anti interference coil

[minersright]

Hi.
This is my first post as I have just became a member, and as I have only a small understanding of electronics and would like to learn a little about coils so I would like to ask a few questions if I may.

#1 Minelab sd-gp detectors do not like electrical interference.(power lines, telephone lines etc.) So there is a after market coil called an anti interference coil that solves this problem. I would like to know how this coil works when the dd and the mono coils don't. I think the coil is wound in a figure eight configiration.

#2 There is also another after market coil that has a switch on it that alters the phrasing, what does that mean?

#3 What is the function of a electronic filter?

Thanks in advance for any advice.
Regards minersright.

[Max]

Hi.
This is my first post as I have just became a member, and as I have only a small understanding of electronics and would like to learn a little about coils so I would like to ask a few questions if I may.

#1 Minelab sd-gp detectors do not like electrical interference.(power lines, telephone lines etc.) So there is a after market coil called an anti interference coil that solves this problem. I would like to know how this coil works when the dd and the mono coils don't. I think the coil is wound in a figure eight configiration.

#2 There is also another after market coil that has a switch on it that alters the phrasing, what does that mean?

#3 What is the function of a electronic filter?

Thanks in advance for any advice.
Regards minersright.

Hi,
figure-8 cancels noise by geometry better than concentric and dd. Take a look here:

http://geotech.thunting.com/pages/metdet/info/coils.pdf

Best regards,
Max

[Reg]

Hi Minersright,

The article on coils gives a good picture and explanation of the figure 8 coil, so the basic answer to your #1 is there.

#2 is a little different. if you look at a DD coil pic on the same article you will see two coils or windings. One is the transmit winding and on is the receive winding. What isn't shown are the wires coming out from each of the two windings. Now, when a signal is transmitted in the transmit winding and the DD coil is not perfectly balanced, there will be a signal coming out of the receive winding. Just what that signal looks like is determined by just how you connect the two receive wires. Lets call the two wires a and b. Now, lets also assume if you connect a to the common and use the signal seen on b, the receive signal looks much like the transmit signal and are in phase, meaning when the transmit signal goes positive, the receive signal will go positive.

Now, if we were to swap a and b, we would see just the opposite, meaning when the transmit signal went positive, the receive signal would go negative. In this case we are 180 degrees out of phase.

Ok, if the normal operation is such that the two signals are supposed to be in phase, then a positive on the transmit will cause a positive on the receive. This would give a normal signal out of the detector. If we were to swap the receive wires, then a positive on the transmit would cause a negative on the receive. I this case, signals would be reversed.

The idea of the switch on the coil is to be able to switch between having large gold produce a negative signal (normal mode) or invert the signal so large gold causes a positive response.

I hope that helps clear up that item.

#3 What is the function of a filter is quite a bit different. A filter is a combination of components arranged such that they enhance certain frequencies or reduce certain frequencies depending upon the type of filter used. The treble and base controls on your radio are basic filter controls. In this case, they can enhance the higher tones, or the lower tones, depending upon which way they are adjusted.

In detectors, filters are used for different reasons. One reason is to try to reduce external noise problems. Noises such as those from the 60 hz power lines is transmitted in the air and is picked up by various components, thus causing one to hear warbles, or whatever the noise might cause. A special filter can be added to block that particular frequency, thus reducing or eliminating the noise problem.

Signals from the ground and signals from buried metal objects generally differ in frequencies. Normally the ground signals are very low response from 0 hz to 2 to 3 hz or so. Target signals can span the same range and go much higher up to 20 hz or so. Now, by adding special filters to try to block the very low signals, we can reduce the ground signals and allow the target signals to pass, thus minimizing ground signal problems.

VLF detectors use this filter technique in their motion discrimination circuitry and in the autotune circuitry. The autotune is sometimes called the SAT also.

PI's use an autotune circuit that acts the same way. One way to tell if the detector has a form of autotune is to listen for a double response when you go over a target. Autotune type circuits use a high pass filter, meaning it will pass higher frequencies easier than lower ones. If the audio signal goes wee-woo or woo-wee the unit has some form of what is called a high pass filter. The first half is the main signal and the second half is the result of going through the filter.

So, once again, the function of a filter is to filter out certain frequencies or allow certain frequencies to pass easier. The difference lies in just how the filter is designed.

Reg