Typical geb disc machine sens

[golfnut]

The sens on machines like IGSL or IDX, Cibola etc is partially restricted by the sens of the GEB ch to a target..



As was explained to me,

CH 1) Disc is set to phase align to a target - so is tuned to max sens of a target alloy.
CH 2) Geb is phase tuned a 'off' of the target alloy phase and 'half off' the ground signal to average GND signal out.

The Comparator Stage has a common threshold for both CH's

The Disc CH always gets more target signal and 'trips' thresh, BUT the LOWER SENS GEB CH may not trip the comparator Threshold and so no tone would be heard...


So in this case, GEB CH sens to alloy is a sens bottleneck.

If you could trip the Comparator on the Disc ch only - you would not suffer the sens bottleck associated with the lower target sens GEB CH. - Sure you would have to remove the ground signal from the Disc CH to avoid continuous falsing of the DISC CH.

Would something like this work ???


S

[Davor]

No. Most of the conclusions are only half way there or just missing the point.
GEB channel is the one that is used for audio extraction, so it is good that GEB channel is deaf to the ferrites. Disc channel is required to identify whether a phase of a target response falls within Fe or non-Fe, and is turned off in all-metal position. In order to have more even discrimination response it is beneficial to boost the Disc channel gain.

You can try to fathom a Verator rig and the way it plays with channels. There are some elements of your drawing there, but instead of summing you'll find multiplying.

[golfnut]

Another way,

If we have A+B

and we want A alone, what would we do..

S

[Davor]

We'd be listening to an all-metal detector.

[golfnut]

Tune Disc to something you want, then invert the groun signal and subtract it from disc CH . ( By adding the inverse)

I see Large vendors do this sort of thing using inverting summers too..



S

[Davor]

Actually what you achieve this way is the same thing as shifting the phase at the switchers. IMHO this is a bad approach because it also influences the frequency response of the gain blocks. Given the sad fact that most of the gain blocks are poorly designed anyway, you'd hardly notice any difference.

Shifting phase at the switchers provides an additional advantage of being completely free from problems related to a huge dynamic range that gain blocks and a discriminator must handle.

Fisher-like shifters could be remedied with additional buffers after the pots, but this is altogether a bad idea because they suffer from reduced amplitude in Disc block, so you lose some of the depth this way, and discrimination will always be soft at small signals.