Vallon VMH3CS Mine Detector

[Ferric Toes]

I think that's exactly what they are doing. It's not 100% efficient, ergo the "tilt" during the on-time.

Presumably, the tilt is the remainder of the the normal L/R charge up time until switch off?

[Ferric Toes]

I wired up a front end amplifier, the same as I have used many times in the past, to give some front end gain. The standard input configuration is via a resistor and a couple of diodes back to back to clip TX pulse and flyback transients. What we see is a standard PI response waveform. Amplifier saturation to plus and minus rails, both 5V; amplifier recovery to flat baseline. Scope trace starts at TX switchoff.

A metal can is then placed on the coil and standard decay observed for both pulse polarities.

The scope sensitivity is increased and the above repeated.

As above with metal can.


What is puzzling is the glitch at 103uS on both polarities. Is this a gating glitch or just an unimportant artifact appearing across the coil? The amplifier recovery with this high inductance coil also seems much too late to detect, say, a 0.3gm nugget; which the Vallon unit does quite easily. O.K., is this a DSP trick where you can sample into the flyback recovery period? Then there is the ground cancelling, how can that be done? Over to the experts in the digital domain, as I am not one.
Nothing changes in the above pictures if I switch to the ground cancelling position. What we need of course is a schematic, but I doubt very much if that is possible.

[KingJL]

O.K., is this a DSP trick where you can sample into the flyback recovery period? Then there is the ground cancelling, how can that be done? Over to the experts in the digital domain, as I am not one.
Nothing changes in the above pictures if I switch to the ground cancelling position. What we need of course is a schematic, but I doubt very much if that is possible.

Eric, what are you using for the FE (I keep exploring for the better mousetrap). As for the DSP trick for sampling into the flyback, I have no idea. I can see how they could cancel out the flyback, but I can't conceive how they can sample into the high voltage and still discern minute changes. For mineralization/ground cancelation you can take your filtered input, feed it into a let's say 0.05 HZ decimating filter and subtract that output from your filtered input. This ground/mineralization filter needs to stop updating if there is no motion, again easy to do digitally, with todays accelerometers IC's.

[Davor]

Perhaps there is some sort of front-end sampling/integration? If so, the "late" capacitor would introduce a glitch when switched on, due to the charge stored from a previous cycle. I have no idea what the schematic may reveal, but I'd do it that way. It is a way to get meaningful samples from deep into the flyback.

Since there is no variation in the periods (iamb, staccato), Paltaglou is off the hook, and the only way to do GB is by early vs. late arithmetic.

No need for EF in a bipolar system. Another bonus.

[green]

I wired up a front end amplifier, the same as I have used many times in the past, to give some front end gain. The standard input configuration is via a resistor and a couple of diodes back to back to clip TX pulse and flyback transients. What we see is a standard PI response waveform. Amplifier saturation to plus and minus rails, both 5V; amplifier recovery to flat baseline. Scope trace starts at TX switchoff.
[ATTACH]36185[/ATTACH]
A metal can is then placed on the coil and standard decay observed for both pulse polarities.
[ATTACH]36186[/ATTACH]
The scope sensitivity is increased and the above repeated.
[ATTACH]36187[/ATTACH]
As above with metal can.
[ATTACH]36188[/ATTACH]

What is puzzling is the glitch at 103uS on both polarities. Is this a gating glitch or just an unimportant artifact appearing across the coil? The amplifier recovery with this high inductance coil also seems much too late to detect, say, a 0.3gm nugget; which the Vallon unit does quite easily. O.K., is this a DSP trick where you can sample into the flyback recovery period? Then there is the ground cancelling, how can that be done? Over to the experts in the digital domain, as I am not one.
Nothing changes in the above pictures if I switch to the ground cancelling position. What we need of course is a schematic, but I doubt very much if that is possible.

Hi Eric, good to see you back on the forum. Does the detector still detect the nugget with your amplifier connected? Do you have a scope trace of coil volts without your amplifier and does it detect the nugget with the scope hooked up?

[Chet]

Hi Eric


In an earlier post you stated that the pulse was running at 6v. This implies that something is occurring to raise the three D cells from 4.5v to 6v. Maybe the glitch at 103us is from a power boost circuit that is synchronized to not interfere with an early narrow sample gate for small objects. Or maybe it’s a power recovery capacitor that is being gated back into the power supply after the receive sample gate ends. The long battery life24-30hrs implies that there may be a power recovery circuit.


Have a good day,
Chet

[Carl-NC]

Presumably, the tilt is the remainder of the the normal L/R charge up time until switch off?

Exactly.

I recalled that someone once told me FTP had a Vallon, so I poked around the dustbin and found it. It's a VMH3 without the CS, which apparently means Changeable Searchcoil, because that appears to be the only difference. The D-cells are dead so I have to go after work and buy some new ones. Then I'll play with it and see what it does.

[Qiaozhi]

I wired up a front end amplifier, the same as I have used many times in the past, to give some front end gain.

How much gain did you have in your front-end amplifier?

We found a similar situation recently with the Felezjoo design, where the front-end gain was 66.2dB, but it was still able to detect some very small targets. The opamp used was an LF357.

[mickstv]

I wired up a front end amplifier, the same as I have used many times in the past, to give some front end gain. The standard input configuration is via a resistor and a couple of diodes back to back to clip TX pulse and flyback transients. What we see is a standard PI response waveform. Amplifier saturation to plus and minus rails, both 5V; amplifier recovery to flat baseline. Scope trace starts at TX switchoff.
[ATTACH]36185[/ATTACH]
A metal can is then placed on the coil and standard decay observed for both pulse polarities.
[ATTACH]36186[/ATTACH]
The scope sensitivity is increased and the above repeated.
[ATTACH]36187[/ATTACH]
As above with metal can.
[ATTACH]36188[/ATTACH]

What is puzzling is the glitch at 103uS on both polarities. Is this a gating glitch or just an unimportant artifact appearing across the coil? The amplifier recovery with this high inductance coil also seems much too late to detect, say, a 0.3gm nugget; which the Vallon unit does quite easily. O.K., is this a DSP trick where you can sample into the flyback recovery period? Then there is the ground cancelling, how can that be done? Over to the experts in the digital domain, as I am not one.
Nothing changes in the above pictures if I switch to the ground cancelling position. What we need of course is a schematic, but I doubt very much if that is possible.

I Eric, just wondering if you could post some pictures of the response to your ironstone target at 1v and 100mv scales.

Thanks
Mick

[ivconic]

...What we need of course is a schematic, but I doubt very much if that is possible.

Slim chances. It is full DSP and all the processing is done in code in MCU.
Schematics with just few parts will tell you nothing.
You could open it and take a close photo (macro).

[Ferric Toes]

Hi Eric, good to see you back on the forum. Does the detector still detect the nugget with your amplifier connected? Do you have a scope trace of coil volts without your amplifier and does it detect the nugget with the scope hooked up?

With my preamp across the coil the detector indicates a fault condition and does not respond. The unit seems to be very sensitive to small changes in the coil circuit e.g. the insertion of a 0.1 ohm resistor in the ground side of the coil at the connector end, shows as a fault and the receiver in the detector no longer functions. I can understand that for mine detection life depends on perfect functioning, but a change of 0.1 ohms in a 3 ohm circuit is a bit fine.

[Ferric Toes]

I recalled that someone once told me FTP had a Vallon, so I poked around the dustbin and found it. It's a VMH3 without the CS, which apparently means Changeable Searchcoil, because that appears to be the only difference. The D-cells are dead so I have to go after work and buy some new ones. Then I'll play with it and see what it does.

That's great; it's too good a detector to be in with the trash. Do you have the full operating manual? I didn't get one with the detector but one should be arriving today. The field operation card I have is so basic it is almost useless. i.e. it doesn't adequately describe ground balancing or sensitivity adjustment. The pinpoint mode is not mentioned at all.
One of the accessories is a 60cm diameter UXO coil, hence the CS. Also I believe there is a probe. I am going to have a go at making a 12in coil in a Whites shell, maybe dual field. That should make a very good coin, ring, and nugget detector both for the beach and inland. Garrett's ATX Extreme seems to have copied, almost exactly, many of the features of the Vallon.

[daverave]

Everything is an area of question!

There are 3 problems at this level, the capacitance, the antenna effect for EMC, and thermal noise.

Over the rds mosfet is high more it heats, so you have a MOSFET with low RDS but also low capacitance, etc ... (many parameters are important) It's very difficult to find it.

We must dissipate this energy, dissipation pad of the mosfet has a surface and this surface will be close to the metal grounded card more capacitance will be great and will vary depending on many physical parameters. (Mass of the case, mass of the ground plane, with large flyback, no mercy!)

We must therefore avoid any metal sink with a large surface parrallel this mass to avoid the capacitive effect).

Unfortunately when a metal heatsink is good, this surface is large ! we must avoid to dispel with metal or electrical conductive materials!

The antenna effect will be more reduced ... and the electrcal noise output from the preamp will be greatly improved. etc....

The fact that Eric is not ground plane on the card at this level that the capacitance decreases relative to the sink, it is a chance, regarding the goldscan 5c case are plastic so it will also in the right direction, but using is kind of materials would have felt a huge noise difference after the preamp .... Best dissipation and almost no antenna effect!

Alexandre

are you saying that a plastic enclosure is better than a metal type...if so what about external noise..surely a metal box is better for pcb screening ??

[Carl-NC]

That's great; it's too good a detector to be in with the trash. Do you have the full operating manual? I didn't get one with the detector but one should be arriving today. The field operation card I have is so basic it is almost useless. i.e. it doesn't adequately describe ground balancing or sensitivity adjustment. The pinpoint mode is not mentioned at all.
One of the accessories is a 60cm diameter UXO coil, hence the CS. Also I believe there is a probe. I am going to have a go at making a 12in coil in a Whites shell, maybe dual field. That should make a very good coin, ring, and nugget detector both for the beach and inland. Garrett's ATX Extreme seems to have copied, almost exactly, many of the features of the Vallon.

Yes, the unit is complete with the carry case, manual, field card, and firing pin OTP (operational test piece). This unit does not appear to be as good as yours. At max sensitivity it will detect the OTP (which tells me it's working within spec) but a US nickel is only 9-10" or so, and it won't detect my 1x foil standard at all. The TDI easily beats this.

I think there is something out-of-whack on this unit. My foil standards are square 1"x1" pieces of foil layered in clear packing tape, with 1x being single layer, 2x has 2 layers, etc. I have 1x-8x continuous (helps identify target holes), plus 12, 16, 24, and 32x. The 1x is invisible. 2x has a negative response, that is, I get a signal as the target moves away from the coil. 3x has a weaker negative response, 4x a decent positive response and the rest look good from there. I'll try to play with it some more today.

Yeah, the Recon/ATX looks a lot like the Vallon. I also have an ATX so maybe I'll try it as well.

[Asgard]

are you saying that a plastic enclosure is better than a metal type...if so what about external noise..surely a metal box is better for pcb screening ??

Metal enclosure is good for emi but no good for capacitance between mosfet / ground PCB / ground enclosure and metal heatsink.

I recommend using a metal box and use a ceramic sink for the capacitance ! You'll see all your problems disappear! heat dissipation, emi, capacitance etc ...

Alexandre