Operating Frequency <> Food for Thought

[tabdog]

This is a post by JB on another forum.

I thought it is intersting and wanted to share it
with tha great folks here.

George Payne designed the first detectors
with ground balance capability, discrimination, ID meters, tone ID and almost
all of the other features on modern detectors. Even David Johnson, who is
undoubtly a genius when designing detectors is concerned, who designed
many of the current detectors including the new Fisher series, several Whites
and Tesoro models and and the Shadow X5 says George Payne is still the
master. Below is an excerpt from one of the articles on my website George
wrote, followed by an article by Gary Finch.

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.

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.

George Payne, Copyright ©2002

George has repeatedly stated that the best frequencies for detecting all
targets were between 10 and 15 khz, meaning frequencies in that range
would respond better on targets from low in the conductivity range to high
in the conductivity range than either a higher than 15 khz or lower than 10
khz. He said the optimum frequency for single frequency detectors was 12.5
khz, and he designed the Treasure Baron series to run at that frequency.
Below is part of article by Gary Finch that corresponds with what George
says.

I've done a lot of research and field tests on this over the years, and the
results have consistently indicated that a detector with an operating
frequency between 10 and 15 KHZ does the best job on a wide variety of
targets, from relics to gold nuggets. This is one reason Tesoro detectors are
so popular as all around units: most of them operate between 10 and 12.5
KHZ, where they are extremely efficient.

My conclusions about operating frequency have come from years of tests,
both the usual in-ground tests on buried targets, (which of course are
always a little subjective, even in the most unbiased), and many scientific
tests on buried targets to determine the amount of voltage response
generated, using an oscilloscope and recorder, etc. (No big deal about this, I
just wanted to learn the truth about it for myself, and had the necessary
equipment).

I've talked with George Payne about this, and this was one reason why he
designed the Treasure Baron as a 12.5 KHZ single frequency unit: after a lot
of tests he came to the same conclusion. This was the optimum frequency
for detecting all metals.

Respectfully Submitted,

Tabdog

[Flapjack]

Thanks for sharing,

[FXDiz]

Good info. Makes you wonder why there are so many detectors under 10 khz freq out there.

[CyberSage]

Very interesting. Thanks for sharing this TabDog. I have just started playing with the 3Khz mode on the DFX in my test garden. With the moderate mineralization in my area it seems to have an edge for depth on coins for me. It does have a tendency to make even pull tabs sound great though. When I use 15Khz mode. It gets a bit noisy. Tiny shards of aluminum and bits of metal really sound off. Very interesting reading, Thanks.

Keep Swing'in
Jack

[rascal]

Interesting reading.

[Rudy]

It is a truism that silver coins are best detected by a detector operating at a relatively low frequency (e.g. 3 KHz), while gold targets are best detected by much higher frequency detectors. That is why dedicated VLF gold detectors operate at the highest frequencies of all VLF detectors.

However, the actual detection depth results obtained can not be solely determined by the operating frequency. The mineralization in the ground matrix is also reacting to the transmitted signal and producing its own response and its response will be very different depending on the nature
(and amount) of the mineralization; ferrous, or conductive salts. In general, low frequency VLF units have a tougher time on ferrous mineralization while high frequency VLF units have a tougher time on the conductive salts.

How well the detector tracks out the ground's response while
enhancing the "contrast" between the target's response and the ground's response is as important as the frequency at which they operate.

[RickO]

Interesting information TD.... and thanks Rudy for your technical inputs... they help. RickO

[Robin D]

Excellent article. Thanks for the info.

RD

[tabdog]

Thanks Rudy,

There is a lot to think about.

I am just learnin, like most of us.

I have not had a chance to try enough different

kinds of metal detectors to know which works best

for what.

I do OK with persistance. I have trouble with weight.

So I got into a somethin I can live with.

HH,

Tabdog

[tabdog]

Hay Jack,

Your DFX is not a two filter machine.

It is multi frequency.

It is a high end machine that can do

much more than I can comprehend.

I think it must be an awsome machine

when one can master it.

I doubt that this old Arkie will ever make

it that far.

HH,

Tabdog

[Rudy]

Originally Posted by RickO

Interesting information TD.... and thanks Rudy for your technical inputs... they help. RickO

You are welcome RickO.

Originally Posted by tabdog

Thanks Rudy,

There is a lot to think about.

I am just learnin, like most of us.

I have not had a chance to try enough different

kinds of metal detectors to know which works best

for what.

I do OK with persistance. I have trouble with weight.

So I got into a somethin I can live with.

HH,

Tabdog

Hey Tabdog,

We are all learning in this hobby. Some have been at it longer, that's all.

[UK Brian]

Something similar recently came up on the Metal Detecting forum and I'll put my reply

One of the top European designers wrote on the subject last year and said the old constraints of frequency no longer applied.
Dave Johnson and John Gardiner (who are chief engineers for the "new" Fisher group) this year wrote

"Frequency is no longer critical (except for 20-30 kHz being avoided because of interference from military communications)".

Good coil and circuit design works round frequency constraints.

Times change, high frequency detectors can go deeper on large items than a low frequency machine with the same coil type and size.

[Detector]

I've always thought the idea of different frequencies advantage was not so much because metal response but ground response. Lower frequencies penetrate deeper in mineralized soil is what I've always heard.

Good article tabdog, but just for the record the DFX is a 2 filter machine. It can be set to use 2, 3, 4, 5 or 6 filters.