NEW !!! Full differential PI front end.

[moodz]

Hi moodz,

What is the purpose of IC2?

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

Hi hobbes ... IC2 is a voltage follower .. you probably figured that .... it isolates the relatively low impedance input on the inverting leg of the diff amp.

We dont want to draw any significant AC current through those caps or there will be balance and freq response problems.

BTW there are much better circuits that could be used here ... the o/p from the diff amp is single ended and pretty much the same as you would get from a traditional design. You could easily bolt the output of the diff amp onto many existing PI designs.

cheers ... moodz.

[moodz]

Differential coil pics ...

You can see the twisted pair in the coil binding.

Centre connection is NOT a centre tap.

moodz.

[moodz]

First amp results.

First amp results for diff circuit.

pic 1 is the pulse output from a very ordinary diff amp.

gain is wired for 1000.

... looks quiet.



Well would you look at that ... there is absolutely NO way I could look at the output of my first amp at 10mV / div on my single sided PI front end.

I have a wireless mouse and keyboard and a compact flouro desk lamp 20 cm from the prototyping area. With my single sided design I had to turn these off in order to take measurements at much greater volts / div.




looks very promising.

I am too tired to go through target shots but I have found I have to use smaller test targets now because my 'standard' ones rail it !

I have changed the circuit previously published. The opamp config has changed and I dropped the coupling caps.

moodz.

[simonbaker]

Regarding the "bifilar coil", I'm wondering if there is a subtle issue.

First, for me, center-tap is very appropriate description because with flat coils especially, the physical layout of the wires is normally not a big deal with regard to the basic coil parameters. So if it's scramble-wound, bi-filar wound, or incremental wound, the topology is very similar -- by sorting through the windings you could convert one coil type to the other without rewinding.

Having said that, I'm wondering if there is an important subtle difference, specificially capacitance. With bifilar wound coils (connected like yours), I think the voltage between adjacent windings is about half the voltage across the coil. With incremental wound coils, each adjacent winding is at a much closer voltage, hardly any difference at all. Would that not effectively make the bifilar wound coil have more capacitance than the incrementally wound?

Or does somehow it all work out the same?...

Regards,

-SB

[moodz]

Regarding the "bifilar coil", I'm wondering if there is a subtle issue.

First, for me, center-tap is very appropriate description because with flat coils especially, the physical layout of the wires is normally not a big deal with regard to the basic coil parameters. So if it's scramble-wound, bi-filar wound, or incremental wound, the topology is very similar -- by sorting through the windings you could convert one coil type to the other without rewinding.

Having said that, I'm wondering if there is an important subtle difference, specificially capacitance. With bifilar wound coils (connected like yours), I think the voltage between adjacent windings is about half the voltage across the coil. With incremental wound coils, each adjacent winding is at a much closer voltage, hardly any difference at all. Would that not effectively make the bifilar wound coil have more capacitance than the incrementally wound?

Or does somehow it all work out the same?...

Regards,

-SB

Hi Simon ... you have a point ... I have an RF background and in that space coil geometry is very critical. A center tapped coil and a bifilar coil are different beasts. Your point about the capacitance and connection options are also valid .... The sort of frequencies we are working with here would certainly be seen in an RF engineering environment. ( though we would probably be trying to suppress these type of pulses not amplify them )

The same configuration could physically be produced with two separate single wound coils ... I have not tried it. The twisted pair would be more transmission like in nature due to closer coupling and distributed capacitance.

moodz.

[moodz]

Hi Simon ... you have a point ... I have an RF background and in that space coil geometry is very critical. A center tapped coil and a bifilar coil are different beasts. Your point about the capacitance and connection options are also valid .... The sort of frequencies we are working with here would certainly be seen in an RF engineering environment. ( though we would probably be trying to suppress these type of pulses not amplify them )

The same configuration could physically be produced with two separate single wound coils ... I have not tried it. The twisted pair would be more transmission like in nature due to closer coupling and distributed capacitance.

moodz.

mmm ... I did a spice model and get the same result but no modelling of transmission lines etc ... just the mag coupling ... so maybe two coils connected the same way would work also... dunno have to try it.
moodz

[simonbaker]

Hi Simon ... you have a point ... I have an RF background and in that space coil geometry is very critical. A center tapped coil and a bifilar coil are different beasts. Your point about the capacitance and connection options are also valid .... The sort of frequencies we are working with here would certainly be seen in an RF engineering environment. ( though we would probably be trying to suppress these type of pulses not amplify them )

The same configuration could physically be produced with two separate single wound coils ... I have not tried it. The twisted pair would be more transmission like in nature due to closer coupling and distributed capacitance.

moodz.

I mentioned it because it seems people are trying to reduce the capacitance of their TX coils. (I'm behind on this, trying to catch up...)

Regards,

-SB

[Unregistered]

I mentioned it because it seems people are trying to reduce the capacitance of their TX coils. (I'm behind on this, trying to catch up...)

Regards,

-SB

simon ... fair call ...

My decay time has not improved significantly .... in fact it is pretty bad on the current results. However I feel that the sensitivity and noise is improved. This was my main goal ... I am not interested in developing some sort of new coil configuration for the sake of it.

regards Paul ( moodz )

[vasster]

First amp results for diff circuit.

pic 1 is the pulse output from a very ordinary diff amp.

gain is wired for 1000.

... looks quiet.



Well would you look at that ... there is absolutely NO way I could look at the output of my first amp at 10mV / div on my single sided PI front end.

I have a wireless mouse and keyboard and a compact flouro desk lamp 20 cm from the prototyping area. With my single sided design I had to turn these off in order to take measurements at much greater volts / div.




looks very promising.

I am too tired to go through target shots but I have found I have to use smaller test targets now because my 'standard' ones rail it !

I have changed the circuit previously published. The opamp config has changed and I dropped the coupling caps.

moodz.

Hi moodz, your work is novel. I'm waiting to see the changes you suggest.
I believe that this is going to be one of the major advances in the art of the PI field.

[moodz]

Thanks vasster ... I am very happy with results so far.

moodz.

[vasster]

'I have changed the circuit previously published. The opamp config has changed and I dropped the coupling caps.'

That means you dropped the caps C1, C3 and possibly IC2?
Can you please post your final circuit?

[moodz]

'I have changed the circuit previously published. The opamp config has changed and I dropped the coupling caps.'

That means you dropped the caps C1, C3 and possibly IC2?
Can you please post your final circuit?

Hi Vasster ... I am currently making daily .. ( nearly hourly ) changes to the circuit ... at the backend. So I will be publishing a field trial ready circuit when I get the following changes somewhere towards completion ...

This is list of changes.

dPI (differential PI ) build status

1. Amp front end ( completed )

2. Remove all switching supplies ... everything runs from single DC supply anywhere between 8 and 40 volts. Actually I have not tried it at 40 volts ... have gone as far as 16 volts .... generating 800 volts differential on coil.

3. Reduced damping resistor to 500 ohms due to slight ringing of coil at 1000 ohms. ( completed )

4. Improved MOSFET drive. ( completed )

5. dsPIC code to sample and process front end amp o/p ( 90 % )

6. Layout PCB ( 10 % )

7. Source coil form, wand and electronics housing ( 0 % )
Does anyone know where I can source coil housing 'blanks' anywhere ??

8. Field trials ( 0 % )

Regards,

moodz

[vasster]

http://www.hayselectronics.com/parts.htm

[Unregistered]

So I will be publishing a field trial ready circuit when I get the following changes somewhere towards completion ...

Really will you share your work here or go commercial like Bugwhiskers?

[Unregistered]

Really will you share your work here or go commercial like Bugwhiskers?

If you have something as good as he has and you have spent a a lot of time and money why shouldn't he go commercial and get something back for his efforts?
I think he has shared a lot already don't you?
And there are those that don't want it to go commercial for obvious reasons!!!
I think that he and moodz should get together and each use the advantages of each others technology.