Super TGSL With H Bridge TX Diff RX Quad Sampling

[Silver Dollar]

I used to work on GPR stuff and the higher frequencies didn't go as deep so I guess it's a tradeoff.

I have read that 12.5 khz is the best all around frequency so wonder why most mfg's have migrated
to 6.5 khz for general purpose? I have a chart around somewhere with frequencies of various targets
I wonder if a magic freq can be chosen so the harmonics hit as many targets as possible?

I'm looking for anything interesting but prefer if jewelry were favored. It is difficult as small and large
objects of a given material discriminate differently....

[Davor]

GPR is so very different from these frequencies, and 6.5kHz is within an octave from 12.5kHz. Multi frequency rigs employ different frequencies for entirely different reason.

Small and large objects do not discriminate differently on a VLF machine unless the thickness of a target is less than a skin (effect) deep. This condition is met for kitchen foil and such. PI machines have this problem because of a large frequency spectrum they cover in a single blow, and for a part of that spectrum thinner than skin depth conditions are met. Some day this will most probably be used for determining the size of a target, but right now it is just hindering a proper discrimination.

[Silver Dollar]

I understand some multifrequency rigs can more accurately discriminate and also extract depth info.
While others choose the frequency with the strongest signal. But I wonder if a single frequency MD
has different depth depending on the frequency? I'm not sure that can be answered definitively as
there are so many variables.

I may have worded my statement wrongly as I meant that small gold will discriminate differently
depending on how deep it is as well as other factors...

[Davor]

Trouble is that people expect too much from the "other factors" even when they are negligible, or simply not there.
Depth info is usually just a gimmick. Many old rigs had it, and it was mainly based on a signal strength measurement. There is a much better way of noticing the depth of a target, and it takes only your ears, half an hour of field work, and a proportional audio. Binary audio simply destroys the effect, therefore I reacted when you suggested such audio for this project.

How does the depth estimation work? Simply - deep targets - regardless of their size- appear much softer when scanning over them compared to the shallow ones. Once you become friendly with your rig, you'll be able to "see" the target in the ground. Surprises? Guaranteed, but that's part of the fun.

You can't expect to experience this effect on rigs with binary audio or clankers with synthesized deedle-doodle audio. Regardless of the number of colours of their displays. The best a display can do is to confirm what you might have heard with your own ears.

[Davor]

I made some changes ...finally.
1. I've put some resistors to soften the audio. I didn't simulate it so proper values might be somewhat different. It may need some extra care.
2. I added a pre-shifter that corrects the Rx to Tx phase to 0° shift, or better say a ferrite shift. It translates all phases equally. It is also used as a fixed "all metal" reference for discrimination in order to maintain good discrimination even at severe GEB shifts.
3. As I needed an extra comparator I simply replaced a dual with quad - it is cheaper than introducing a LM311.
4. I corrected negative supplies for 4053 and the phase shifting comparators. They all use GND as negative rail, but 4053 has Vee pin connected to -5V to be able to cope with negative signal.

[Silver Dollar]

Thanks for the updates!

I forgot the transmitter is referenced off GND. Oops!

I'm going to have trouble fitting the extra comparator on my board. Maybe I can piggyback the 393...

I still want to tweak the audio a bit. Volume changes might make us loose deep signals so maybe a
slight frequency change to indicate amplitude of the GEB channel? I'm not sure about biasing but
perhaps a FET fed from the GEB signal that parallels a resistor across the oscillator R?

Something like this?

[Davor]

I'll see it later on. I'm not sure a VCO will do much good if you already have a proportional audio. Using several modes of indication may turn confusing.

[Davor]

Well, if there must be a VCO, it should be hooked to pin 7 of U3.
Also I'm not entirely happy with PP circuitry. Something is wrong with input. It should be DC isolated from a mixer, and it must be referenced to signal ground. The mixer presents itself as a low impedance DC source (air signal x GB phase) to the PP circuit, so the servo (U3a) can't do much about it.
I suggest the changes as in the attached correction. Values might be a little bit off.

[6666]

I have a chart around somewhere with frequencies of various targets

SD if you find it could you put it here please.

[Thomas]

I also have a request, Could we see an image file, I cannot see .SCH files.

[6666]

Try this.

[Davor]

Just a tip - when you wish to save a drawing or a diagram with only a few colours, such as a desktop, use .gif or even .png, but not jpeg. This whole image could have fit into less than 1/10 of this size AND without any loss in quality. Jpeg is good for photos, but sucks for everything else.

For reading .sch files go to http://www.expresspcb.com/ and download a free software. It is far from the best software around, but due to a very limited number of options you learn it in an hour. For this reason many people use it for sharing schematics.

[Silver Dollar]

Here's some info about target frequencies I have kept for reference;

"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.

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.


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


Here is a list of items that certain frequency's respond best to certain metal. This information comes from George Payne, the engineer that gave us all the ground canceling, target ID'ing, discriminating detectors that everyone bases their designs on today.


Best frequency to use:
US Dime = 2.7 KHz
Gold Coins & other US Coins = below 10 KHz.
Us Nickel & gold Jewelry = 16.5 KHz
Gold Nuggets = above 20 KHz

[Silver Dollar]

Thanks for the help on the schematics. I'm more of a technician than an engineer!
I realized the FET to tweak the oscillator frequency should go on the other resistor.
Here are the schematic and a PNG of the change. I'm calling this Version 4.0.

[ashefr10]

hello , thanks for the work , what is the AVENTAGE over classical TGSL?