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SD 2200D waveforms

A

Anonymous

Guest
Attached please find SD transmitt waveform I captured and drew without opening the control box. The impulses durations change slightly as a "detecting channel" is beeing selected.
Piotr
 
Hi Piotr,
What's up? Sighted your posting earlier - I'm back to reflect - it isn't there!!
Went to another forum where you had posted a hand-drawn sample of a Minelab product. Don't tell me you're being got at!! I want to see what you posted - my initial impression was you had left out one 60uS Tx. The tim ing still related to 32768 crystal.
g.
 
Hi Piotr and all,
This is the coil current waveform for the SD2200. It was taken by inserting a 0.01 ohm resistor in the ground connection of an 11in mono coil. The scope measured the voltage across this resistor. The horizontal scale is 100uS per division and the vertical 10mV /division. You can clearly see that the negative going current is inductance limited and is still increasing steeply at the cut-off point. The peak current on the large pulse is 3.5A while the smaller pulses are a third of this. The trigger point at the left of the screen is the cut-off of the previous transmitter pulse. The magnetic field from the coil will follow the shape and amplitude of the current.
Eric.
 
Lets see, if the magnetic field follows the waveform, and a sawtooth generates odd and even order harmonics, then we get BBS (now FBS). Looks like lots of inductance, almost got a linear ramp. Seems like some time constant objects could be cancelled, since the current in the object is still building at turn off.
Candy stated this in patent 5,576,624.
quote column 18 line 50
"Some of these particular first order objects would not be detected, that is there would be a "hole" in the response of the detector. However, if the duration of the interrogating pulse is changed, the form of the FRD {ferrite relaxation decay} changes while that of the first order object remains the same, the respective signals are now dissimilar and that part of the received signal due to the first order object is easily discerned."
See how easy it is.
JC
 
Hi JC,
The inductance is only 300uH but the resistance is very low - about 0.6 ohms for coil and cable. Hence coil time constant is 500uS. Time for full current growth is 2.5mS so you can see why a 230uS TX pulse hardly gets started.
Eric.
 
Hi Eric,
Great photos and graphs. You've taken the forum another step foward, by being able to post images.
Always have to ask myself why a designer did what he/she did.
Think someone mentioned this before, but maybe the sawtooth better meets radiated emission limits, for one or more of the following countries.
Maybe to get more out of the battery (detection/amphour).
Maybe this works more magic on the metal for some reason not understood by mortals.
Needed the extra turns to get it to respond like the competition.
Alway curious
JC
 
Thank Eric for this great forum and for the graph very interesting. My question is relative of the inductance of the coil SD2200,you say only 300uH BUT the resistance is very low 0.6 ohms, How you can you reach a inductance of 300uH with few turns of low resistance wire per meter( because of the capacitance) ? Do you think they use some kind of circular thin FERRITE to enhance this inductance ? I feel the same of all of you about the marvelous 35 years old !!!! OP amp 709 ,I try many other new and expensive new amp without succes o perhaps of my few electronic kwowledge. have a good day Eric and all the Forum,Alexis.
 
Hi Alexis,
No, there isn't any ferrite. Just a simple coil of thick Litz wire and heavy gauge conductors in the cable.
Eric.
 
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