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Pulse Frequencies- your choice?

A

Anonymous

Guest
Quite a few designs we see for pulse induction detectors use relatively high frequencies to generate the pulses they use.
There is a lot of talk about "skin effect" and overcoming inherant problems in processing ground noise.
The SD series generate two pulses, typically these are 2khz and 7khz for the SD2000 and 2khz and 9khz for the SD2200 there is also a correspondingly large sample time.
Given the excellent sensitivity and performance in mineralised ground conditions of the SD series, it got me curious as to why using low end frequencies isn't a favoured choice for PI...or is it?
There would appear to be no problems with small and large target sensitivity using these freqs and also that the design of a more managable ground balancing system can be implemented with the appropriate coil design.
How many of the PI designers here have worked with similar low frequencey pulses in their designs..and what performance results or problems did you have?
-gaz
 
Hi Gaz,
Fundamentally, the signal from an object is the same whether a PI detector operates at 1 pulse per second, or 10,000 pulses per second. The very first PI detectors used pulse rates of less than 100, while today
 
Thanks for the outline Eric <IMG SRC="/forums/images/smile.gif" BORDER=0 ALT=":)">
Your description clearly outlines the functionality of a typical PI setup.
on this point..
<BLOCKQUOTE><STRONG>Originally posted by Eric:</STRONG> What is important from the objects point of view, is the duration of the transmitter pulse and it
 
Hi Gaz,
I suspect that the drop in current as you increase the pulse frequency is due to the high Q coils used. Increasing the Xtal frequency will also shorten the transmitter pulse and as the current is inductance limited, it will not rise to as high a peak value. The drop in mean TX current by this mechanism, is not compensated by the increase in current due to the higher pulse frequency. If you look at the TX current in a SD, it is a good representation of a sawtooth waveform.
Eric.
 
"After doing some more general measurements, there doesn't appear to be any major power losses in the 7khz pulse in proportion to the lower frequency pulses of 2khz when received at the RX test points I checked, although I'm sure this may be the case in some PI systems."
As has been pointed out before, the 2200 long pulse draws approx 3 times the current of the
short pulse. The GP equalises the current in each pulse.
Candy's original explanation of multiple short
pulses was to equal their time to the long pulse for noise considerations and to make it cleaner
to sum portions of these for his subtractions.
If you look at his principal in it's simplest form, ie that metal response is not pulse length dependant and ferrite is, then there is probably a variety of ways to sample and proccess the pulses as to satisfy this relationship but in order to close the holes that the 2000 suffered, it seems necessary to make several samples of each pulse and check them against the other.
The formula for subtracting the ferrite and promoting the metal suggests that more short pulses the merrier, but as he has gone from 4 to 3 then the advantage if any, must be small I gather.
 
Hi Robby
<BLOCKQUOTE>the 2200 long pulse draws approx 3 times the current of the
short pulse. </BLOCKQUOTE>
Agreed, the current drawn by the long pulse is greater due to it's "on time", what I described was that there appears to be no losses in amplitude measured in the received high and low pulses when measured at the receiver from the coil.
<BLOCKQUOTE>The formula for subtracting the ferrite and promoting the metal suggests that more short pulses the merrier, but as he has gone from 4 to 3 then the advantage if any, must be small I gather. </BLOCKQUOTE>
True, I think he prefered to go for 3 pulses to assist in faster processing and auto ground-balance in the 2200.
To compensate for the losses in only sampling 3 pulses, he increased the frequency of those smaller pulses from 7 to 9 khz, this also improved the sensitivity to a new range of target shapes.
This is one reason why the 2200 can pick up some targets that the unmodified 2000 can't do so as efficiently.
Doing so also increased the incidence of more false calls that are hard to differentiate by ear when locating small targets in hot ground.
Sensitivity was improved on the bench more than in any other SD with the 2200, however I feel to the point where the increase in sensitivity and pushing the design limitations of the ground-balance system, it is now running at borderline for the often encountered "hot ground" conditions in West Aussie.
I suspect some added/modified filtering may help to keep ground interferance levels to a point where the smaller targets are still easily identifiable within the target envelope during processing, this should only create marginal loss (if any) in sensitivity, but we'll see. <IMG SRC="/forums/images/wink.gif" BORDER=0 ALT=";)">
 
Gaz,
The Minelab SD's take advantage of the following to Balance the ground:
****** The decay time of ferrite ground is dependent on the duration of the transmit pulse ******
OK, So a long transmit pulse provides a receive decay from the ground which is longer than the decay from a short transmit pulse. Targets such as gold nuggets and coins provide the same receive signal decay time for either long or short transmit pulses.
The SD's sample the receive signals from the long and short pulses at different times after the pulses end. By subtracting a ratio of the signals sampled from each pulse you can cancel the effect of a ground signal by making both samples the same voltage with the ratio control. Receive signals from most targets will have a different decay time than the ground and will not cancel.
The ground signal is tuned out as well as some targets which have a similar decay time. OK, this might be a problem except that we have two different samples. The decaying ground signal from the long pulse is a lot longer than that of the short pulse while all targets decay at the same rate without regard for pulse width. The long and short pulse signals are seperated into two seperate channels. Both channels are seperately adjusted to balance the ground.
A simple comparator circuit in the SD automatically senses which signal is the largest and switches either the signal from the long pulse or the short pulse channel to the audio circuit.
A low frequency signal is also provided by sampling the receive signal just prior to the next sequence of transmit pulses. This signal includes the effects of swinging the coil through the Earths magnetic field as well as some signals from hot rocks etc. This signal is subtracted from the two channels to reduce the effect of hot rocks and swinging the search coil.
 
HiYa Dave,
I think we just explain the same operation a bit differently.. <IMG SRC="/forums/images/biggrin.gif" BORDER=0 ALT=":D">
tho..
<BLOCKQUOTE>A simple comparator circuit in the SD automatically senses which signal is the largest and switches either the signal from the long pulse or the short pulse channel to the audio circuit. </BLOCKQUOTE>
I'm not sure this system is actually in use in the final design.
It would appear from my tests that filtering of both pulses is the prefered method where both frequencies are processed in a linear fashion and then simultaneously sent to their respective G/B ccts and then onto the tone/threshold/audio cct for "mixing".
It's kind of like having a "sterio detector" with one channel for small targets and the other for large. <IMG SRC="/forums/images/smile.gif" BORDER=0 ALT=":)">
 
Gaz,
Sounds like the mixing method was all they needed. Now here is something to think about, the SD's don't work in our North Carolina ground! The soil here is actualy a bright orange volcanic clay. When it's dry, you can't stick a knife in it. When it's wet, the stuff sticks to everything like epoxy glue!
Although a VLF can tune it out OK, the SD2200D and the new GP could not handle it when it was wet. Both worked to some extent when the ground was dry but the depth they got was very poor.
There was a long string of messages concerning orange clay ground a while back. Many people in Australia reported problems. I was wondering if you have experienced any orange clay ground?
 
Hi Gaz and Dave,
Both the SD2200 and the SD2100 that I had, have a three position toggle switch. In the middle position, the two ground balanced channels are mixed, and in either of the other positions the earlier channel or the later channel are connected separately to the audio circuit. I think I am correct in saying that on some ground, only the later channel is usable, albeit with a loss in sensitivity to the smallest nuggets.
Eric.
 
Dave, you're absolutely right and yes I think I may have experienced the dreaded "clay" you mention. <IMG SRC="/forums/images/biggrin.gif" BORDER=0 ALT=":D">
Considering your location I assume the Basalt type "clay" you described is similar to that which is found in our salt lakes here, albiet here the color can be bluish, red, brown or green in color as well.
If the geological composition you have described contains a high level of sulphide and alkaline mineral components(which have unusually high levels of ground noise properties) it can have an extreme performance detrament to the standard SD series when using Mono (or to a lesser degree DD) coils.
Firstly, have you ever used an original or modified SD2000 in these conditions?
-gaz
 
Gaz,
I have never even seen a real live SD. The info about their not working in our soil conditions came to me from the manager of the Cottonpatch gold mine which is about an hour from here. I also heard the same thing from several prospectors in the area.
I learned how the SD's work from the patents. Now there is a tough bit of reading for you!!! I believe that only Bruce Candy could take something so simple, write so much about it, and still leave you wondering just what the heck you just read! He should have been a politician.
 
Yes Gaz, he has definatly pushed this close to it's limit. A large problem is that part of the effect of each pulse is handed onto the next and
he says that he overcomes this by adjusting gains to suit. It would appear that he would soon maybe have to go to his prefered method of using a bi-polar pulse in order to clean this up. As I see it, the gains would be worthwhile for a variety of other reasons also.
I would be interested to hear how others do their
testing on mock ups of these or mods as I have to drive out to the bush with a battery operated cro
and any bench tests on my designs at home need the sample time wound back to at least 15-20 micro seconds or later to actually hear a response. The cro's screen looks more like a picture of a complex puppet show including the strings when sampled early.
Talking about target response, I just got back from the Vic goldfields and got one speck that
went less than .1 gram. It wouldn't toe or heal
on a CT14"DD before digging but I dug one .15
grammer that can't be detected in air in the
coils centre. I have a 4+ grammer from Clermont that the 2000 totally ignores but the 2200 picks it up ok. Unfortunatly they don't make DIN standard nuggets for testing PI designs.
 
Hi Dave
Thats called a thousand ways to skin a cat
Regards Frank Wallis
 
lol @ Dave, <IMG SRC="/forums/images/biggrin.gif" BORDER=0 ALT=":D">
Yep, it's a good read.. *shudder*
Patents by nature are intended to cover a broad range of aspects and don't necessarily represent the specific nuts and bolts workings or every principle used in each model type that is marketed.
The patent was intended to cover all of the SD/GP series (and possibly others) and can incorperate any combinations within the patent that Candy may choose to impliment in order to repackage and sell the things..
The company wasn't floated without a business plan <IMG SRC="/forums/images/wink.gif" BORDER=0 ALT=";)">
 
Frank,
I don't know about skinning a cat but after reading that particular patent I wanted to unwrap a Candy!!! The point is well taken about how the Minelab patent is designed to be very broad based. However, After writing up a number of my own patents I can assure you that the text of the Minelab patents was designed to confuse the reader. Whatever the text contains it will get through as the patent examiner does not normally read anything but the claims that are being made.
A US patent (and Australian) is supposed to teach someone skilled in the art the invention being patented. The Minelab patents do not adhere to this principle which goes against the spirit of the whole thing. I have never seen as simple an invention covered in such a blanket of BS in all my life! With this said, the trick to figuring out Candy's patents is to start by reading the claims being made. Remember, The meat of any patent is in the claims and not the text.
 
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