Doubble d's and pulse delay

[Anonymous]

Hi all
Will a properly cancelled dd type coil allow for very short pulse delay's when used with PI detectors?
My thinking on the subject is that provided that the signal receiving is done only with the coil that is not transmitting then it should be possible to use very short pulse delays and thus allow better sensitivity to small gold. Is there a problem with my thinking?
Cheers
Steve D

 

[Anonymous]

Hi Steve,
Balanced coils, whether DD or concentric, do give some improvement in the shortest delay that can be used, as the saturation of the front end amplifier is reduced. The biggest factor though, is the time that it takes to switch the transmitter field off. This applies whatever type of coil arrangement is used. The stored energy in the magnetic field has to be almost completely dissipated before sampling can start. The higher the pulse current and the larger the TX coil inductance, the greater time this will take. This energy is largely dissipated in the damping resistor with the normal L/R time constant. Coil/shield/cable capacitance also has an effect as the L, R, and C then become a damped tuned circuit. The value of the damping resistor is then adjusted for minimum ringing and overshoot (critical damping).
Eric.

 

[Anonymous]

Hi Steve,
I have built several DD coils and I was never really able to fully "balance or null" a DD coil to the point that I could reduce the delay to almost nothing. I am not really sure why but there was always some signal present that would limit the minimum delay.
What I have found is the DD design makes it much easier to reduce the overall delay, to a point. It does get more difficult below 10 usec for some reason. At least, it does with the PI's I have experimented with. I am not sure just what is going on at this level.
Now, it also appears that approaching the perfect "null" point also impacts the ability to detect extremely small gold. At least, it appears that way in the experiments I have tried. As a result, I do not try for a perfect null. By extremely small gold, I mean gold a grain or two or less in size. Physically, in the pinhead or smaller size.
My problem is, I don't have a perfect environment where I can eliminate noise variables, etc. So, I have to rely on simple testing as best as I can at my home until I can run more extensive tests away from the typical interferences that occur in a city. Even then, there always seems to be some form of noise around.
Now, to simplify my answer, yes, a DD coil design does make it easier to reduce the delay some. A DD design also increases the effective sensitivity to real small stuff by concentrating the "detection" area to primarily the overlap zone. Thus I find it easier to "hear the response from real small gold targets when I use a DD coil.
Reg

 

[Anonymous]

Eric,
Would pumping higher voltages into the detector as with the regulated add-on Modifications for the Minelad SD's cause a greater pulse current and therefore muck-up the sampling times.
Gary.