turned off.
#2 The lingering effects of the eddy current are measured.
In the Explorer II operation the target continues to be stimulated while the target response is being analyzed.
Keep in mind that I am not expert like Eric Foster.
Glenn
Cody and I have a basic disagreement on whether or not the Explorer II is a PI machine. Even though both technologies use time domain analysis of the received signal, this is not what characterizes a PI detector.
I think bbsailor's comments about a radar pulse are quite good. That is the same analogy that I use. I think the basic PI concept is that there are two separate and distinct operations.
#1 The target is stimulated and then the transmitter pulse is turned off.
#2 The lingering effects of the eddy current are measured.
In the Explorer II operation the target continues to be stimulated while the target response is being analyzed.
Keep in mind that I am not expert like Eric Foster.
Glenn
Prospector Al
New member
Excellent post Dave. I recall from EE-101 that the Fourier analysis
of a square wave yields a fundamental and harmonics with decreasing amplitude. For a perfectly square wave-shape, the number of harmonics is infinite. So, the people who say they are transmitting at 17 or 28
frequencies at the same time aren't telling a lie--they're simply trying impress the public with what Willy would call B.S.
I can easily make a detector that transmits at 177 frequencies at the same time! Any usefulness of such a scheme would depend on the analysis of the returning signal. The relative magnitudes of the different frequencies and their phase shifts have been changed owing to the interaction with the soil and the target.
It takes a very sophisticated system to derive any useful information form the returned data. A program running on a laptop computer might do it.
There were other excellent posts on the subject--Willy's questions should now have been answered: Yes, Willy, there is B.S. out there, but sometimes it is the wording of facts that causes confusion...
Prospector Al
of a square wave yields a fundamental and harmonics with decreasing amplitude. For a perfectly square wave-shape, the number of harmonics is infinite. So, the people who say they are transmitting at 17 or 28
frequencies at the same time aren't telling a lie--they're simply trying impress the public with what Willy would call B.S.
I can easily make a detector that transmits at 177 frequencies at the same time! Any usefulness of such a scheme would depend on the analysis of the returning signal. The relative magnitudes of the different frequencies and their phase shifts have been changed owing to the interaction with the soil and the target.
It takes a very sophisticated system to derive any useful information form the returned data. A program running on a laptop computer might do it.
There were other excellent posts on the subject--Willy's questions should now have been answered: Yes, Willy, there is B.S. out there, but sometimes it is the wording of facts that causes confusion...
Prospector Al
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Anonymous
Guest
n/t
A
Anonymous
Guest
AH Electronics in the early ' 70s in their off resonance detector which has been described as not BFO, TR, or PI, transmitted a square wave. It discriminated on the frequency shift that occured from the return signal from the target.
Captain Kirk
New member
Vlad,
I think your statement is an interesting one. I have to admit that referring to the Explorer II as a "VLF" detector caught my attention. When does the operation cease to be "very low frequency? In the Explorer II manual (on page) 7 it states:
"...Low signal frequencies (e.g. 1.5kHz) penetrate deepest, but sensitivity to smaller targets is low.
On the other hand, higher frequencies have a more shallow depth of penetration but high sensitivity to small targets.
The Explorer II's multiple-frequency operation provides the advantage of both"
Could you give me the reference that makes the statement that the Explorer II is a VLF detector?
HH,
Glenn
I think your statement is an interesting one. I have to admit that referring to the Explorer II as a "VLF" detector caught my attention. When does the operation cease to be "very low frequency? In the Explorer II manual (on page) 7 it states:
"...Low signal frequencies (e.g. 1.5kHz) penetrate deepest, but sensitivity to smaller targets is low.
On the other hand, higher frequencies have a more shallow depth of penetration but high sensitivity to small targets.
The Explorer II's multiple-frequency operation provides the advantage of both"
Could you give me the reference that makes the statement that the Explorer II is a VLF detector?
HH,
Glenn
Captain Kirk
New member
Prospector,
I think that your comments about the infinite number of frequency components of an actual square wave (or rectangular pulse) is valid. I think that you will also agree that you can limit the higher end frequencies by controlling the leading and trailing edges of the "square wave"
HH,
Glenn
I think that your comments about the infinite number of frequency components of an actual square wave (or rectangular pulse) is valid. I think that you will also agree that you can limit the higher end frequencies by controlling the leading and trailing edges of the "square wave"
HH,
Glenn
A
Anonymous
Guest
sic "Multiple frequencies operating simultaneously sends and receives much more information to the Explorer-2 than a standard one or two frequency VLF detector."
Operating at frequencies above the VLF range is a good idea for sales, because these frequencies give extreme air test, but do not do well in ground with just mild minerals. The highest frequency of their Eureka gold detector is 60kHz, which is far below the 100kHz claimed on the Exp-2.
Operating at frequencies above the VLF range is a good idea for sales, because these frequencies give extreme air test, but do not do well in ground with just mild minerals. The highest frequency of their Eureka gold detector is 60kHz, which is far below the 100kHz claimed on the Exp-2.
A
Anonymous
Guest
Either people do not understand detector principles, or they are intentionally misleading.
You have 2 basic types being used in the hobby field today, Pulse Induction, which is described as time domain, or Transmit Receive which is frequency domain or continuous wave.
I have no doubt that if Minelab's multi frequency VLFs were PIs, they would be advertised as such, if for no other reason than a sales ploy to say they have a discriminating/identifying Pulse.
If you do a search on patents you find that in 1972 someone filed a multi frequency patent, and later on, TR-IBs were made using this principle.
However back at that time, A.H. Electronics came out with a detector known as "off-resonance"-the Pro, and was the first to discriminate conductive targets. It used frequency shift to determine discrimination points, and as such depth was not affected because no voltage change occurred in the single winding loop. Off-resonance has been described as not TR, BFO, or PI, but a different design altogether which used only one oscillator.
You have 2 basic types being used in the hobby field today, Pulse Induction, which is described as time domain, or Transmit Receive which is frequency domain or continuous wave.
I have no doubt that if Minelab's multi frequency VLFs were PIs, they would be advertised as such, if for no other reason than a sales ploy to say they have a discriminating/identifying Pulse.
If you do a search on patents you find that in 1972 someone filed a multi frequency patent, and later on, TR-IBs were made using this principle.
However back at that time, A.H. Electronics came out with a detector known as "off-resonance"-the Pro, and was the first to discriminate conductive targets. It used frequency shift to determine discrimination points, and as such depth was not affected because no voltage change occurred in the single winding loop. Off-resonance has been described as not TR, BFO, or PI, but a different design altogether which used only one oscillator.
Captain Kirk
New member
n/t