field test unit no 001 "model T"

[mickstv]

Mick ... good pickup the circuit .... I use now is different but for reasons other than blocking .... all will be revealed in the next schematic.

PS whilst I have been waiting for the post demod ideas about the DC amp to crystallise ... I have been retesting the timing and descrimination response for different targets and made a major discovery ..... the descrimination is dependant on the timing and balance point .....in fact there are optimum timings and balance points where this occurs ... this has other implications which are a bit hard to describe here ( and probably should be described in a patent )

moodz.

That is starting to sound like you might need the frontend to be auto adjusting and a set of timing options for different target types maybe ?



Mick

[mickstv]

Hi Guys,

I knocked up my own CAT5 based coil on the weekend on the weekend and have assembled the very bare essentials of the coil drive with just a pot for the discrimination balance. Although its all pretty rough as guts, lots of noise, and the sensitivity didn't seem very good. I did at least get to see the discrimination Moodz has been demonstrating for myself. So yeah just in case you were starting worry Moodz, its not an effect entirely confined to your lab The 3 pictures in order are, side of my computer, large chunk of aluminum, no target. Both channels effectively 2v/div (10x\100x probes)

Hi Midas, good to see the positive results. I've decided my problem it's some sort of strange issue with the analog CRO setup, so I might go and have a look on EBay for a digital replacement.

Have you tested the frontend with any other targets like say some PCB board/copper, lead and steel pipe. Just wondering what sort of results you get.


Mick

[Midas]

A few pretty pictures..

Hi Guys,

I knocked up my own CAT5 based coil on the weekend on the weekend and have assembled the very bare essentials of the coil drive with just a pot for the discrimination balance. Although its all pretty rough as guts, lots of noise, and the sensitivity didn't seem very good. I did at least get to see the discrimination Moodz has been demonstrating for myself. So yeah just in case you were starting worry Moodz, its not an effect entirely confined to your lab The 3 pictures in order are, side of my computer, large chunk of aluminum, no target. Both channels effectively 2v/div (10x\100x probes)

[moodz]

Moodz, the picture of the NP fet you put together it looks like you have pin8 tied to pin3. That can't be right because Pin8 is D1 and pin3 is S2. Or am I missing something.



Mick

Mick ... good pickup the circuit .... I use now is different but for reasons other than blocking .... all will be revealed in the next schematic.

PS whilst I have been waiting for the post demod ideas about the DC amp to crystallise ... I have been retesting the timing and descrimination response for different targets and made a major discovery ..... the descrimination is dependant on the timing and balance point .....in fact there are optimum timings and balance points where this occurs ... this has other implications which are a bit hard to describe here ( and probably should be described in a patent )

moodz.

[moodz]

I believe no one has commented yet, but the copper fill on the PCB should be controlled. It should be connected to some potential, perhaps using separate guards for the different areas if necessary. Floating copper fills only act to conduct disturbances between traces instead of shorting them to the connected potential - usually ground.

If a minimal capacitance is wanted, the fills should be left out entirely.

Just a couple cents, this is more in the general electronics area but it might result in noise control improvement for this interesting project.

Good point ODM ... I have not actually got to the point of laying out a board yet because the circuit keeps changing ... but you are right this will have to be a consideration with the layout particularly if surface mount is used and closer spacing of pins and traces etc.

moodz.

[mickstv]

Mick .. before I get onto the amp .. I just wanted to make sure that NP FET pack you are using was OK ... I had not actually tried it when I recommended it to you .. only went off specs .....

The result is PERFECT ... works better than the discretes.

Tick that one off ...

I mounted the chip on a little SOIC board with some "pins" that go into the sockets where the blocking fets plug into.

The demod outputs in the pic below are close to, but not balanced ... the descrim balance control setting is 2619.

moodz.

[ATTACH]17290[/ATTACH]
[ATTACH]17291[/ATTACH]

Moodz, the picture of the NP fet you put together it looks like you have pin8 tied to pin3. That can't be right because Pin8 is D1 and pin3 is S2. Or am I missing something.



Mick

[ODM]

I believe no one has commented yet, but the copper fill on the PCB should be controlled. It should be connected to some potential, perhaps using separate guards for the different areas if necessary. Floating copper fills only act to conduct disturbances between traces instead of shorting them to the connected potential - usually ground.

If a minimal capacitance is wanted, the fills should be left out entirely.

Just a couple cents, this is more in the general electronics area but it might result in noise control improvement for this interesting project.

[mickstv]

Unfortunately no .. ferrite responds as iron ... but weaker ...... you can still balance it out though. Playing with the timings and balance may do something ... dont have time at the moment ....

moodz.

If ferrite responds like iron I can't see that as being a problem. So we'll call it cured, but we wont really know what is going to happen until we have a few units out testing.



Mick

[moodz]

Sounds good. I'll put that resistor in as well and see what happens. Is the frontend still immune to ferrite ?



Mick

Unfortunately no .. ferrite responds as iron ... but weaker ...... you can still balance it out though. Playing with the timings and balance may do something ... dont have time at the moment ....

moodz.

[mickstv]

Sounds good. I'll put that resistor in as well and see what happens. Is the frontend still immune to ferrite ?



Mick

[moodz]

... I re-tested at 50 us and 100 us Tx pulse with excellent and consistant descrim results ....

moodz.

[moodz]

Hi Paul, I'll bung another in tomorrow and see what happens. What was/is the mod for the descrim so I can fit that as well.


Mick

Mick ... add a 1K resistor in parallel with the damping fet like so ....

moodz.

[mickstv]

Mick .. before I get onto the amp .. I just wanted to make sure that NP FET pack you are using was OK ... I had not actually tried it when I recommended it to you .. only went off specs .....

The result is PERFECT ... works better than the discretes.

Tick that one off ...

I mounted the chip on a little SOIC board with some "pins" that go into the sockets where the blocking fets plug into.

The demod outputs in the pic below are close to, but not balanced ... the descrim balance control setting is 2619.

moodz.

[ATTACH]17290[/ATTACH]
[ATTACH]17291[/ATTACH]

Hi Paul, I'll bung another in tomorrow and see what happens. What was/is the mod for the descrim so I can fit that as well.


Mick

[moodz]

Mick .. before I get onto the amp .. I just wanted to make sure that NP FET pack you are using was OK ... I had not actually tried it when I recommended it to you .. only went off specs .....

The result is PERFECT ... works better than the discretes.

Tick that one off ...

I mounted the chip on a little SOIC board with some "pins" that go into the sockets where the blocking fets plug into.

The demod outputs in the pic below are close to, but not balanced ... the descrim balance control setting is 2619.

moodz.

[mickstv]

Mick ... I have been able to reliably recreate the fault and a fix is in place ( very simple ). I will do some more testing and include in next drawing. The problem was to do with the damping mosfet latching up during turn on and never being within control range of the damping control voltage It also causes the flyback voltage to reach excessive voltages which can damage the blocking fets.

moodz

[ATTACH]17285[/ATTACH]
[ATTACH]17286[/ATTACH]

Sounds good cant wait to try it out. I've got 6 blocking fets left so hopefully I won't nuke the rest.



Mick