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Anonymous
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Ref: US 5,537,041 Candy
The schemes described below do not infringe the above cited patent because the claims of the above patent exclude pulse induction. If anyone knows of any patents which the following schemes might infringe, please post the information. Thank you.
VARIABLE DURATION TRANSMIT PULSES
It is customary to use pulses which are all identical-- one transmit-receive cycle is the same as the next. The Minelab SD's use different length pulses, apparently as an aid to getting rid of maghemite.
The basic idea is that the receiver response curve will vary, depending on the duration of the transmit pulse. This provides somewhat the same information as can be obtained by demodulating fixed-duration pulse responses at several different delays, but the effects are slightly different, esp. when comparing maghemite to metallic targets. Of course one can have transmit pulses of two or more lengths, and also demodulate different delays.
VARIABLE OFF-TIME
Varying off-time also produces changes in receiver response, if everything else is kept the same.
In general, it is the practice to sample earth field just before the beginning of a transmit pulse, and subtract it from the earlier target detection pulse. The difference between these two samples increases on high-conductivity and ferrous targets, as the off-time is increased. However, beyond a certain point, sensitivity is usually not improved because increasing the delay means that fewer samples per second can be low pass filtered and turned into an audible target response.
If it were not necessary to subtract the end of the "tail" in order to cancel earth field, then crowding pulses closer together would improve, not decrease, sensitivity to high conductivity targets because the tail of one pulse/sampling interval can leak into the next one, adding to it. (This is not true for bipolar pulse systems, which were are ignoring here.)
I propose the following scheme.
1. Of successive off-times, have at least one out of two be short ones. Sample target signals for detection and discrimination. Pretend that earth field doesn't exist.
2. Of successive off-times have, have one out of two, or less, be of long duration. Near the end of the off-time take a sample of a duration sufficient to earth field balance the (presumably more plentiful) target signal samples.
The advantage of this scheme is that the benefit of long off-time (less impairment of sensitivity to high-conductivity targets) is combined with the benefit of frequent target signal sampling and current overlap from one flyback to the next (improved sensitivity).
--Dave J.
The schemes described below do not infringe the above cited patent because the claims of the above patent exclude pulse induction. If anyone knows of any patents which the following schemes might infringe, please post the information. Thank you.
VARIABLE DURATION TRANSMIT PULSES
It is customary to use pulses which are all identical-- one transmit-receive cycle is the same as the next. The Minelab SD's use different length pulses, apparently as an aid to getting rid of maghemite.
The basic idea is that the receiver response curve will vary, depending on the duration of the transmit pulse. This provides somewhat the same information as can be obtained by demodulating fixed-duration pulse responses at several different delays, but the effects are slightly different, esp. when comparing maghemite to metallic targets. Of course one can have transmit pulses of two or more lengths, and also demodulate different delays.
VARIABLE OFF-TIME
Varying off-time also produces changes in receiver response, if everything else is kept the same.
In general, it is the practice to sample earth field just before the beginning of a transmit pulse, and subtract it from the earlier target detection pulse. The difference between these two samples increases on high-conductivity and ferrous targets, as the off-time is increased. However, beyond a certain point, sensitivity is usually not improved because increasing the delay means that fewer samples per second can be low pass filtered and turned into an audible target response.
If it were not necessary to subtract the end of the "tail" in order to cancel earth field, then crowding pulses closer together would improve, not decrease, sensitivity to high conductivity targets because the tail of one pulse/sampling interval can leak into the next one, adding to it. (This is not true for bipolar pulse systems, which were are ignoring here.)
I propose the following scheme.
1. Of successive off-times, have at least one out of two be short ones. Sample target signals for detection and discrimination. Pretend that earth field doesn't exist.
2. Of successive off-times have, have one out of two, or less, be of long duration. Near the end of the off-time take a sample of a duration sufficient to earth field balance the (presumably more plentiful) target signal samples.
The advantage of this scheme is that the benefit of long off-time (less impairment of sensitivity to high-conductivity targets) is combined with the benefit of frequent target signal sampling and current overlap from one flyback to the next (improved sensitivity).
--Dave J.