Geotech

All true. What I was talking about was the problem of old analog discriminators. Where setting the discriminator to reject pull tabs might reduce the depth on silver quarters by half even though they are no where close to the disc cut-off. It was well-known back then that to maximize depth, turn the disc as low as possible. Digital designs don't have that problem, but they do have the problem you bring up. Every disc or notch setting can have (and probably does have) a "brick wall" cut-off. With digital discrimination it's a none-or-all situation, unlike analog disc where it's a none-or-a-lot-less situation. With digital disc, a bad target right on the edge of the disc setting will get broken up, but so will a good target just on the other side of the edge. Like you, I prefer to use no disc and listen to tones. I also prefer mixed-mode where I can hear the AM signal at the same time. Makes for busy audio but you get used to it, and I think it is actually easier to listen to than a bunch of broken up crackling made by a digital discriminator.

This is called "surface blanking." It has been featured on a few detectors but I don't recall the specific models. Garrett did it years ago, and I think either Compass or Discovery as well.

Yes, both magnitude and phase are calculated from the response vector, but only phase is used in disc and notch. Magnitude is used for the depth meter and audio loudness. That's one of those holdovers from the analog days. Those people are probably 60 or older. Back then, discrimination was set by rotating the demodulator phase until a given target started producing a negative response. It was then discriminated. Problem is, conductivities that were progressively closer to the disc point were progressively weakened. Newer designs do it digitally without altering any responses.

From the Better Late Than Never dept... Neither Eric nor Bruce invented PI. Hard to nail it down to a particular person, here are some references Eric posted on Geotech: 1. 'On the Induction of Electric Currents in Cylindrical and Spherical Conductors' by Horace Lamb, 1884. 'a much simpler treatment' (than sine waves) 'that of the currents induced by the sudden cessation of a previously constant field'. 2. 'Apparatus for geophysical prospecting' (US Patent 2,278,506) 1942. TX pulse and RX switching by a motor driven camshaft which operated switches. 3. 'A Conducting Sphere in a Time Varying Magnetic Field'. J.R. Wait 1951. Both sine wave and step fields are covered. 4. 'A Method of Detecting a Mass of Non-Ferrous Metal Located at Depth in the Earth'. J.H. Wescott, 1955. 5. 'A Pulsed Bomb Locator'. F.B. Johnson 1956. 6. 'Method of Geophysical Prospection'. J.R. Wait (US Patent 2,735,980) 1956. 7. 'Eddy Current Method for Measuring the Resistivity of Metals' C.P. Bean et al. 1959. 'The measurement is made by noting the rate of decay of flux from a bar situated in an external magnetic field that has been rapidly reduced to zero'. Eric was working on PI development in the 1960s at Oxford and wrote some journal articles of his own.

In the old days, the response of any target that was discriminated (whether using a linear disc or notch) was just blanked out. That is, if you were listening to a threshold audio, it would go silent over a disc'd target. In digital detectors, you can do whatever you want; make it grunt like a pig, or give it a Price-Is-Right "loser jingle." The processing price of discrimination is negligible. A detector can reasonably resolve target phases to around 1°, and you could easily notch targets down to that resolution. That is, you could have 180 notch settings and control each one, even giving each one a different tone if you like. The White's V3 did this.

Internally, there is no difference. Notch is just a narrow band of discrimination, and all discrimination is based on target phase. And tone ID works the same way, where tones are assigned based on the phase response. With disc & notch, you just assign it "no tone at all" or, maybe, assign it a grunt sound.

Metal detectors typically use "integration" as a way to improve SNR. It's basically averaging which is a form of accumulation. It can be done in analog or digital. With MF there have been two broad approaches to processing the multiple frequencies. One way is to treat each frequency as a separate single frequency metal detector. That is, each frequency channel is individually ground balanced and individually gives a target response. The results from each channel can then be compared/correlated to determine what the target is. This is what the DFX & V3 do. We'll call this "channelized" processing. Another way is to combine certain signals from each channel to create "composite" signals. One composite signal is typically the ground balance signal, another the salt balance signal, and target information is extracted from the composite signals. The Fisher CZ series uses this method, as do BBS/FBS. Composite processing tends to offer only a single VDI and a single graphable result, whereas with channelized processing you can get multiple graphs and multiple VDIs. This is why the V3 can give you all 3 spectrums. With channelized processing you can treat each channel as a single narrow-band detector and tightly filter it. With composite processing, once the composite signals have been created they now have a blend of frequencies with noise mixed from different bands and you've got what you've got. It's possible that this is what is going on in the newer all-digital MF designs. It would be interesting to see if, say, a DFX or V3 struggle as much. However, another factor is that designs seem to continuously push up gain and sensitivity so maybe a DFX would be quieter but also not go nearly as deep. I design metal detectors for a living. No, I don't have any competitor's code but I can deduce some things from how the detector operates. I also look at competitor's patents which are often revealing.

All single frequency, multifrequency, and PI designs use accumulation. No real difference there. The real difference is that a SF design is narrow band and MF and PI are wide band. A narrow band design allows you to more tightly filter the incoming signals which helps suppress EMI. Not only can you bandpass-filter the preamp, you can even bandpass-filter the RX coil. Can't do that with MF and PI, they have to run more or less wide open, although the preamp is usually lowpass-filtered at around 100kHz. That helps knock down radio interference. Frequency shifting is really only effective for a single dominant EMI, and mostly for line noise (50/60 Hz). The problem is that the demodulators (part of the accumulators) can cause, say, 60Hz to "fold" into the baseband signal to where you can hear it. A slight frequency shift (maybe even a few Hz) causes this effect to become randomized so you no longer hear it. However, even though it's randomized it can still affect the detection and cause what's called "silent EMI."

In what way have the detectors failed?

Minelab has routinely removed part numbers in the past. Less so now. The FET is an SI7464.

Yes, the charger normally gets warm, and if the battery is especially low the charger will get especially warm. For a battery that has been on the shelf for a year or more, I would closely monitor the charger to make sure it doesn't overheat.

Yes, "P" is fine. Means "Preset."

Correct, Chris officially "retired" from ICMJ, I think a year ago now.

"Signal Balance" is just a poorly chosen name for "Gain". Turn it up until the detector becomes unstable, then turn it down until you have good stability.

Also Todd Marshall at Centerville Electronics NW, he was head of the Service Dept at White's. A fortunate thing with the TDI-SL is that it had a good reliability record. The biggest problem was that the 12" coil would develop a seam crack and leak water. If the camo version has the HiQ coil then that eliminates the main worry. These things are still selling on eBay for $600+ used so, say, $700 for the new one or $600 for the demo would be a good deal. If you don't like it, you can probably get all your money back. OTOH, the dealer probably has $800 in each one and may be reluctant to take a loss.

The short answer in 'no,' detectors are designed around the coil impedance characteristics. The long answer is, it's possible but very difficult. First off, some coils are internally narrow-banded with a tuning cap and will only work with one frequency. DFX/MXT coil are not, they are wide-band so that's good. In most cases the TX coil inductance and resistance need to be fairly close so it may require winding an impedance-matching transformer. Depending on the detector design, the same may be needed on the RX coil (it would on the CZ). Even then, it will be difficult to get the phase response correct so disc will likely be off.

It should not make any performance difference. I know that Reg and some others have just shorted the cap and I have never heard of a failure. I'm just offering a word of warning.

R86 slightly limits the max gain. Removing it increases the max gain by 10%.

Here is a diagram for clarity. I've labeled components both for the SL and the [TDI/TDIpro]. Note that only the SL has test points. I have never tried this mod so I don't know what the results will be. The purpose of C42/R75/U7d is to eliminate DC offsets and reduce bounce-back noise but maybe it's overkill. If all you want to do is short C42 then I recommend lifting pin 10 of U7 [or removing Q16] or cutting the trace.

The AQ coil is currently on my TDI Pro, but the same can be done to the SL. For either you would need to reprogram the micro because that's what sets the timing. Only the original 200 TDIs had a timing circuit you could easily modify.

Output of the opamp. The C42 mod should have little to do with coil size. The purpose of C42 is to remove DC offsets before the final gain amp, which has to do with the circuitry and not the size of the coil. As Melano says, as you crank up the gain of the final amp (with the GAIN control) the DC offset gets gained up and can cause noise issues. I've never done this mod so I can't otherwise comment on its effectiveness. The absolute values of C20 and C21 don't matter much, even 10% caps are fine. Just make sure they match each other within 1%.

The caps can be just about any film-type: polypropylene, polystyrene, PPS, etc. What you want is a cap with low microphonics. Then you want to match the 2 caps to 1% or better. So if you get 2% caps, order 4 or 5 of them and then hand match two of them with a cap meter. On C42, I would not put a dead short there, rather put a 1k resistor across it. The reason is, there is a switch (U7d) that grounds that node and by shorting C42 the switch now grounds the opamp output (U12a). That's called "opamp abuse" and no one wants to be accused of that.

As it turns out, the factory TDI coils are a major performance limitation. I've adapted an Impulse AQ coil to run on the TDI and can then reduce the minimum delay to 6us. It's still not gonna run like a Minelab, but it is a little more respectable.

On the bright side, the patent expires Nov 3, 2024.

Sorry, ignorance of the law isn't much of a defense. Plus, it's documented that everyone here knows of the patent issue. I don't think ML would ever come after an individual who brings a Q80 into the US, but if they did, the first thing they would do is issue a letter of infringement and demand surrender of the unit. That would probably be the end of it, unless the infringer would rather dance with ML than kiss $700 goodbye. P.S. -- Discussion of intentional infringement, such as encouraging someone to go to Canada and buy a Q80 to bring back, can be considered "inducement to infringe," also illegal. All I'm sayin' is, "Be careful what you say in public."