The Preamp Gain (PAG) setting can be viewed as being available for adjustment so that it can be turned down – not turned up. Here's what I mean by this.
If there were not a need to deal with mitigating factors, the PAG setting could be fixed internally in the DFX at 4. If this were done, two problems could occur. First, in areas where there is very strong mineralization, "
VERLOAD indications might occur due to just the mineralization. Being able to turn down the PAG allows operation in soil that would otherwise not be possible. The other related problem area is electromagnetic interference (EMI). To better understand this issue, let's look a bit closer at what the PAG thingy does.
The PAG setting controls an amplifier in the DFX that is very much like the pre-amps used in older audiophile systems. These pre-amps are amplifiers that boost the very small voltages, coming out of a phonograph needle or a magnetic tape pick-up, up to a voltage level that the stereo's amplifier, which drives the speakers, can use.
On the DFX, the pre-amp is responsible for boosting the very small currents on the receive coil in the search head up to a level that can be more easily processed by the DFX's computer circuits. In cases where the EMI is very strong, this noise will take up a significant portion of voltage range that the pre-amp is providing to the DFX's computer. One would say that the EMI is using a considerable portion of the dynamic range of the amplifier – only the range of voltages above the highly amplified noise and the max ("
VERLOAD) level is available for the computer to extract information.
In these cases, where the EMI is relatively strong, the A.C. Sensitivity (ACS) can be reduced to a level where the spurious audio and display indications stop occurring. Doing this doesn't address that fact that the DFX's computer is limited as to the range of useful information that is available out of the pre-amplifier. Also, the EMI noise, although it can be scaled down with the ACS below a level where it stops causing false indications with the search coil in the air, this noise is still at the base of the signal coming from the pre-amplifier – buffeting the stronger target signals and adding to their instability.
To address these issues, the PAG setting can be turned down. It doesn't really have a detrimental effect on depth since in these circumstances the limiting factor is the competing EMI signal noise.
Out of time for now,
Enjoy.