The Wisdom of George Payne - Part 1

"Why a VLF ground balanced detector has less sensitivity to higher conductive targets in the all metal mode." by George Payne

The target signal returned to the receive coil can be thought of as composed of two components, one we call x and one we call r. The polarity of the x signal (its direction) tells us if the target is ferrous or non-ferrous. The r signal has only one polarity. Also, the ratio of the x and r signal tells us the target’s phase. In addition, the signal magnitude (which relates to sensitivity) of both x and r are a function of operating frequency.

A VLF detector by its very nature is only designed to respond to the r signal and ignore the x signal. Since the ground reaction primarily produces a x signal in the receive coil the VLF detector does not pick-up the ground but only responds to the r signal of the target. Therefore, the VLF detector only needs the r signal for proper operation.

However, for discrimination we need to measure the x signal and the r signal to determine what the target is. Since we are using the x signal then we have to contend with the resultant ground signal pick-up.

The x and r target signals are frequency dependent and obey very predictable characteristics when the operating frequency changes. We know that the x component decreases as the operating frequency decreases. Above a certain frequency the x component reaches a maximum. The r component acts differently. It is maximum at one particular frequency and decreases if you go up or down in frequency. We call the special frequency at which the r signal is maximum, the target’s “-3db” frequency. It also turns out that at the -3db frequency the x signal is one-half of its maximum value. This special frequency is unique to each target and is different for different target.

The higher the conductivity of the target the higher will be the targets -3db frequency.  Conversely, the lower the conductivity the lower the -3db frequency. The -3db frequency of the high conductivity target will also make the r signal peak at a high frequency, normally well above the operating frequency of the VLF detector. This will make the high conductivity target have lower sensitivity on the VLF detector because the r signal amplitude drops if we are significantly below the -3db frequency. Simply put, maximum sensitivity on a VLF detector would be if we position the operating frequency directly at  the target’s -3db frequency. For example, a dime and penny have a -3db frequency of about 2.7KHz. This is where their r signal peaks and would be the best frequency for picking them up using a VLF detector. However, a silver dollar has a -3db frequency of 800Hz. Nickels, on the other hand, have a -3db frequency, where its r peaks, at about 17KHz. Targets like thin rings and fine gold are higher still. Clearly there is no one frequency that is best for all these targets. The best you can do is have an operating frequency that is a compromise.

All of the discussion so far pertains just to “r reading” VLF detectors. If you now add in the discrimination requirement if gets really confusing. Remember, to obtain discrimination we need to read both the x and the r signal components. As I said the best response to the x signal is not the same as the r signal. We need to be at an entirely different frequency for x. Generally for best discrimination we need to have an operating frequency well above the targets -3db frequency.

As you can see the ideal frequency for each target is different. In addition, for best performance the operating frequency to read x should be different from the frequency to read r. The best we can do is reach a compromise frequency. Generally we can say that high frequencies are best for low conductivity targets and low frequencies are best for high conductivity targets.

********************************************************************************************

I have say I like the way he explained this issue - nothing like hearing from the so called inventor of motion detection.

One thing I learned is that the Gold Bug DB or G2 should be great nickel hunters.

Bryanna - Nebraska

Current - New to me but not new MXT Pro and T2 SE2 - Previous Minelab Sovereign GT, Minelab Safari, Whites DFX, Whites Eagle Spectrum
Smile its a good for you!

Good read !

And yes George is the most in tune Guy to ever design a detector....


See where He states that 17kHz is the ideal freq for Nickle's...

I talked on the phone to Troy Galloway one time about optimum freq's and He said He designed His Shadow X-5 at 19kHz frq because he said after extensive in lab testing the 19 resonated the nickle the best....

I lean towards high freq machine myself because of the affinity they have for low conductor's...They really are the best choice for iron debris unmasking ..Think if a co-located target even if it's a quarter and a nail ..It is seen as a low conductor to a detector in a compostie snapshot...

I myself absolutely love machine's above 17 khz for working iron...Seriously they will produce when all else fails...

I believe small buttons and small gold coins will resonate at even a higher freq....

That's the beauty of the DEUS actually seeing what happens when freq's are selected ...even on the X-terra you can see thing's happen when you use different freq's...

Remember when I stated the 3kHz Xterra coil was a silver killer....Well funny He say's the dime resonates the best at 2.7...

Look back at the best coin hunter's they run around 5-8 kHz...Now we know why ..help's discriminate better but still attain coin affinity freq's...

Good post ..

He has talked of releasing an updated mark 1 on a side note..

Keith

Modern DSP-processor based machines, such as the T2/F75 have the ability to circumvent some of the 'ideal frequency' issues. Doing the X,R processing in mathematics allows more options and less compromises than traditional techniques. For example, the existence of JE (Jewellery) mode on the F75, and the generally good performance at picking up silver/copper/bronze coins despite being some way off 'ideal' frequency (13KHz vs. 1 - 3KHz).
Last year I said I was going to try and measure the 'natural' frequencies of some targets, as, other than this Payne document, there is virtually no mention of them. I failed at my first attempt, the second was better, I measured 3KHz for a wheat cent and 12 to 13KHz for a nickel. I'm not sure what to make of the discrepancy with the nickel, maybe my next attempt will be better. One thing worth noting, though, is that targets are not highly tuned, hence matching target and machine is not a critical requirement.
Edit: I did find a reference online to a nickel's natural frequency being 12.5KHz, but I can't attribute it to a reliable source.



Edited 2 time(s). Last edit at 07/31/2012 03:41AM by Pimento.

Yes they Do Pimento....But it is still an eye opener to see a target resonate on an ideal freq!

Good post

Keith

The V3i has three freq no surprise after reading George's article the choose 2.5khz for silver and 22.5khz for the smaller conductors like gold nuggets. 7.5khz must be middle conductors like brass. Makes you wonder why the v3 does not hammer on Silver but for some reason not as hard as minelab FBS.

Bryanna

Bryanna - Nebraska

Current - New to me but not new MXT Pro and T2 SE2 - Previous Minelab Sovereign GT, Minelab Safari, Whites DFX, Whites Eagle Spectrum
Smile its a good for you!

the FBS has the advantage of all freq's at once being transmitted in some harsher ground you will see better discriminating result's...Low freqs suffer poor discriminating problem's But high freqs suffer poor discrimination also...the middle of the road freq's disc the best......the V3i probably sees the coin but is mis I.D.'d alot more often than the F.B.S. Alot of machine's can get deep its the user's who choose to dig deep or not....another reason why you can not learn to dig deep target's or unmask with a detector in a few days of use....

Thats the only advantage I have ever found with F.B.S. is better I.d. at depth especially in mineral......they are not neccesarily deeper they just I.D. better...More user friendly...less target chasing ...and In a park or yard that is a good thing...relic hunting deep wood's you can chase alot of signals that the F.B.S. machine's probably would not allow you to become enthused about.


A minelab Musketeer is extremely deep on High conductor's...Some people who kill deep silver hunt with the Musky...and with excellent result's...but there again it's a dedicated 5kHz machine...With a 5kHz aligned coil and a whole platform built around 5kHz......some what lacking discriminaiton though...


Keith

It seems from reading this that the ideal detector is a detector with multiple frequencies that you can choose on the fly, a la V3i or XP Deus. Provided, of course, that the operator is skilled enough to maximize the machine's capabilities.

I read somewhere else, that the lowest frequencies are optimum for high conductors only under certain conditions. In particular, conditions that lead to wrap-around. I could be remembering it wrong though...

Pinpoint twice, dig once

Hey pulltab..yes high freq machine's will tend to wrap high coins into iron on some platfrom's...and lower freqs really dont do this...

The DEUS will wrap high condcutor's into iron on the high freq but not a problem at 8 or 12 or 4 kHz...so there's some merit to your statement..

But some high freq machines dont do this....A GMP which is on the same freq as the DEUS' High freq will not wrap high conductor's but the DEUS will..

Keith

Just remember if a unit likes nickles many gold rings lay in this general area....Rule of thumb has always been high conductor low freq. and low conductor high freq.

Certainly arguable with some of the units I have used but a CZ with a five and fifteen gets the deep silver and may be the best nickle unit I ever used...

Using an F5 with a 7.8 freq. does the above well in both instances so you can see a compromize freq. further adds to the truism of George's writings....

Now a multi freq unit like the Explorer series may be the best silver unit out there but certainly not the best nickle or gold ring unit which certainly confuses me...

Dan's post.. ((( Now a multi freq unit like the Explorer series may be the best silver unit out there but certainly not the best nickle or gold ring unit which certainly confuses me... )))

Dan,
Could part of this be due to the fact that some Explorer and E-Trac user's get locked in on finding silver ??

And I wonder what the ratio of nickels to other coins are lost ??

The AT Pro with it 15 kHz seems to like nickels to, out of all my nickels with it since Feb. 2 have been war nickles, 35 % silver, and read the same as a newer Jefferson, at 53...
My friend dug a Buffalo last month with his E-Trac because he started digging the nickel slot.
Saw another friend with his Explorer SE pro dig 2 V nickels...along with other nickels...
I think my last Buffalo was with a CZ.....

I could be wrong,, but, I don't think any 1 unit or coil can do it all.
Even Tiger has more than 1 club....