instability in the wave tail of a PI detector

[Hesamavr]

I'm designing a PI detector.

I have a problem with the decay curve which relates to its instability. it oscillates like a thread in the breeze. The circuit is not something special. like other PI detectors with 390R damping resistor.

First I thought it might be because of the damping. So I decrease it but it did not solve the problem. I attached a video of the waveform. The DSO voltage has been adjusted to 200mV to make the instability more obvious and the wave tail is clear on the screen.

Video:

http://uupload.ir/view/clmu_video_(2).mp4/

[Qiaozhi]

I'm designing a PI detector.

I have a problem with the decay curve which relates to its instability. it oscillates like a thread in the breeze. The circuit is not something special. like other PI detectors with 390R damping resistor.

First I thought it might be because of the damping. So I decrease it but it did not solve the problem. I attached a video of the waveform. The DSO voltage has been adjusted to 200mV to make the instability more obvious and the wave tail is clear on the screen.

Video:

http://uupload.ir/view/clmu_video_(2).mp4/

Firstly, it looks like your coil is underdamped.
Secondly, try rotating the coil so that it's parallel with the ground, and not perpendicular. The noise in the waveform may be externally generated.
PIs can be quite noisy when tested indoors.

[Hesamavr]

Firstly, it looks like your coil is underdamped.
Secondly, try rotating the coil so that it's parallel with the ground, and not perpendicular. The noise in the waveform may be externally generated.
PIs can be quite noisy when tested indoors.

You mean I should increase the damping coil? As I know if we see a ringing effect, we must decrease it. correct me if I am wrong. The high frequency indoor or supply noise is understandable, but what about that instability?
below is the circuit. There are two resistors which both are 390R

[Carl-NC]

Doesn't look all that bad to me, especially if this is indoors on a work bench. Demodulation and a bunch of integration will help a lot.

[pustareka]

Держи !


nastroika R5
http://www.geotech1.com/forums/attac...9&d=1527156368

[Qiaozhi]

You mean I should increase the damping coil? As I know if we see a ringing effect, we must decrease it. correct me if I am wrong. The high frequency indoor or supply noise is understandable, but what about that instability?
below is the circuit. There are two resistors which both are 390R

[ATTACH]44499[/ATTACH]

What are the details of your coil? Inductance and resistance?

I've just noticed that you have your coil connected to the MOSFET source instead of the drain.
Have a look at Geotech Baracuda REV-A as an example. This is no doubt the cause of your problem.

[Andi68]

The diode at the gate of the Mosfet has to be turned!

[Hesamavr]

What are the details of your coil? Inductance and resistance?

1.7R and 380uH. Spider coil. 20cmm diameter

[Qiaozhi]

1.7R and 380uH. Spider coil. 20cmm diameter

I've just edited my post (#6) while you were replying.

[waltr]

I've just noticed that you have your coil connected to the MOSFET source instead of the drain.
Have a look at Geotech Baracuda REV-A as an example. This is no doubt the cause of your problem.

Good catch.
Yes, this could be the problem. The MOSFET turn on/off is the Gate Voltage relative to the Source.
So the Source should be held at a constant potential.

[Hesamavr]

Mannnn. this is a P channel Mosfet and the load should be connected to source.

[Qiaozhi]

Mannnn. this is a P channel Mosfet and the load should be connected to source.

Wrong. The load should always be connected to the drain.
As waltr says, the source needs to be held at a constant potential.
Look at the Geotech Baracuda for an example (attached), and chapter 12 of ITMD.

[pustareka]

Mannnn. this is a P channel Mosfet and the load should be connected to source.

Учи матчасть !! Lern elektronik !!

http://www.geotech1.com/forums/newre...reply&p=250606

The diode at the gate of the Mosfet has to be turned!

andy68 nein, uskorava vikljucenie (spid up turn of time P-MOSFET) !

[Hesamavr]

Wrong. The load should always be connected to the drain.
As waltr says, the source needs to be held at a constant potential.
Look at the Geotech Baracuda for an example (attached), and chapter 12 of ITMD.

I'll check your suggestion. Thanks.

[Hesamavr]

Учи матчасть !! Lern elektronik !!

http://www.geotech1.com/forums/newre...reply&p=250606





andy68 nein, uskorava vikljucenie (spid up turn of time P-MOSFET) !

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