Brought to the Top

[Anonymous]

I took the liberty of bringing this post to the top as it was an answer made today, to a point by Dave Emery back in January, and in my view, quite an important bit of information. Eric.
I don't know what the mods do for the 2000. I do know that people who own them love them like a baby. Modification of the short pulse width may well be what the mod does. Please let us know if you find out. All the best, Dave. * * *
Hi Dave, back in '96 I developed those Xtal mods for the SD2000.
Beleive me, it was pretty scarey messing with a less than year old SD2000 that cost $3670 at the time. lol
Those early mods simply increased the freq of both the pulses.
The SD2000 has one long and 4 short pulses. (the SD2200 has one long and 3 short).
The resident Xtals in the SD2000 are 2.0meg and 2.2 Meg.
If you add a simple switching CCT you can add the other two Xtals below, or change the ones on the board.
The Xtals used to increase sensitivity are 2.5meg and 3meg.
Only one Xtal is used at a time.
This dramtically increases the sensitivity to small targets and improves allround sensitivity as well.
This is great for working DD coils.
The possible drawbacks are when using Mono coils in hot ground as it can be extremely noisey.
I have since developed a new system of mods on the SD2000 series using Mono Coils that permits excellent sensitivity in hot ground with fast balancing and minimal "noise" problems, I call it GRM1.
There's only 32 of these mods being "tested" out there, all with no problems, (some of the Aussies here may have come across them)... I may even officially release them one day
Gaz.

 

[Anonymous]

Hi Gaz,
I admire your nerve to try to modify such an expensive detector. I am sure I wouldn't have tried such an experiment for fear of what might happen. This is great information you have provided that explains a lot. I know, I have been asked before if I knew what was being done on the mods, and I certainly wasn't sure. Now I know.
Also, thank you Eric for finding the post. I went down about half way and gave up.
Gaz, I am curious if you tried even faster crystals to see how they worked? I would suspect you would reach a point where either the cpu failed or the delay for sample was simply too short and caused a failure.
Logically, the timing is software based so increasing the crystal frequency would increase time between clock cycles. Just what the limits are would be interesting to know.
It would be interesting to know the resultant delay before sampling as the result of these changes versus the original timing. Does anybody know the delay before sampling for any or all of the SD's or the GP for that matter?
With people claiming to be able to find 1 to 2 grain nuggets with these detectors, I have to assume the delay is less than 10 usec. My experimenting with a 10 usec delay indicates that it is tough to detect something less than 3 grains in size.
The Minelabs do have a higher powered pulse than I am using, and that would help, but I am not sure just how much. I would expect this to increase the signal to noise ratio, but if the target's response is gone before the sample is taken, even the strongest pulse isn't going to make much difference.
Again, thanks for the information. Keep up the experimenting, with your willingness to take chances, who knows, you may find the ultimate answer needed to make that "ultimate" detector.
Reg

 

[Anonymous]

Hi Reg and all,
I compared the sensitivity of the SD2200 and the 2100 to the Deepstar that I make. Both used a 11in mono coil, and on small nuggets, cupro nickel coins and thin rings there was almost no difference. I put the SD mono coil on a Deepstar with a modified TX circuit to give the same conditions, and looked at the coil recovery waveform. It barely recovered by 15uS, so any faster sampling is going to take you down the recovery slope. You can probably win a bit, as Gaz has obviously done, but you are starting to sample in an unstable area which will give more noise and drift.
Some of the claims I have seen for detecting small nuggets with the SD's take a bit of believing, but gold does vary greatly in its conductivity, and hence detectability. A small piece of very pure gold can give the same signal as a larger impure piece.
Eric.

 

[Anonymous]

<BLOCKQUOTE>Gaz, I am curious if you tried even faster crystals to see how they worked? I would suspect you would reach a point where either the cpu failed or the delay for sample was simply too short and caused a failure. </BLOCKQUOTE>
Yes I used higher Xtal Freqs to the point where the standard unit "crashed" and ceased to function properly at all...usually you go "out of bounds" in the ground balance dept.
The Xtals that worked for good all round performance in the conditions we experience in Aussie are the 2.5 and 3 , although you can go higher (around 4meg) if you like depending on the ground you intend to work.
However you will soon run into groundbalance problems if you push the normally available coils and Sd's too far.
Below is a picture of the SD2000 signal without any modifications.
The CRO settings are 5u/s @ 2Volts, increasing the clock freq simply shortens the duration of the whole cycle and lowering the freq stretches it.
This helps you find different target shapes and sizes depending on what the freq is used, you do not get any real "power increase".. it's simply a matter of picking up gold that other detectors don't "see" as well.
I beleive Minelab have used this factor throughtout the SD range with some additions and alterations in the amount of RX signal the SD2200 can handle and the extra features in the latter series.
The SD2100 uses the same RX components as a Sd2000 with an onboard VCO/pulse gen instead of Xtals hence the SD2000 is ideal for us to play with <IMG SRC="/forums/images/biggrin.gif" BORDER=0 ALT="">.
All the SD series produce the same amount of output power.
The biggest advantage in shortening the pulses are a better small target response using the coils currently available.
If you go too high in base Xtal freq the larger pulse will begin to flatten out in it's decay time (bottom of waveform), this effect appears to decrease the large target response while increasing small target response overall.
It would appear the slower decaying waveform and longer pulse is naturally required for larger targets , however this can be marginally compensated by increasing physical wire thickness in the coil which helps increase large target response, however as with most things in life, what you increase in one area you will lose in another.
The main benefits of making coils to suit the variations you can produce in the pulses is to have specific coils for specific target sizes..... and I've been having quite a bit of fun and success in the field developing both. <IMG SRC="/forums/images/biggrin.gif" BORDER=0 ALT="">

 

[Anonymous]

oops, forgot to attach the pic...hope this works <IMG SRC="/forums/images/biggrin.gif" BORDER=0 ALT="">

 

[Anonymous]

<BLOCKQUOTE>
Some of the claims I have seen for detecting small nuggets with the SD's take a bit of believing, but gold does vary greatly in its conductivity, and hence detectability. A small piece of very pure gold can give the same signal as a larger impure piece.
Eric.
</BLOCKQUOTE>
The SD series can be incredibly sensitive to small target response and .1-.2 grams nuggets are not uncommon in my experience using the 8 inch mono.
The 11" will call on most small stuff but of course, not as well, therefore using an 8" around Aussie is a must for flogged areas. <IMG SRC="/forums/images/wink.gif" BORDER=0 ALT="">
In my experience it's the shape and orientation of the nugget in the ground, plus the ground mineralisation that are more predominate factors for West aussie conditions.
Gold content composition would definately be a factor in some areas of the world, however most of our gold is better than 90% and located in hot ground, so the small differences created by the varying content may be very hard to discern in the field.
I have made a few specifically tuned coils to extract as much benefit from the mods and standard SD's so as to "see" different target shapes, which is in my view the best way to cover our goldfields here.
This is done by going a little "out of bounds" to the general setup of the standard coils.. most members here would be doing the same I see. <IMG SRC="/forums/images/biggrin.gif" BORDER=0 ALT="">
Here's pic of my 60cm Mono, 35mm and a probe for the SD series.
The design shape of these coils produces a wider and more linear detecting range across the face of the coil with comparable sensitivity, to that of a standard round mono when wired the same.
This basically helps you to not miss small gold that may only be detectable in the centre of a round mono at depth when griding.
I found the smoother round end edge shape also helps limit "hot spots" across the coil tips in highly mineralised ground.
They work very well in the field. <IMG SRC="/forums/images/smile.gif" BORDER=0 ALT="">
(gotta love making thermoformed plastics though.. phew what a drama and learning curve! <IMG SRC="/forums/images/biggrin.gif" BORDER=0 ALT=""> )

 

[Anonymous]

You are exactly right about the shape and orientation of the nuggets in the matrix, heres a few dinks I found in one area that produced several hundred detectable pieces, I would clear an area (I thought) move the material around in the same spot and some more whispers would appear, This material was right on bedrock and when you got all of the cobbles cleared away down to the sediments the signals started appearing, after a while I just gathered up most of the material and sluiced it...Lots of great info on this forum I am just one of the readers and really enjoy the comments....Geo

 

[Anonymous]

gaz how`s the auto ground balance Mod.I hear your working on for the sd2000 going ???
kris in Vic.

 

[Anonymous]

Hiya Kris,
Largely the auto ground balance has been put on the shelf due to the inherant losses signal processing causes.
With the current GRM1 mod you only need to adjust one control 1/4 turn over a wide variety of mineralised ground anyway and I feel that retaining the raw processed signal doesn't introduce any inherant losses.
If I get around to ensuring the normally good response time of the SD2000 can be saved after applying any introduced signal processing I'll get back into it again ..but right now I haven't seen the need.
<IMG SRC="/forums/images/smile.gif" BORDER=0 ALT="">
-gaz

 

[Anonymous]

Eric, Thanks for bringing the message to the top.
Gaz, Great to hear from you. I can imagine that making the mods on a brand new SD was akin to operating on one's own mother! Without doubt, not for the faint of heart.
OK, I keep going back to VLF/TR theory. All the prospecting VLF's use very high frequencies. For instance, the Goldbug II uses a frequency of 71.01KHz.
What we are seeing is that a small conductive target gives us a better signal when we force the induced currents to flow at the surface of the target. This of course is the well known skin effect.
The losses to high frequency signals are much higher at the surface of a target. This is why many radio coils used to be silver plated. What we need is for Dave Johnson to explain the reasons why T/R's work so much beter with small conductive targets when they operate at a high frequency?
No doubt we are going to find that a PI can also benefit from some form of high frequency pulsing.
Sure, our PI's rectangular transmit pulse contains some energy from harmonics that go to infinity. The question is this, would the detector work better with a continuous high frequency signal?
The minelab SD's use samples taken from the long pulse to form a low frequency information channel and samples taken from the high frequency pulses to form a high frequency channel. As the detector is more sensitive to small nuggets with the high frequency channel then it looks like the mod to make the pulses even shorter should indeed increase the detectors sensitivity to small gold.

 

[gazdet]

Hi Again Eric and Dave and Everyone else from back in the day!,...

Wow..

It seems like forever since I have visited this Forum and posted about my SD2000 Mods (as Gaz).. I hope you all have found heaps of Gold since then!
Looking forward to checking out everyone's experiences with the later machines.

Cheers Gaz