IDX-PRO+VDI

[Qiaozhi]

i use new 12v battery pack
i just get 4.75v from regulator

When debugging an electronic circuit, you should always check the power supplies first. This is just common sense.
Don't worry about the transmit oscillator until you have all the supply voltages going to the right pins.

Any more "dog" references will be deleted.

[gtdavid]

Ok I got a bit more current to the tx.
What I did was lower the resistance on the 39 ohm resistor . The change was not noticeable as far as detection distance got thow. .
With all my playing around I did notice the 39k resistor in front if u8 affected discrimination would increasing this resistance make the idx more resistant to heavy mineralized soils? ?

[seadoggy]

Anyone have done Mr.Bill mode??do i need to change resistors R24 whit 2.2Megomhs,,R7 resistor whit 51 omhs,R32(220k) resistor replace whit 375k running it through a toggle switch with the 220k???

[golfnut]

Chatter or motor boating

My IDX over the time Ive had it has occasionally appeared to chirp or chatter - it drifts in quietlyat first and builds and then drifts away again. The backgorund noise of the unit is superimposed on the repetitive 'chatter'.

The unit looses sens during this and has been bugging me for ages - my IGSL did a bit of this too.


I managed to scope it while it was in the chatter state and you could see the background noise begin to ripple and then become a slowly building sinus and it gets big at the putput of U3 at comparator inputs.


The period was 120ms or 8Hz - AHa, the centre frequency of the IF filters. Stacks of gain with a narrow passband, yes we've made an 8Hz oscillator.

I dont know how to fix this yet.


I may move the center freq of the first filter pair down and the second pair up a little to split them up and raise the stability.


S

[Koala]

Mine intermittently chatters also with my homemade coil but fine with factory 8". If I alter the RX capacitor slightly the chatter does not appear but loose some depth. Can work for long periods without chatter but when it starts if I turn the sens down it goes away and can then turn the sens back up. MD seems more stable if the disc is turned up slightly.

[golfnut]

Cheers Koala, heres what I did. Im using a commercial coil, or my own - chattered with both..


Here what Ive just done..


----------------------------------

I added an extra 1u to one of the filters - not much different.. and then to both, it just made the oscillation period a little longer or lowered the frequency of chatter it a bit.


I knew if I put my finger on U3 I could stop all noise and oscillation/repetitive chatter and still get a good air test range. So a light 'load' could help.


What I did next was add resistive loads to U3 amp outputs 4k7 between pins 14 to 12 and 1 to 3

Adding these teriminations has, in limited testing, made all the difference to the trouble I was having. I cant notice any air test degradation.

Heres hoping.


S

[Koala]

Cheers Golfnut not near any electronic equipment at the moment. Taken the MD with me though not had too much time to test. Managed to spent a couple of hours on the beach in the dark and only a few modern coins and bits of scrap aluminum. Damp sand was no problem. Very impressive depth. Using a full size spade and had to dig some proper holes to retrieve some targets.

[golfnut]

Been playing with a nel coil on my IDX - working ok now. The Nel Hunter ground balances real well on saturated lush grass (something my coils wouldnt do). I got the one for a Tesoro Tejon/Lobo etc Im running around 13kHz ( Tx needs cap in Box unit)


As coil is Hunter 8x12" its less deep than the 10"DD - to get back to my old depth Im going to have to use ALL Metal.




As I dont wish to dig a thousand nails per session I need to get an idea of what the alloy phase is without too much design effort time cost..

I may try a mod to reuse the PP Channel as a second Disc Chan.

Then I have a dedicated 3rd CH offset to listen for later stuff e.g. out towards silver.

Inorder to assess the relative amplitudes on Disc1 and Disc2 (without AtoD's, processors, code etc) - I would amplify the deifference between them a bit and use the result to biias a fet.

If done right the Fet would be a variable resitor - shunted across one of the resistors forming the audio Tone frequency.


Fe could be low, Au/bronze etc mid, and Silver Hi tone or the other way around as you like it.

Preliminary sketch..


If the pot up near U6 was adjusted to one extreme the tone would be on all the time - i.e. like squealer mode, sometimes make unit and user very sensitive to small targets.

error -- Fet needs to be on the Top resistor to affect the RC time const of the RC , or just use fet in place of resistor?
S

[Aiko]

Guys, I have build two coils - one concentric and one double D. On the concentric I have 90 degree phase shift, but on the DD coil I have 180 degree shift. Is it normal? I suppose this comes from the construction, because all parameters of both coils are identical. Both coils works similar on depth and discrimination and the VDI numbers on identical targets are very close. I am confused how this detector works almost identical with 90 and 180 degree phase shift.

[Davor]

Those phase shifts are static responses on top of which you have motion responses of the targets. Detector processes the latter, hence it is of no concern what static response you have.

[Merc]

Those phase shifts are static responses on top of which you have motion responses of the targets. Detector processes the latter, hence it is of no concern what static response you have.

Would it not affect the all metal/pin-point mode?

[Aiko]

Thanks Davor! But why I have so different phase shift, when my coils are identical as wire size and inductance. As well why everybody talk about the phase shift if it is not so important?

[Davor]

Would it not affect the all metal/pin-point mode?

If you pay a close look at the schematic, you'll notice that pp channel is also motion compensated via C16. Even the hard core static-response detectors have a "tune" function by which you zero the static coil response, so again the static response is of no practical concern (within reason).

Thanks Davor! But why I have so different phase shift, when my coils are identical as wire size and inductance. As well why everybody talk about the phase shift if it is not so important?

It appears that many coils are factory-tuned to some static phase shift in the air as per some tuning procedure, most probably designed to reduce "air signal" in proximity of ground. Tuning a coil over real ground does not seem practical in industrial environment. It is beneficial to have the least possible "air signal" in the working conditions, e.g. proximity of some average ground, so that the least of the Tx noise and harmonics is reflected this way into Rx.
Coils respond differently as per the nature of Tx to Rx linkage. It can be inductive or capacitive, and real part can be in phase or counterphase, so resultant phases can assume anything within 360°. Concentrics have a lot of area where Rx and bucking coil are running together, hence a lot of capacitive linking. DD-s have less common area and less capacitive linking. The only way of reducing this linking is by means of balanced Tx or Rx (or both) operation and bifilar winding, where each winding has wires at opposite potential over a signal ground. As these effects are not too much pronounced at VLF, no one seem too much interested in reducing this kind of linkage.

[Qiaozhi]

Guys, I have build two coils - one concentric and one double D. On the concentric I have 90 degree phase shift, but on the DD coil I have 180 degree shift. Is it normal? I suppose this comes from the construction, because all parameters of both coils are identical. Both coils works similar on depth and discrimination and the VDI numbers on identical targets are very close. I am confused how this detector works almost identical with 90 and 180 degree phase shift.

This is normal. The phase-shift between TX and RX depends on the coil construction. If DD and concentric coils are constructed for a particular detector using the same TX and RX inductance values, they will be 90 degrees out of phase with each other. This is discussed in ITMD on page 132 of the Coils Experiments chapter.

The correct phase-shift is important, otherwise the detector may not be able to ground balance, and the discrimination settings will be offset from their correct positions.

[Aiko]

Many thanks for the explanation guys