Full m/lab SD2000 schematics

[ZED]

Front end receive gates

Hi Kev

The logic cct is good in itself in terms of generating pulses but it does not lend itself
very well when it comes to trying to change pulse widths or creating extra pulses not
to mention moving pulses about the place.

A MCU would be the ticket provided you had the flexibility to modify pulses or create
extra pulses,especially out on the field,plus the MCU would need to be able to generate
more than one pulse at exactly the same time.????,,,,not exactly a high point with sequential critters.
Could explain why m/lab dont use a MCU to generate there pulse trains.

The pic below shows the pulse train for the front end receive fets,both fets open and close at the same time,the cct that controls the front end receive fets gets its signal from the logic cct {same signal as the trans cct},the signal goes into a R/C circuit so as to create a delay,this R/C cct is also controlled
by other ccts,the delay is used to prevent the front end receive fets opening during the high voltage
fly back,this is my interpretation anyway.

Zed

[ZED]

I new i should of used arrows

G`day Bugwhiskers

"On your timings sheet Zed you show a figure of 300uS delay to the final ground channel sample. The timing figure should be from the last of the short 60uS pulses as it is providing the energisation for that sample."

Yes Bugwiskers you are right,the delay of 300 u/sec after the last 60 u/sec
transmit pulse is correct,now ! i have a little itsie bitsie line where the last 60 u/sec trans pulse finishes,its sitting on the same line as the 240 u/sec sample,in my infinite wisdom i thought this would suffice but deep down i new,i should be using arrows.

I would like to hear more on your ideas about the late 240 u/sec grnd sample,i find it a very interesting and curious sample.

Zed

[bugwhiskers]

Hi Zed,

The ATMEL Tiny26 would be able to provide all the logic to control all the timing functions easily. It's just an endless loop that has delays then sets or resets the output pins.

example:

start:
setb portd,1 (turn mosfet on)
ldi cnt,56 (load counter with 56)
dl1:
dec cnt (sub 1 from counter)
brne dl1 (if cnt is not zero go to dl1
clrb portd,1 (turn mosfet off)

here you put more delay loops and pins on and off that control analogue switches or fets

jmp start: (go back to start and do it all again)

If you run the Tiny26 at 20MHz then the resolution of the delay loops is 50nS, more than enough.

regards
bugwhiskers

[ZED]

sound good

Bugwhiskers it sounds very good,only one problem,i am deaf,blind and stupid
when it comes to digital,always on the look out for a good book but they seem to specialize on a particular mcu and i dont know which one to go for.

Zed

[bugwhiskers]

The ML ground balance method whilst nifty has flaws. The worst flaw is that certain sized targets at certain depths and GB settings have reduced visibility ie holes in the response.

The method relies on the premise that the ground decay follows the TX on time, ie longer TX = longer ground decay time. Targets on the other hand as long as they are fully energised will generate the same decay. The ML GB method is about discriminating the "perceived" ground element and subtracting it from the total signal.

If you look at the timing diagram you will see a short ground sample taken after a long TX and a long ground sample taken after a short TX pulse.

What is even more interesing is some folk are modifying the SD2000 and getting better results than the latest model. If someone can come up with a GB method that achieves the same result (or better with no target holes) then they can start looking at an early retirement.

regards
bugwhiskers

[bugwhiskers]

I just had another thought about the "holes". As the GB method is about eliminating anything (ground or target) whose TC changes with a different TX pulse width then a target that hasn't been fully energised falls into that category.

regards
bugwhiskers

[ZED]

Bugwhiskers the big problem with all systems of grnd balance is when the
target signal starts to become equal in amplitude between the first sample
and the second sample plus the grnd balance sample,looking at the second
sample and the grnd balance sample as one sample and the first sample as
an other,when the target signal becomes equal in both then the target does a
dissapearing act,also known as windows
Now if the detector has two or more channels for detecting then one can
cover to some extent the effect of the window or bring out the next model
detector where the windows are shifted and detect the targets the previous
model didnt see because of its windows and then say how much
more sensitive the newer model is.

But seriously we need to find a way of getting a grnd sample that has no
target signal in it.

Zed

[bugwhiskers]

Hi Zed,

You are not the first person who has voiced that conspiracy theory. Recent testing of the 4000 in Sensitive/Smooth mode showed the response falling off either side of a 10 gram nugget. Would ML be worth the $70 million it is being sold for if their first detector (SD2000 in 1995) saw every nugget within 3 feet of the surface.

regards
bugwhiskers

[ZED]

"Would ML be worth the $70 million it is being sold for if their first detector (SD2000 in 1995) saw every nugget within 3 feet of the surface."

Ummmm is that a trick question ?.

Zed

[ZED]

Bugwhiskers

"As the GB method is about eliminating anything (ground or target) whose TC changes with a different TX pulse width then a target that hasn't been fully energised falls into that category."

sorry havent figured out quotes yet.

Yes that could be argued but there are many circumstances where targets
fall through the window,another is the distance of a target relative to the coil
the grnd matrix and the target TC,a target may have a TC that puts it just outside
of a window when close to the coil and yet when the distance
between the target and coil increases with the resulting drop in target signal
it may rapidly fall into the window due to the signal amplitude and target TC.
I have a nugget around 3 grams and often when i generate my own timings
i see this nugget change in sensitivety from 3" to 13" using an 18" mono.
All because of the timings.Now i must say that having two channels does
nip this problem in the bud.

Zed

[bugwhiskers]

Hi Zed,

I believe the SD2002 has 4 channels, maybe this was an attempt to get rid of the holes.

Zed sed
"But seriously we need to find a way of getting a grnd sample that has no
target signal in it."

Why not kill 2 birds with 1 stone. By discriminating PI targets using conductivity as the basis the ground won't enter the equation as it's conductivity is very low compared to the good stuff.

regards
bugwhiskers

[ZED]

Oh yeah you reckon

Sorry Bugwhiskers but i beg to differ,my current experiments involve large amounts
of coil current and sampling during the fly back and i have a hot rock
that gives off a conductive response from 20 to 100 mv straight off the coil,
no pre amp,now im not saying that will be the case with a standard pi but it
leads me to believe that a conductive response is something that i will have
to deal with as i design and explore.

Zed

[Unregistered]

Hi ZED.
Thanks for the circuit. Has tried in a simulator timing, it works.
I thank, once again.
Vladimir D.

[kev]

Thanks again ZED

Hi Guys,
Thanks for the FET timings, I thought they'd be off during the pulses.
A micro does an excellent job of timing without jitter if you utilise autonomous functions. On mine I use the PIC 16F88. A frequency and delay adjust pot is connected to separate A to D channels. I use a delay loop function for the pulse, sample and delay timings, but between the target sample and the emi/earth field sample I write an offset to the 16 bit timer and then read and process one of the A to D channels, and afterwards wait for the timer to rollover. I can do heaps during this time.

I have about 6 functions that I call in a set order, they are basically the same, except that one reads the delay pot, one reads the frequency pot (changes pulse length too), the others are copies of this but in one it toggles the audio chop after processing the A to D, so while the unit is runnig at 7kHz, this function is called every sixth time so that the audio is 580Hz. Another toggles the charge pumps, while another called about every ten cycles monitors the battery voltage via another A to D channel.

Because all the housework is done while the counter is autonomously doing the counting, the latency in Main() is equivalent, I have very accurate stable timings, so an SD2k micro control would be very workable indeed.

Cheers
Kev.

[kev]

Narrowing the holes

Hi,
I've just been thinking about what you Guys have said above regarding detection "holes" or windows. A mircocontrolled SD2k would allow complex pulse trains to be easily generated, which would narrow the holes, maybe even eliminating them. It would be simple to apply a train such as this a 120us pulse followed by four 30us pulses, then a 240us pulse followed by four 60us pulses, then a 480us pulse followed by four 120us pulses the back to the start again. This would have the advantage of hitting large and small gold at the same time, and reducing the likelihood of a gold signal appearing identical to the ground.

Cheers
Kev.