GEB on IGSL/IDX

[golfnut]

In another post, someone suggested that a detector could be built by only using one channel, i.e. the DISC channel. If you try this experiment you will find that it works ok in an air test, but will react horribly to ferrite, and will be unusable in the field

I agree here.


One thing though - i thought for a uniform ground matrix and level sweep the ground signal frequeny is DC.

If it was then it would be outside of the Disc filter passband and would not pose a problem.


I guess the problem is that the ground varies a lot during a sweep and breaks through into the disc filter passband and would be a problem.

S

[Davor]

But don't the GEB channels still limit the detection sensitivity?

Yes, it is noise limited
It seem as if the GEB channel is completely misunderstood. In reality a GEB channel is an all metal channel, so it is maximum metal zero ground, and that's why tone signal is extracted from it. Furthermore, sensitivity of GEB channel is maximum at exactly 90° from ground phase, and funny that you mentioned it, small gold is found right about that angle. So GEB is about as optimal as it gets.

On the other hand, Disc. channel isn't. Let me explain.

A switching detector is in effect an analogue multiplier. It multiplies a square wave with an incoming signal. Such multiplication produces some high frequency components, but more importantly a DC signal that is related to the incoming signal level, and the phase. With only a slight approximation, we may say that the output of such multiplication after filtering is cosine related to the RMS value of the incoming signal. Cosine means full value for 0°, zero for 90° phase difference, and vanning values in between.

Let's assume for simplicity that we have a gold response at 80° from Disc sample phase. cos(80°)=0.174 and when converted to dB against the value in a GEB channel (0.985) it is -15dB. It means that you'll get a positive response on small gold only for 15dB++ in GEB channel.

Shortly, Disc. channel is in considerably more trouble than GEB.

My suggestion ... go for a dual discrimination just as IGSL does, and overlap the discrimination channels. You'll hit two flies with a single blow:
- you effectively get a very useful third "tone" indication for Al foil, modern nickel rich coins and small gold
- by overlapping, the disc. channel vector is skewed a bit further, so that 80° from the above example becomes 60-70°, and GEB to Disc. ratio becomes -4.7 to -8.7dB, hence your depth increases for at least 10%

I already overblown the gain in Disc. channels in my IGSL to compensate for the Disc. to GEB ratio, and as a consequence my IGSL is neither losing sensitivity at low levels, nor exact discrimination indication in full span from low to high levels in the overlapped region. I just love it.

[golfnut]

Ive recently been detecting with my IDX (at 11kHz) and my experience with it is this.

Large iron is frequent in UK - three max signal overloads per sweep is common where I detect.

With Disc set to reject small iron, (which it can - or it can reduce to a clipped tone - or a signal one way and not the other) and to accept all alloys above iron, the Rx easily gets swamped by large iron that it simply cannot reject.

I dug a 12guage brass in at spade depth face up for mest detect. It did get it, but the sound came from 'the back of the box'





Im formulating a wishlist for a solid UK detector.

Prerequisites are.

Solid 4quadrand detectors on target alloy receivers - to maximise S/N, Sens etc.

1)a Rx separately gated at phases for max sens to Silver, with unique tone ID
2)a Rx separately gated at phases for max sens to bronzes, with unique tone ID
3)a Rx separately gated at phases for max sens to gold, with unique tone ID



Single ended detectors like on IDX for Iron and GEB

4)Rx separately gated at phases for Iron, with unique tone ID
(high sense just is not required for tractor parts - avoids long saturated periods)

5)GEB Rx (I hope to use a common geb Rx as gnd signal is gnd signal right?)


I would expect to A/D read all channel outputs and to be able to select the larger outputs and play the relevant target tones.

On thing I fancy - If you got bronze and Iron together - Play the Id tones one after the other. I.e. Growl,Ping not one mixed in with the other..

I guess this is shaping up like a little like a goldmaxx power.


S

[Davor]

Except for 2) my IGSL does it all
You can get all that you listed with a proper VDI.

[golfnut]

I did my IGSL like that too. I made my Fe ch into a non Fe and split the phases into gold and silver.

But I had other problems with my IGSL which made my version a non starter.

S

[Qiaozhi]

In another post, someone suggested that a detector could be built by only using one channel, i.e. the DISC channel. If you try this experiment you will find that it works ok in an air test, but will react horribly to ferrite, and will be unusable in the field

I agree here.


One thing though - i thought for a uniform ground matrix and level sweep the ground signal frequeny is DC.

If it was then it would be outside of the Disc filter passband and would not pose a problem.


I guess the problem is that the ground varies a lot during a sweep and breaks through into the disc filter passband and would be a problem.

S

Even if the ground matrix was uniform, you would be reliant on the user having to keep the coil at a constant height above the ground, and you could only search in places where the ground was completely flat. The DISC channel changes amplitude in one direction for non-ferrous targets, and in the other direction for ferrous targets and ground.

[golfnut]

That explains why on mine which is wired reversed -

Gives a double tone each way for a target. See image.


The double tone does give a clear ID for a real digable target - and I get 8.5" on shotgun brass in mineralised clay - measured properly in the field - placed FlAT- FACE UP

I swapped my Rx wires over at lunch so I will retest tonight on way home. I expect Single beep indistinguishable from Fe target.


It also explains that when I had small pael meters on X and Y for Iron the meters moved the same way.

[golfnut]

Should get this tonite

[golfnut]

This...

....

[golfnut]

No, this....

.....

[golfnut]

Model of comparator on IDX

Run it. See OP go to 5v



Make V5 -100mV and OP only goes to 250mV

AHA all dropping into my hollow head..

Im a year behind you all but there non the less.

[golfnut]

Now with the model ....

late again steven....

[golfnut]

Reducing Iron breakthrough on IDX

With the current circuit for IDX, I tweaked the model for the Iron case.


So put -100mV in as a signal the OP goes to only 220mV . This stil breaks through as a tone as there is further amplification in the IDX.

To half the breakthrough of Iron - alter the resistor value of the 47k - the one in series with the diode.

Make it 1k and output only gets to 100mV on iron target. USEFULL And of course rail on a non FE.



Not tried in field - add a 1k2 in parallel to existing hardware for quick test....

S

[Davor]

Your sim is a bit wrong. You need comparators instead of op amps there - they have open collector output.

The diode polarisation OR type is wrong. In IDX you have a zenner in that place, and the polarity is as you put it in your sim. IGSL and TGSL have a 4148, but reverse polarised.

The role of that diode is merely preserving the input of a subsequent circuitry from a minus rail on input - not too many op amps are happy with their inputs being slammed to the rail.

[simonbaker]

But then the GB channel would not be doing its job of ground balancing. Also, if you simply move the GB sample pulse to the same position as the DISC sample, you will just end up with two DISC channels connected in parallel.

In another post, someone suggested that a detector could be built by only using one channel, i.e. the DISC channel. If you try this experiment you will find that it works ok in an air test, but will react horribly to ferrite, and will be unusable in the field.

I agree, but I'm talking about a special case. If it's true that Gold is 90 deg from ferrite/typical soil, then if we set the single channel sync pulse to cancel ferrite, we automatically will optimize the signal for gold. In other words, both channels would be set the same as our current GEB ground balance, not the DISC channel. Davor calls this "All Metal" mode, because I guess that is how ALL METAL mode is implemented in typical MDs. I would agree, although for the TGSL, ALL METAL mode uses a curious shaped sync pulse for some reason that I have never quite understood. And of course the caveat of using ALL METAL mode for hunting gold is you must hunt where it is unlikely other metals are present.

So maybe the TGSL has a "Gold Hunter" mode all this time, called ALL METAL mode. But I would think about modifying the ALL METAL mode to use a normal square-shaped sync pulse instead of the squirrely triangular sync pulse that I observe -- the triangular sync pulse would seem to drop the signal level in the DISC channel. This may be intentional by Tesoro, for threshold compatibilities or something, but for a gold hunter, I want more signal.

So, to make a cheap, simple Gold Hunter, for searching in remote areas, it seems we should be able to rip out the whole TGSL DISC channel, tie the final comparator DISC channel input "high", and you're done. Maybe even remove that final comparator completely and just listen to the raw signal from the GEB channel.

-SB