what is Self Adjusting Threshold

[DJSUOL]

In addition, The graph shows the signals from the integrator and the final gain stage of my surf PI circuit. It can be seen that some small spikes occur and why the output is not constant when there is no metal object?

[DJSUOL]

You mean the output of integrator keeps increasing instead of being saturated to a certain value？

[kt315]

why the DC signal is not blocked by it
---
because we say DC but in reality it is slowly changed AC. SAT always has relation to sound level and does the level going to zero.

[DJSUOL]

Sorry I am a little confused now. For example, in the surf PI design, the DC voltage from the integrator is directly fed into a high-pass filter, why the DC signal is not blocked by it? Is it because that the output of integrator is still an AC signal and the time constant for the high-pass filter is larger than the integrator? In addition, when a target is present, the output of integrator will increase so the change could be fed to the final gain stage. If the metal detector keeps detecting the same target, the output from the integrator will remain steady so the voltage will become the value before. Am I right? Thanks for your explanation.

[DJSUOL]

Yes. I found some designs about SAT but I do not know the detailed background about it. Actually, I even do not know how could it tune the signal out when the coil keeps detecting the same object.

[waltr]

Could you please tell me where I can find the details about this part? I could not find the details to introduce the related circuit design from the Internet

Which Part????
SAT circuit?
Look at many of the common PI detector schematics for the SAT circuit. The Hammer Head is a good one.
You use LTSpice----- Simulate JUST the SAT circuit and inject with a single pulse.

[DJSUOL]

Could you please tell me where I can find the details about this part? I could not find the details to introduce the related circuit design from the Internet

[waltr]

Thanks for your reply. Could you please tell me about that why the coil with a specific inductance such as 400uH is chosen for a better performance? What is the influence of the inductance to the performance?

Trade off between: magnetic field strength & coil decay time.

[DJSUOL]

Thanks for your reply. Could you please tell me about that why the coil with a specific inductance such as 400uH is chosen for a better performance? What is the influence of the inductance to the performance?

[waltr]

Simply it is an AC coupled gain stage. Op-amp with a cap in series to the input.
Any change of Voltage into the cap causes a change in the output Voltage but if the input Voltage does Not change then the output returns to the Voltage before.

[DJSUOL]

I found that the SAT is mentioned once during the PI part and it says that a high-pass filter is used for SAT. But I do not find more details.

[kt315]

it works but only with ITMD into eye. good book with lowest price.

[DJSUOL]

I found that in some PI designs, a high pass filter could be used to implement SAT. How does it work?

[kt315]

searching it online.... but not this site. https://www.geotech1.com/pages/metdet/info/hmdw.html

[Davor]

That's merely a fancy name for a crawling-slow motion mode.