SAT: Everything you need to know

[landman]

I did a search and found that the title "SAT" did not produce a single entry. So let's do this right. I would like to know everything you all know about S.A.T.'s or self adjusting thresholds as they relate to VLF's and more specifically to PI's. I'd like to know what it is, why it is and how it works and most importantly how it can impact your machines performance when adjusting it.

 

[Reg]

Hi Landman,

Your question could take a booklet to fully answer, but the simple response is the SAT or autotune as I am used to calling it, simply tries to maintain a constant threshold level. This is accomplished by using a basic high pass filter and adjusts the discharge rate of the filter. The detector signal is sent through the filter and the output from the filter is then added to a constant level signal, set by the threshold control. A high pass filter will generally take an increase in amplitude and make a hump type signal out of it. When the target is removed, this same hump will be generated in the opposite audio level. So, passing a rising signal through a filter will create a shorter increasing audio and on some detectors a wee type signal and when the target is removed, the signal changes to a woo, or quieter signal. On some detectors this simply translates to an increase in audio followed by a decrease in audio level. Now, when you are adjusting the SAT speed, you are basically adjusting how fast the filter responds which translates to how fast the hump type signal returns to the basic threshold level.

Now, keep in mind that any signal from a target would have a certain amplitude if nothing were to influence it. Unfortunately, the ground signal or other signals may add to or subtract from this signal as a coil is passed over the ground. Fortunately, the ground signal is generally a slow response and is dependent upon the sweep speed of your search head. As such, the signal is quite slow. Pass this ground signal through a high pass filter and some to much of this signal is eliminated. Speed up how fast the filter circuitry responds and more of any slow signal is reduced.

On the down side, the target signal, which normally is a much faster response is also reduced some but not as much as the ground signal. The reason is if a high pass filter is used, it allows faster signals through easier than slow ones. So, a target response is reduced but the ground signal is reduced by a greater amount.

As you speed up the SAT, you are further reducing the ground signal, but because the filter speed is increasing, you will also be reducing the target response also. So, there will be some depth loss as the result of using the sat, especially at faster rates.

A detector with a SAT can be easily determined by simply taking a target and passing it over the coil, but stopping the object at the center of the coil. If the signal rises and doesn't change, then no SAT is involved. However, if the signal rises and then decreases back to the normal threshold, there is a SAT involved. Just how fast that signal rises and returns back to the normal threshold will be determined by the SAT adjustment.

This is a basic explanation without going into too much technical detail.

Reg

 

[landman]

So is an all metal non motion detector simply a detector with out a SAT?

 

[Reg]

Hi Landman,

Yes, a non motion detector does lack a SAT or at least a fast or adjustable SAT. Some non motion mode units use a very slow autotune to adjust and compensate for various forms of signal drift. Normally, this is so slow it is hard to tell it is there. In the early days, even warming of the coil or even components would cause the threshold to change levels. This used to get annoying, so a very slow compensating form of autotune or SAT was added to keep the threshold constant.

Reg

 

[UK Brian]

Hi Landman

To add to what Reg has already posted it would seem that to get the best performance from a P.I. then the S.A.T. adjustment should be at its initial low setting.

In fact on many beaches, especially black sand ones, its better to increase the setting. With the Goldquest SS I use it mainly at the mid point of the range. This has the added advantage of tightening up the target response so you are more able to feel the shape of the buried item which helps in the I.D. process and saves wasted digging.

 

[landman]

So slow = low and it is the ideal in the best conditions but like you said does not allow for sharper edges when over a target so speeding it up (fast) will help it form a desirable signal for you. What conditions would you increase the knob from slow to all the way to fast or in other words increasing the SAT? Mineralized or salt ground that is squawking at you? Adjust the SAT from slow to fast before you touch the Pulse speed?