Does single coil circuit suffer from EFE beep ?

[6666]

Hi Gary
just wondering with your single coil detector
does it suffer from the EFE beep at the end of each swing of the coil ?
Thanks.

[chemelec]

What is an EFE Beep?

[homefire]

Earth Field he's talking about I think.??

[Qiaozhi]

What is an EFE Beep?

EFE means "Earth Field Elimination".
Obviously the elimination of the Earth Field would not cause a beep, so he should have simply said "EF".

[6666]

Hi Gary
My brief simple question was just asking you ,
does your single coil, single sample PI design give a false beep at the end of a coil sweep.


The Earth field or earth field effect, is caused by swinging a coil through the earth’s magnetic field and inducing a small voltage into the coil, this can sometimes be a problem with PI designs that only have one coil and only take one sample and have a high gain receiver circuit., and the small voltage induced into the coil can give a false target signal beep at the end of a coil sweep.


The EF problem in high gain receiver circuits was over come by the invention of the second EF sample and used for example in the surf PI.

[chemelec]

Hi Gary
My brief simple question was just asking you ,
does your single coil, single sample PI design give a false beep at the end of a coil sweep.


The Earth field or earth field effect, is caused by swinging a coil through the earth’s magnetic field and inducing a small voltage into the coil, this can sometimes be a problem with PI designs that only have one coil and only take one sample and have a high gain receiver circuit., and the small voltage induced into the coil can give a false target signal beep at the end of a coil sweep.


The EF problem in high gain receiver circuits was over come by the invention of the second EF sample and used for example in the surf PI.

Not that I have ever had happen, but my detector doesn't beep.
It is a variable frequency tone.
but I think that would be more likely to occur with a motion type detector, not a non-motion one

[6666]

Thanks for reply, the word beep is just my generic description of a target signal.

[Qiaozhi]

Gary - Do you have a description of the operation of this detector?

It looks like the two coils are pulsed in an interleaved fashion, and the coils are in anti-phase. This would produce a bipolar TX output. Even though the preamp is configured as a differential amplifier, the output appears to only be sampled on one phase, so I don't see how it could eliminate the Earth field. Unless this circuit is doing something completely different. Perhaps you can explain.

[Qiaozhi]

I think I've figured it out:
In a normal single coil bipolar design, the preamp output will contain both positive and negative signals. This requires the two signals to be sampled separately, before being fed into a difference amplifier. In a design with two separate TX coils, the preamp can be a difference amplifier, and then you can sample both pulses with a single sample gate. In both designs the Earth field is automatically eliminated because it always has the same polarity, whereas the target signal will have a different polarity depending on which pulse is being sampled. So the answer is that Gary's design does eliminate the Earth field, and it has nothing to do with being non-motion as opposed to motion. The Earth field will still affect a non-motion design, as it's the motion of the coil itself that causes the problem.

[dbanner]

Twin loop treasure seeker(Robert and David Crone):"...if we were to subtract the outputs of the two coils the ground effect from each would be cancel out."

[dbanner]

"....By similar reasoning, medium and long wave radio broadcasts would cancel out as the field strength of these signals does not change significantly in 100mm and each coil will receive same amount of interference."

[dbanner]

Likewise earth field