DFX vs. XLT

[Detector]

My understanding is that the idea of using multiple frequencies is to get a better or more accurate response from the target. That is to say 16 frequencies excite a target better than one, and the response is received by the 2 best frequencies 3 and kHz.

I'm with Tony. I really don't think the multiple frequency has no purpose other than to waste energy and a sales gimmick. One thing I can say with confidence. I've had both the Spectrum Eagle and XLT and the DFX is the best I've had by far.

[del]

i've had both xlt and dfx and the dfx is better at finding smaller non-ferrous targets like buttons and gold related items and at greater depths plus it is a better beach detector ( very stable in the wet black sand ) the xlt would give alot of false signals in wet beach. plus at the present there are by far better coils(dd) for the dfx than the xlt . these are just my opinons to share

[Jason]

Wow, that WAS pretty technical. So from what I understand is that the 16 frequencies hit the target and excite it as you say, then the target produces harmonic frequencies which the DFX uses 2 to better translate the data it recieves? Is this what y'all are telling me? WHat about something like the Infenium that produces 96 frequencies.

[Jason]

Hey auseeker, what is that you are holding in your picture?

[Detector]

I use the term excite from a lack technical electrical terminology and to simplify a concept for us layman.

Originally Posted by Jason

Hey auseeker, what is that you are holding in your picture?

Jason, glad you asked. They are two large dungeness crab! Our season opened in July and they were the first ones caught this year. Also was the only picture I had to post. Good luck and Happy Hunting......

[OkieDigger]

Mmmm, crab...now I'm hungry!

[Rudy]

Originally Posted by TonyinCT

OK so in between all that goobly gook you just stated that apparently none of us understand , why do Minelab and Whites and Garrett for that matter bother "wasting" their energy producing these detectors that transmit multiple harmonic frequencies if they are of no use ? It makes no sense to me to produce a detector with this technology if it doesn't work or is wasted energy.!
And I stand by my statement that the detector "sees" more of the target in the ground. Which I suppose if you turned on a light you could see more since you were "illuminating" the area.

Tony, we were discussing the DFX. The Minelab (Explorer) is a different beast altogether. The dectection used in the Minelab is significantly different from the DFX (never looked at the Garrett so I can't comment on it). The ML detector channels operate in a phase locked manner to the transmitter. It detects on three different frequencies (fixed). The other harmonics are alsom... wasted.

You can believe all the marketing hype if you wish. It really doesn't matter. You can hunt successfully without understanding what is going on inside the detector.

[Rudy]

Originally Posted by Jason

Wow, that WAS pretty technical.! So from what I understand is that the 16 frequencies hit the target and excite it as you say, then the target produces harmonic frequencies which the DFX uses 2 to better translate the data it recieves?! Is this what y'all are telling me?! WHat about something like the Infenium that produces 96 frequencies.

Jason,

The harmonics are produced by the detector's transmitter. The targets respond with a field at the same frequency they are being excited with. Since we are hitting them with multiple frequencies simultaneously, they respond in kind. The energy they respond with at each frequency is roughly a function of how much energy they received at that frequency, their resistance and inductance.

Once again, what is important is not how many frequencies the detector hits the target with, but how many frequencies it is capable of listening to(processing). In the case of an MXT, it is just one frequency, in the case of a DFX it is two different (and fixed) frequencies.

[Detector]

OK so from what Jeff Foster says both the 28 frequencies of the Exp II and the 16 frequencies of the DFX are somewhat for marketing. BUT! the harmonics are a by-product of the technology(natural) so they are not using any more energy(battery).

The real advantage is using 2 frequencies, and this is what gives the detector the edge over single frequency detectors.

Quote from Jeff Foster

First, some background info. A White's competitor has marketers that have been going on for some time about how their products use many frequencies ? 28 seems to be the current technology's claim. It is my understanding that the gist of the technology in that product is detailed in U.S. patent 5537041. In this patent (in section 9, lines 57 to 60), the inventor makes clear that this technology does not use multiple frequencies (like the technology discussed in U.S. patent 4868504). It works based on "time-domain" processing.

So, is it incorrect to say that the competitor uses 28 frequencies, if the patent for it says it is not a "frequency-domain" machine? Here's the rub. In the early 1800's a Frenchman named Joseph Fourier proved that any repeating wave-shape, like what might exist on an plucked guitar string, can be mathematically described as some number of simpler "pure" harmonics added together. For the time-domain machine in question, Fourier's math means that although the machine uses two pulses of different widths, these pulses are comprised of 28 harmonics.

So what does this all mean so far? Well, it appears that the characterization of the competitor machine as a 28-frequency machine seems not to directly, in my opinion, relate to the main gist of the machine's technology. This isn't to say that the technology is bad ? in my opinion it is very good technology ? but, in my opinion, the marketing seems to skirt the main operational character of the machine.

Given this backdrop, where White's competitor is leaning on Fourier's concept to market a time-domain metal detector as a 28 frequency detector, what can White's do? They can describe the operation of their detector by playing the same game.

The DFX generates the 3kHz and 15kHz signals to drive the search coil with digital electronics. Digital electronics deal only with ON and OFF - 1's and 0's ? so the mixed 3kHz and 15kHz signal that drives the search coil is not smooth. It is because of these rough edges that the DFX produces the 16, or so, (Fourier) harmonics.

In both cases, in my opinion, characterizing one machine as using 28 frequencies and the other as using 16, does absolutely nothing in terms of understanding the true operation of the respective technologies. It is my understanding that one machine is a time-domain machine, which, in my opinion, would be better described as not using any frequencies and the other uses 2 frequencies.

[Jason]

The next to last paragraph of that quote makes sence.

[DJS]

Originally Posted by Rudy

Because the receiver electronics are not tuned to them, so the signals emitted from the target at those frequencies are not being sensed by the receiver electronics inside the DFX.! The DFX receive channels, like on most VLF detectors, operates in the frequency domain and not in the time domain.

The harmonic frequencies are produced by the transmitter part of the detector simply because it is putting out rectangular current pulses into the coil.! Any repeating pulse train can be mathematically described as a series of sinusoidal waves, starting with the fundamental frequency (the pulse repetition frequency) and a series of harmonic frequencies with ever decreasing amplitudes.! This is known as a Fourier series.! See for example:
http://mathworld.wolfram.com/FourierSeries.html! or

http://en.wikipedia.org/wiki/Fourier_series

Because the amplitude (ie. energy) of the generated harmonics decreases quite rapidly as one gets further away from the fundamental frequency, the DFX transmitter plays a little trick.! Instead of putting out a pulse train with a constand repetition rate, if you look at the transmit signal on an oscilloscope, you'll see that the pulse train is really not constant.! Every so often you'll see pulses coming! out with a significantly different width. This changes the Fourier spectrum of the transmitted energy so that there are two energetic frequencies (ie. like two fundamental frequencies) and a whole slew of harmonics of progressively decreasing energies.! As you can probably guess, these two frequencies correspond to the two frequencies the DFX's receive channels operate on.! !Actually, there are three channels: the DC or all metal channel, the 3 KHz and the 15 KHz channel.

No. By broadcasting multiple frequencies the transmitter is "illuminating" the targets with a broad spectrum of frequencies, but it can only "see" the target response in only two of those frequencies. The rest is wasted energy.

It's like if I was to broadcast a message on all TV channels, but my TV is tuned to channel 4.

The harmonic frequencies are a natural byproduct of using a transmit circuit that produces a train of pulses, as explained by Fourier above.! It is a fairly inexpensive and reliable circuit.! The XLT, MXT and many other detectors use a similar circuit for the transmitter.! The XTerra circuit is different and it is designed to produce a sinusoidal wave rather than a pulse train. As such, it emmits very few harmonics.

I hope I didn't get to technical above.!

The take away is that what is important is not what is transmitted, but what the receiver is designed to accept and process.

wow, I had to go have a beer after trying to understand all that...


actually I did understand it for the most part(I think), I really dont know 100% how metal detectors work nor do I know much about transmitters/ receivers, but I can see what your saying

I'm not agreeing or disagreeing with anyone but after reading that I kind of have a question(probably a very dumb question)... if a MD puts out all those frequencies but only uses a couple, and different frequencies react differently with whatever is in the ground then couldnt you build a detector that constantly "scans" all those frequencies(like a police scanner) and locks on to which ever one(s) was strongest to get its information?
it would seem to be alot more efficent way of using the transmitted frequencies and getting the best information about what was being detected to me... but maybe I'm not understanding the principles of how it works, or maybe it just isnt practical or cost effective...

[Brian]

Multifrequency is just a marketing ploy.
Harmonics are used to suggest many frequencies are used and often that the result is more depth and better discrimination.
All modern P.I.'s are multifrequency (the same as the Infinium) but few offer any discrimination.
European (non P.I.) depth tests were all dominated by single frequency machines....twin and multifrequency didn't get a look in.
Minelab can claim as many frequencies as they like but its whats processed by the machine that matters, which is only two or three. In Minelabs case you end up with detector thats very slow in use. Whites bought the rights to a discarded Minelab patent and decided quite correctly that only a couple of frequencies are needed and the option to run as a single frequency machine is very important. Where I live twin frequency mode is poor and Whites themselves suggested it should be reserved for wet sand use only.
If you want real depth on wet sand and haven't got a high power P.I. then a DFX, Fisher CZ or Explorer/Sovereign is essential. On land I just don't see the need.

[Detector]

I wouldn't go so far as to say multi-frequency is just a marketing ploy because the DFX & Exp II are multi-frequency. They just exaggerate on the number a bit. Not all that uncommon. I take it the difference is between a "time domain" vs "frequency-domain"

[Rudy]

Originally Posted by Detector

OK so from what Jeff Foster says both the 28 frequencies of the Exp II and the 16 frequencies of the DFX are somewhat for marketing. BUT! the harmonics are a by-product of the technology(natural) so they are not using any more energy(battery).

The real advantage is using 2 frequencies, and this is what gives the detector the edge over single frequency detectors.

Quote from Jeff Foster

Thanks Detector,

I don't have Jeff's book since I don't own a DFX, but from what you quoted, it seems that both Jeff and I are in perfect agreement.

[Rudy]

Originally Posted by DJS

....
I'm not agreeing or disagreeing with anyone but after reading that I kind of have a question(probably a very dumb question)... if a MD puts out all those frequencies but only uses a couple, and different frequencies react differently with whatever is in the ground then couldnt you build a detector that constantly "scans" all those frequencies(like a police scanner) and locks on to which ever one(s) was strongest to get its information?
it would seem to be alot more efficent way of using the transmitted frequencies and getting the best information about what was being detected to me... but maybe I'm not understanding the principles of how it works, or maybe it just isnt practical or cost effective...

That is a good question!

It can be read two ways. I'll answer the first one that came to mind:

1. Since the transmitter is putting out all those harmonics, why not have a receiver section that scans them all?

The answer to that one is that the energy in the harmonics being put out are significantly less than the fundamental, so, there will be very little target signal to receive and extract to make it worthwhile. Also the depth at which you would be able to detect the target at these harmonics is likewise diminished.

For example, say the transmit section is set up to produce a square vawe pulse train operating at 3 KHz and, at this frequency it pumps 1 unit of energy into the ground. The first harmonic is at 6KHz and will have an amplitude of 1/2. The second harmonic is at 9 KHz and will have an amplitude of 1/4 and so on.

As you can see from the above, the energy from the harmonics decreases rapidly thus target signals from those harmonics will likewise be very small.

As I mentioned in an earlier post, White's and Minelab get around this decreasing energy problem by producing a pulse train in the transmitter that is more complex.

[Detector]

I don't have Jeff's book since I don't own a DFX, but from what you quoted, it seems that both Jeff and I are in perfect agreement.

Well I wouldn't quite say "perfect agreement"

Broadcasting at frequencies that are not used in the receiver section are just a waste of energy and battery juice. Only the frequencies the receive section are tuned to count.

From what I'm reading these unused harmonics are a natural part of all such frequencies. In fact it would take more energy to filter them out than let them exist.

[mxtswinger]

Many thanks folks for answering the call . I think I've got it pegged now. I've read tons of posts from DFX/Explorer users and I don't recall anyone mentioning the "multiple frequencies" as the reason for their success.

Good Swingin

[Brian]

Its all got away from which is deeper/better. If you want to cover land and wet beach then the DFX will work on the wet sand (XLT can be made to work but you lose performance).
Drawback of twin and multifrequency is it increases the problem of external noise/interference. Thus Minelab introduced noise cancelling. Also increases iron sensitivity.
With the DFX for best results on gold and nickel you should use a high frequency, for copper, bronze and silver low. Trouble is how do you know whats under the ground ? So you run both frequencies at the same time but that appears to reduce general sensitivity/depth.
By the way I don't think it runs at 3 and 5 khz but more like 2.7 and 14.7 or 8.
And even 3 khz is to low. They should have stuck with the 6.59 of the XLT/Spectrum.
The DFX has adjustable filters which is great, BUT, if you don't know what your doing its all to easy to lose performance. I've had the DFX and always kept an XLT. In a test bed the DFX can normally be made to go deeper than the XLT on a set target but in the field (on land) the XLT is for me the better machine. Its a bit like deciding if you want to cover a acre field all over to eight inches with an XLT or on occasions hit nine with the DFX but the rest of the time it could be eight, seven, or even six.
XLT is easier to use, DFX requires work and thought. Change your filters to improve performance and you don't react by changing your sweep speed to suit then you've done more harm than good. So its down to wether you want an easy life or not.

[Jason]

Thanks Brian. You answered my question perfectly. Wile all the rest was very interesting to read and I learned alot, some of it was too technical for me to understand. While I'm not a beginner, I'm also not an expert either. The DFX seems to be more for some one with lots of experience at the operations of a MD and want to tweak some of the options for their own style of detecting. I think I will go with the XLT which was the forerunner in my research anyway.