NEW !!! Full differential PI front end.

[moodz]

Hi Moodz,
what about effecet of size of metal to hight of peak.
does the hight of peak change according to metal size.or size just effects width of peak?
Also what about effect of distance ( target to coil) to the hight of peak?

our variables are too much, we have to consider them all .

what do you use after this above circuitry, multiplexer and S&H caps then outer Ato C circuitry ( counter related to chip)?

Hi unreg,
the size and distance to target both effect the height of the peak 1/d squared approx. The baseline to the right (where most designs start sampling) of the peak also increases but not as much as the peak response.
There are targets that do not move the peak at all ... but do move the baseline. If I move the sampling point earlier the peak will decrease for copper targets but wont register ferrous at all.
These are all bench tests ... may not even work effectively on real ground.

My backend processing is fairly conventional .... multipoint sample and amplification fed to CPU.

moodz
I am working on a much faster monocoil ( < 5 uS delay ) and the use of novel amplifier usually employed in HF DSP radio.

[moodz]

This is the ONE. It works. Its differential. I am happy.

[moodz]

scope pics moodzPI differential front end v300a

Green measured at point b
White at point A

obviously there is no amplification here.

There are also more elaborate / different ways to apply the TX drive sigs.

pic 1
NO TARGET
flybacks peak at +100 and -100 volts.
note the convergance of each decay to the baseline.


pic 2
Close up of decay baseline
NO TARGET


pic 3
Close up of decay baseline
Copper target
Note the changed convergence of the +/- decay lines
Just what a diff amp likes
Note how the change is symmetrical


The diff amp section is currently under construction .. stay tuned.

moodz.

[hobbes_lives]

Nice work. That's definitely something I'll be experimenting with at some point.

[Carl-NC]

Yes, now that's differential! I played with this a few months, got almost 1000v diff., and faster settling. Has promise.

[moodz]

Yes, now that's differential! I played with this a few months, got almost 1000v diff., and faster settling. Has promise.

Thanks Carl & Hobbes ...

I did some comparisons to the single ended designs I looked into and I am getting nearly twice the voltage differential as I did with single. Rather nice as the mosfet only sees half of that voltage as it sits on only one of the hot sides. 750 volts approx differential ( ie 350 + on the mosfet with respect to ground. Settling was improved but not by a factor of two. Any improvement is good right ?

Anyway I made some more changes as the previous circuit wasnt so hot ( or rather too hot ) and I blew up a couple of amps. Also got better balance by getting rid of the two damping resistors and replacing with single resistor. compare schem below to one posted previously.

BTW I am not an agent for CA3130 amps ... Its what I have most of.

moodz.

[simonbaker]

Yes, now that's differential! I played with this a few months, got almost 1000v diff., and faster settling. Has promise.

The thing I don't get about these "differential" designs is that it seems to me that both coils will receive the target signal -- so the target signal will essentially become part of the common mode signal and be canceled out by the differential circuitry.

With PI designs, target isolation is normally done by time difference. In VLF designs, target isolation is done by "nulling" the coils.

If this differential design works for PI detectors, then wouldn't we be using it also for VLF designs to avoid all that messing nulling?

(One of the big advantage of PI designs is the ease of making the coils -- no finicky nulling.)

Can anyone explain how this center-tapped coil and differential circuitry avoids canceling out the target signal?

Regards,

-SB

P.S. I'm probably missing the point of the differential circuit - I thought it was to try to cancel out the big TX pulse that gets into the RX circuit.

[porkluvr]

This would be so much more clear if there were dots to indicate proper coil phasing. What is it, the new convention to leave evertbody guessing? Too much trouble, or condescension?

Aww, forget it. Everybody knows what bifilar wound means so phase dots would only be redundancy.

[moodz]

The thing I don't get about these "differential" designs is that it seems to me that both coils will receive the target signal -- so the target signal will essentially become part of the common mode signal and be canceled out by the differential circuitry.

With PI designs, target isolation is normally done by time difference. In VLF designs, target isolation is done by "nulling" the coils.

If this differential design works for PI detectors, then wouldn't we be using it also for VLF designs to avoid all that messing nulling?

(One of the big advantage of PI designs is the ease of making the coils -- no finicky nulling.)

Can anyone explain how this center-tapped coil and differential circuitry avoids canceling out the target signal?

Regards,

-SB

P.S. I'm probably missing the point of the differential circuit - I thought it was to try to cancel out the big TX pulse that gets into the RX circuit.

Weeeeeeeeeeeeeeeell .... how do I explain this.

Simon ... even a single coil is 'differential' it all depends on what your point of reference is for measuring the tx or rx signal. With so called non differential designs one side of the coil is connected to ground or supply and the other side to a switch. Well by connecting one side to ground or supply you have just made your differential coil unbalanced. ie you will have to measure everything with respect to ground or supply.
Actually during TX the design above is unbalanced in a technical sense as the mosfet only drives one coil effectively ... trace DC through circuit with switch ON. However the field induced in the TX side also induces a current on the other side and the circuit balances.

Be aware also that we are not balancing mag fields here ... ie nulling out like a VLF where the coils have to be physically offset.

There is only a single coil of twisted pair used here.
First conductor is called say A - a
Second is called B - b

So when you have wound your coil you will have two ends made of two conductors .... AB and ab

The so called centre tap ( I dont like that term ... there is no 'centre tap' .. it is a single biflar coil ) is really a cross connect between pairs.

ie you take the 'a' wire at one end of your coiled pair and connect it to the 'B' wire at the other end this forms the centre tap in the schematic.

going on with the story ... during recieve you can imagine that the mosfet disappears as it is turned OFF. look at the coils are connected in additive series ie A - aB - b so any induced signal in each coil will add. If they were connected AB - ab the coils would be in parrallel so this would be the same as a single coil ... and if they were connected A -ab - B they would be opposed ... so any induced EMF in one coil would be equal and opposite thus balancing out .. but resulting in no recieved differential signal .. but you would have a common mode signal ( ie same at both ends ).

Lots of systems would not work if the differential circuit did not exist.

eg ethernet, telephone etc etc.

moodz

[simonbaker]

Weeeeeeeeeeeeeeeell .... how do I explain this.

Simon ... even a single coil is 'differential' it all depends on what your point of reference is for measuring the tx or rx signal. With so called non differential designs one side of the coil is connected to ground or supply and the other side to a switch. Well by connecting one side to ground or supply you have just made your differential coil unbalanced. ie you will have to measure everything with respect to ground or supply.
Actually during TX the design above is unbalanced in a technical sense as the mosfet only drives one coil effectively ... trace DC through circuit with switch ON. However the field induced in the TX side also induces a current on the other side and the circuit balances.

Be aware also that we are not balancing mag fields here ... ie nulling out like a VLF where the coils have to be physically offset.

There is only a single coil of twisted pair used here.
First conductor is called say A - a
Second is called B - b

So when you have wound your coil you will have two ends made of two conductors .... AB and ab

The so called centre tap ( I dont like that term ... there is no 'centre tap' .. it is a single biflar coil ) is really a cross connect between pairs.

ie you take the 'a' wire at one end of your coiled pair and connect it to the 'B' wire at the other end this forms the centre tap in the schematic.

going on with the story ... during recieve you can imagine that the mosfet disappears as it is turned OFF. look at the coils are connected in additive series ie A - aB - b so any induced signal in each coil will add. If they were connected AB - ab the coils would be in parrallel so this would be the same as a single coil ... and if they were connected A -ab - B they would be opposed ... so any induced EMF in one coil would be equal and opposite thus balancing out .. but resulting in no recieved differential signal .. but you would have a common mode signal ( ie same at both ends ).

Lots of systems would not work if the differential circuit did not exist.

eg ethernet, telephone etc etc.

moodz

Ok, I was barking up the wrong tree. Would you summarize again what problems are you trying to solve with the differential design?

Regards,

-SB

[Unregistered]

Ok, I was barking up the wrong tree. Would you summarize again what problems are you trying to solve with the differential design?

Regards,

-SB

for me ... noise.

and balancing front amp.

and getting better sensitivity.

and possibly improving decay times.

moodz

[Unregistered]

This would be so much more clear if there were dots to indicate proper coil phasing. What is it, the new convention to leave evertbody guessing? Too much trouble, or condescension?

Aww, forget it. Everybody knows what bifilar wound means so phase dots would only be redundancy.

pork ... u r right if it is bifilar the dots are redundant. The dots used to be there I will have a look at my library symbol.

For reference the dots would be at the capitalised ends ( ie A & B ).

moodz

[porkluvr]

For reference the dots would be at the capitalised ends ( ie A & B ).
moodz

Thanks, Moodz. Sorry about the apparent attitude. I woke up with my head feeling twisted and got off to a bad start. Then I really stepped in it because I meant to go anonymous before making my last post. (Big WHOOPS.)

But, really, it's not only because of your scat, my angst has been building - I get out of sorts when people have multi-winding coils; where it would be sooooooo simple to eliminate ambiguity by simply employing phase dots - instead of assuming the reader knows which way the multiple windings should go. But, NOOOO.

I feel like the Lone Ranger because it's like nobody, but NOBODY knows what a phase dot is, anyway. (Somebody, please help me. Where's a doctor - I think I may need... a LOBOTOMY.)

I've got it. The two windings are continuous * from top to bottom. I know you want to concentrate on less mundane issues than coil phasing... even when phasing is at the heart of the issue... .

(I'll be alright, tomorrow.)

edit:* I had it wrong. My original post stated the windings were anti-phase until I re-read what you wrote and made a change. You don't have to, but please, please find library parts with dots and use them. Maybe it'll catch on, but I won't hold my breath. I wouldn't have made a good RF engineer since I can't even properly phase a bifilar coil just because... .

[moodz]

Thanks, Moodz. Sorry about the apparent attitude. I woke up with my head feeling twisted and got off to a bad start. Then I really stepped in it because I meant to go anonymous before making my last post. (Big WHOOPS.)

But, really, it's not only because of your scat, my angst has been building - I get out of sorts when people have multi-winding coils; where it would be sooooooo simple to eliminate ambiguity by simply employing phase dots - instead of assuming the reader knows which way the multiple windings should go. But, NOOOO.

I feel like the Lone Ranger because it's like nobody, but NOBODY knows what a phase dot is, anyway. (Somebody, please help me. Where's a doctor - I think I may need... a LOBOTOMY.)

I've got it. The two windings are continuous * from top to bottom. I know you want to concentrate on less mundane issues than coil phasing... even when phasing is at the heart of the issue... .

(I'll be alright, tomorrow.)

edit:* I had it wrong. My original post stated the windings were anti-phase until I re-read what you wrote and made a change. You don't have to, but please, please find library parts with dots and use them. Maybe it'll catch on, but I won't hold my breath. I wouldn't have made a good RF engineer since I can't even properly phase a bifilar coil just because... .

... fair cop .... Redrawn with dots and hopefully clearer .... cheers from moods

[hobbes_lives]

Hi moodz,

What is the purpose of IC2?

It looks like your balanced design is getting better each time.