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Four Tips to Help Your Detector Auto Tune or Noise Cancel

09 Jun 2010
Metal detectors are sensitive instruments designed to detect small changes in magnetic field. Nearby metal changes the field as intended but so do far off thunderstorms, electric motors, electric fences, RF transmitters, power lines and nearly everything that a modern society plugs into an electrical outlet.
Many of Minelab’s metal detectors have a function that allows them to locate and avoid these interfering signals. In some detectors we use the term ‘Noise Cancel’ while in others we use the term ‘Tune’, ‘Auto Tune’ or ‘Manual Tune’. These terms all refer to the same thing, that is the process where the metal detector or the operator searches through the available tuning range, looking for the quietest point at which to operate the detector.
With appropriate settings in some detectors, such as the pulse induction GPX range of detectors, the operator can hear the noise directly, while metal detectors, like the E-TRAC or X-TERRA range of detectors largely mask this noise from the operator until it becomes quite bad. This discussion is largely centred about the pulse induction class of detectors used for gold prospecting although many of the general principles hold true for treasure hunting detectors as well.
During an ‘Auto Tune’ process, the detector sweeps through the entire tuning range, picking the best point to operate. The selected operating point combines the features of quiet operation and a quiet neighbourhood so that if the noise shifts a little then there is less chance that the noise will be heard by the operator.
The thing to remember is that, while prospecting, you have set up the detector to minimise noise in order to make small targets easier to hear. This is not necessarily the best configuration to use during the tuning process, as the noise is also harder for the detector to find. Many of the following points are designed to make any noise more obvious in order for the detector to most easily find and avoid it.
Based on my experiences, the following could be of some help.
Hold the coil in the orientation that it will normally be used. This ensures that the detector will experience the same noise during the tuning process as when detecting normally.
Lift the detector away from nearby metal and from the ground. Any nearby metal will make the environment seem noisier than it really is. Don’t forget that the ground can appear to be a target during this process as well, even if well Ground Balanced. If the detector is away from targets and ground then a little coil movement won’t make a significant difference to the Tuning outcome.
Change settings to enhance noise if required. If operating at a lower gain or in Cancel mode it may be advantageous to increase the gain, or switch to Mono or Double D mode. Similarly changing the Motion setting to a faster setting will allow any noise to be more easily located by the detector. You can safely switch these settings back after the Tune is completed.
Enhance any difficult noise sources. If you are still able to hear some remnant interference after an Auto Tune then you may need to move the coil to enhance those signals either by turning around to face the noise source or by tilting the coil away from horizontal. Remember that large signals will be located and avoided easily. It is the small and intermittent signals that are difficult to locate.
Remember to Ground Balance after the tuning process is complete as a new operating point may not be as well Ground Balanced.

Comments

Excellent reading Phil and almost exactly the method I use in the field except I leave my GPX-4500 in the Manual Tune position of the menu at all times making manual changes a few points at a time to get the "Tune" position exact, I often find the auto Tune is sometimes out by a few points either way even when using the faster Motion setting and elevated Gain method so I feel it needs that final manual tweak to get the threshold silky smooth. ..... Jonathan Porter
Posted By: Jonathan Porter on June 09, 2010 08:10pm
I have a GPX-4500 and i would like to know how much effect does the gain have on the depth? How much depth do i loose by dropping the factory preset down from 8 to 3 to get it quiet enough be able to hear?
Posted By: lcd 24 on June 14, 2010 12:34pm
lcd 24, I would say that you in this instance it sounds like you get more depth by turning the gain down. All the gain in the world is for nothing if you can’t separate the targets from the background. Sometimes it is possible to reduce the noise level by adjusting the Stabilizer or Sweep Speed setting but this of course depends on the source of your noise.
Posted By: Phil on June 15, 2010 11:51am
Jonathan, thanks for the comment. That’s a good example of using manual tune. The Auto Tune is searching for a wide quiet area rather than the absolute quietest. There may be quieter settings that are passed over in preference for a more stable operating point.

I was disappointed to have missed you in Victoria a few weeks ago.
Posted By: Phil on June 16, 2010 10:48am
If you are in a very noisy area and are required to tune regularly, it may save some time to set up a Search Mode as a tuning mode. Simply set the Motion to Fast, and increase the Gain to 15. Then when you want to perform an Auto tune, just select the "Tuning" Mode, press Auto Tune, and when completed go back to your preferred search Mode. Saves a lot of time and works great!
Posted By: Nenad on June 29, 2010 11:09am
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