Please Eric :please:

[Kev]

Hi Eric, or is it Mr Toes,
Been reading back a bit about the history of the PPD1 etc., Would it be possible to re-post the diagram of the front end of the PPD1 disc channel, originally posted here;
http://www.findmall.com/read.php?34,129970,129970#msg-129970

It's a great resource this forum, thanks again Eric for sharing your knowledge, and thanks Find's for hosting it.

Ta
Kev.

 

[Kev]

Hi Eric,
When I use a 60us pulse width on my GQ, I get droop after 50us. I've got a 3300uF condenser in circuit and when I add another 1000 it just adds a wee leg up for a microsecond.
Eric you said one time that it's often due to battery impedance, which would make sense, since the extra cap is also probably not able to suck anymore from the source.
I'm running 12V would 14Volts help a bit, I don't need to go any wider than 60us at this stage, but that begs the question, how do you go about selecting batteries that can supply the likes of the GS5?

Here's a pretty grotty printout of what's going on.
http://www.geocities.com/kejamnz/misc/223.pdf

Cheers
Kev

 

[Eric Foster]

Hi Kev,

Is this the RX waveform? Is it upside down? RX usually saturates positive. I usually find it helps to have a look at the coil current waveform. Insert a small resistance, <1 ohm into the ground end of the coil. Look at the waveform across it on a scope, and if the current is dropping off toward the end of the pulse, you will see it. I can't see a 60uS pulse being long enough to cause droop with a Goldquest electronics, if you have a good battery pack. Most of the pulse current is drawn from the capacitor, which for Goldquest values, has a discharge time constant of 1mS. Your 60uS pulse will not make much impact on this. Coil R + series resistor + Fet R across 3300uF.

Eric.

 

[Kev]

Hi Eric,
Yes it's the output of the preamp. I wondered about that, could my +5V smoothing be faulty? I'll look at the coil waveform, but I also noticed that when I parallel a resistor with the 39R coil series R and brought it down to 33R that the pulse appeared to hold up about 5us longer, I guess that rules out my +5V supply.

I'm using a pulse train of 20, 40, 60us

Here's the pulse train;
http://www.geocities.com/kejamnz/misc/221.pdf

If my pulse is drooping, I wonder if this is what is making it very sensitive to crumbs and not the varying pulse width?

Cheers
Kev.

 

[Eric Foster]

Hi Kev,

Quick question - are you using a DD coil? If so, there is an easy explanation.

Eric.

 

[Kev]

Hi Eric,
I tried your coil and everything was OK, and bump it dawned on me 300uH divided by 33 ohms = 9us, multplied by 6 time constants = 55us. Yes it's a DD and I guess I'm observing the current flow dropping as the Tx coil becomes fully energised.

This is good, now I know that I need a 60us pulse to reach the flat top of this coil.

Thanks Eric for putting up with my lumbering comprehension!
Kev.

 

[Kev]

n/t

 

[Eric Foster]

DD coil is not very well balanced. If your windings are accessible, you can move the overlap slightly and observe what happens within the transmit pulse time. You should be able to get a straight zero line, except for spikes at the start and finish of the pulse.

If you windings are not accessible, you can position a small piece of ferrite near the crossover, to alter the balance. Again you should be able to get a straight line. Wave ferrous and non-ferrous objects over the coil and observe some interesting effects.

Eric.

 

[Kev]

Hi Eric,
Thanks for the tips on balancing. When I was 'balancing' this coil I found that if I passed hotrocks and soil samples from a particular prospecting area over the coil while I adjusted the overlap I could get a reduced response from these while still receiving a reasonably good response from small stuff, except when it was directly under the overlap and within an inch or two. Most of the gold where I go is not on the surface. The others I've tied to balance correctly don't operate as well in this difficult ground.

I don't know why this is the case.......

Cheers
Kev.

 

[Kev]

Hi Eric,
No way can I centre that line, anyway I have another coil almost ready for balancing I'll check it out then, but I did see even with this coil and large ferrous and non-ferrous objects, the decay from the spike went in opposite directions momentarily. Now I see why one needs to sample on the Tx on time since it gives a stable and centred Rx (correctly balanced coil of course)

I guess that a well made DD wouldn't need to be electronically balanced like the co-axial on the PPD1?

Your an excellent teacher Eric, I just wish I'd not been so lazy and investigated other clues that you have given over the past!

Regards
Kev.

 

[Eric Foster]

Hi Kev,

That is something worth investigating. Is the balance for null electrical coupling the same as minimum response to a uniform sheet of mineralised ground? I think it should be, but I am not sure. Minimising hotrocks is a different, as they are a more localised signal.

I've just remembered that with the Goldquest in its standard 3pin plug configuration, it is impossible to get a zero line null. The best is a flat line a few hundred millivolts up from zero. This is caused by the common ground lead between the plug and the circuit board. To get a zero line null you need to ground the RX coil braid back to the RX ground, and the TX coil braid, back to the TX ground on the board, by using a 4 pin connector. The two are, of course commoned on the pcb. It is quite difficult to get a true zero line, as it is caused by residual currents flowing in any part of a common ground. This appears as an offset voltage to the receiver amplifier.

Eric.

 

[Wirechief]

Hi Kev, By saying flat top does that mean saturated? I hope your doing well and I'm staying steady at it myself. CU later. John-Wirechief.

 

[Kev]

Hi Eric,
Ah that's what's happening in my GQ, thanks for that Eric!
The areas concerned that I am searching are rich in Fe minerals within smectite and kaolinitic type soils. I believe that this soil throws a normally well balanced coil out to the point of compromising it's normally good surface insensitivity to mineralisation, but that a coil slightly off balance comes into balance (roughly) in the presence of this soil type. The effect is not as dramatic with hot rocks but, the best response reduction point for the soil samples also gives a good reduction in response from a number of volcanic eruptive cobbles found in the same area, the commonest being lamprophyre, this means that I don't detect these as deep as I normally would with a commercially available coil.

I've got some homework to do....
catch you later.
Kev.

 

[Kev]

Hi John,
Yes correct. The Tx coil current rises according to the time constant of the coil and drive circuit which is a result of it's inherent and exterior R,L & hopefully minor C . 63% of maximum after the first time constant, 63% of the remainder after the second time constant etc.,
What I took so long to realise was, the droop was the current induced in the Rx coil dropping as the current in the tx coil approached maximum, or saturation, which is usually expected after 5 or 6 time constants, depending on how closely approximated your calculation.

The steady state current doesn't change so we shouldn't see any current on the Rx coil because it's the change in current di/dt of the Tx coil that sets up the field, that ultimately cuts the Rx coil, and induces a current to flow in it.

Hopefully I've got this right now John, but.....
Regards
Kev.

 

[Kev]

Hi Eric,
Just wondering? The non-inverting input of the amp is connected directly to 0V, wouldn't I also need to add a 2k2 between pin 3 and 0V to see the coil balance at the amp output?

Thanks
Kev.

 

[Eric Foster]

Hi Kev,

Can't find the PPD front end diagram at the moment. Will post it when it turns up.

Have a look at http://www.arizonagoldprospectors.com/ and go to the online forum. There is a discussion on DD coils, which has some relevant information to your experiments.

Eric.