NEW !!! Full differential PI front end.

[moodz]

Well .... will the differential coil ever deliver practical results ? ... a chance remark by Simon in an earlier post has sparked ( no pun intended ) the latest development on the differential circuit. What would you say about a 3 uSec decay with very ordinary components on a standard ( well a standard differential ) coil made with AWG 22 copper magnet wire ... no fancy teflon here ... though I have been pricing it on ebay.

Pic 1 ..... 3 us decay .... about 500 volts p/p Tx pulse is 100 uSec 12 volt supply ... this coil before the circuit mod was running at best 12 uSec.
Hmmm I wonder if we will actually be able to sample at 3 uSec at the base of those pulses ???



Pic 2 ... whew ! look at that first sample point ... no problemo. Scope set to 5 volts per division.



moodz

[moodz]

I need to get an LTSpice file for that circuit to observe it better and to make sure we're talking about the same circuit. Can you post or link to one for reference?

To make sure I understand -- are you saying that during charging, equal current is flowing in both coils at all times?

Regards,

-SB

Hi Simon .... no I mean equal flux ... the flux is the 'charge' similar to the electric field in a cap ... as the cap charges the field between the plates gets stronger ... so by analogy as the current gets greater during the TX pulse the flux gets stronger ...

however ... check out my next post .... I wanted to check your commentary about capacitive inrush current ... look what the coil response decay does now.

Regarding the circuit ... have a look at the last one I published it is the one I am working from ... except for the very latest results just mentioned.

Regards,

moodz

[simonbaker]

Ok ... think of it this way.

Voltage is the conjugate of Current.
Inductance is the conjugate of Capacitance.

When a pulse first hits a capacitance it appears as short circuit / Inductance appears as open circuit.

Maximum voltage is at time=0 for inductance time=max for capacitance.
Maximum current is at time=max for inductance time=0 for capacitance.

Inductance stores energy as mag field ( flux ) ... capacitance as electric field ( charge )

Many people associate charging with capacitance ... however you also 'charge' an inductance by storing a flux. If the inductance is a closed loop and is superconducting the flux will remain constant and continuous. ( plenty of demos of this on youtube )

Intuitively because of this conjugate relationship there is an analogous configuration for capacitance as for inductors.

So if we charge two caps in parallel then connect them in series we get twice the voltage and max current at time=0 where R source = R load.

If we 'charge' two coils in parallel then connect them in series we get twice the voltage and max current at time = max where R source = R load.

The critical factor is that because the flux is 'charging' both coils there is no sudden flywheel coupling ... that flux energy build up due to Ldi/dt is equally cutting both coils.

moodz.

I need to get an LTSpice file for that circuit to observe it better and to make sure we're talking about the same circuit. Can you post or link to one for reference?

To make sure I understand -- are you saying that during charging, equal current is flowing in both coils at all times?

Regards,

-SB

[moodz]

here is quite a nice lecture ...

http://www.wtc.wat.edu.pl/dydaktyka/fizyka-wykZielinski/lecture1.ppt

[moodz]

Ok ... think of it this way.

Voltage is the conjugate of Current.
Inductance is the conjugate of Capacitance.

When a pulse first hits a capacitance it appears as short circuit / Inductance appears as open circuit.

Maximum voltage is at time=0 for inductance time=max for capacitance.
Maximum current is at time=max for inductance time=0 for capacitance.

Inductance stores energy as mag field ( flux ) ... capacitance as electric field ( charge )

Many people associate charging with capacitance ... however you also 'charge' an inductance by storing a flux. If the inductance is a closed loop and is superconducting the flux will remain constant and continuous. ( plenty of demos of this on youtube )

Intuitively because of this conjugate relationship there is an analogous configuration for capacitance as for inductors.

So if we charge two caps in parallel then connect them in series we get twice the voltage and max current at time=0 where R source = R load.

If we 'charge' two coils in parallel then connect them in series we get twice the voltage and max current at time = max where R source = R load.

The critical factor is that because the flux is 'charging' both coils there is no sudden flywheel coupling ... that flux energy build up due to Ldi/dt is equally cutting both coils.

moodz.

[simonbaker]

Simon with the first statement you got to remember that the mag flux built up by coil 1 during tx cuts through coil 1 and 2 equally. When Tx turns off only the changing mag flux can cause any current and it causes an equal current in coil 1 and 2. ( Has too if you think about it because there is only one current through the damping resistor ) Coil 2 does not amplify the magnetic field ... physics 101 conservation of energy rule. But what it does do though is you get twice the voltage. ie Flux from single coil 1 is built up during TX however flux decays through coil 1 and 2 in series during RX
... hence twice the voltage... but not twice the energy.

moodz

I think it is not so simple to account for current because of parasitic capacitance. At switch-off, I believe there is a large amount of current diverted momentarily to parasitic capacitance that does not pass through second coil right away. Coil 1 already has current in it, while Coil 2 needs time to build up it's current. I refer to "coupled flywheel" analogy from previous message. Eventually, current through coils becomes about the same, but on a very short time scale, the currents are not identical at first (I think).

I agree twice the voltage -- and we agree target does not see voltage, only magnetic field. So to me, advantage of differential design is maybe faster turn-off (because of higher voltage), but not increased mag field (I think -- to be proved). But faster di/dt could help with small targets.

That's what I think so far, still pondering this. I haven't had time to run Spice sims to dig deeper. Good topic for discussion, much appreciated.

Cheers,

-SB

[moodz]

Hey, good questions! Especially last one for center-tapped coils.... I need to ramble a little...

blah blah (edited )

I am guessing that with a tapped coil, when you turn off the current to coil1, the voltage spike arises and coil2 starts building current while coil1 starts decreasing current, with capacitance absorbing the bulk of the current from coil1 until coil2 is up to speed.

So the magnetic field does not instantaneously double like I was considering before. In fact, I am now sceptical that coil2 amplifies the magnetic field as far as the target is concerned.

blah blah (edited )

Decorum prevents me from blabbing further in this message.

Cheers,

-SB

Simon with the first statement you got to remember that the mag flux built up by coil 1 during tx cuts through coil 1 and 2 equally. When Tx turns off only the changing mag flux can cause any current and it causes an equal current in coil 1 and 2. ( Has too if you think about it because there is only one current through the damping resistor ) Coil 2 does not amplify the magnetic field ... physics 101 conservation of energy rule. But what it does do though is you get twice the voltage. ie Flux from single coil 1 is built up during TX however flux decays through coil 1 and 2 in series during RX
... hence twice the voltage... but not twice the energy.

moodz

[moodz]

.... now I am getting confused.

Ok ... how about we model in spice a primary of N turns coupled to 2N turns secondary. This could be done with a Trifilar coil. One filar is the tx coil and the other two are cross connected so they end up in series as a 2N secondary .... hey that sounds like a really good idea to try for real .... this may provide more clues.
I will tentatively predict that the trifilar coil will produce identical results ( except for the DC bias ) as the bifilar as all we are doing is splitting out the TX and RX functions.
Then if that works try an N filar coil remembering that the damping resistor will have to be R x N where R is for one coil.

moodz

[simonbaker]

Simonbaker & moods,

the idea of the fully differential front end is intriguing. Experimenting only with the differential input preamp already got my head spinning. Things are just not the same anymore.

About stimulating the target:
What counts are ampere turns. Trade turns for amperes or the other way around, it is still the same.
Now, when we have a finite amount of energy stored in the inductor, we need to apply this energy in a way that is most efficient in stimulating the target.

Simonbaker has compared the target with a bell before.

What makes the bell ring better, a very short, hard blow, or a softer longer blow? (considering the same amount of energy)
I think it will depend on the size of the bell. Large bells ring at a lower frequency (TC?) The ideal bell stroke has to be adjusted to that frequency.

Now, what happens when you hit the large bell in quick succession twice?
You kill the ringing.

Analyse the Flyback. Does it not go in one direction first and then in the other direction? Which way stimulates and which way kills the eddy currents?

Tinkerer

Hey, good questions! Especially last one for center-tapped coils.... I need to ramble a little...

1. First, need to revisit what happens when turn off TX pulse to a center-tapped coil.

I became convinced (previously) that because current cannot change instantaneously, that the second half of coil would instantaneously get all the current diverted to it.

But wait a minute! My argument should also apply to the second half of the coil -- it's current cannot change instantaneously either!

What we have is the equivalent of an irrestistable force meeting an immovable object. The theoretical solution is tricky because we need to obey energy conservation. Can we suddenly have twice the magnetic field? Or must somehow the current reduce to half when the second coil becomes "energized" so the turn-amperes stay the same? (This is probably a freshman physics problem that is obvious to freshmen).

It is equivalent to suddenly coupling a spinning flywheel to a non-spinning flywheel -- what happens (besides the gear box turning to dust)?

The real world answer is that something has to give. With flywheels, some mechanical part becomes elastic to absorb the otherwise infinite force spike that would occur. In our circuit, capacitance takes up the slack. But with flywheels, we know in the end they must share the original energy of the first wheel.

I am guessing that with a tapped coil, when you turn off the current to coil1, the voltage spike arises and coil2 starts building current while coil1 starts decreasing current, with capacitance absorbing the bulk of the current from coil1 until coil2 is up to speed.

So the magnetic field does not instantaneously double like I was considering before. In fact, I am now sceptical that coil2 amplifies the magnetic field as far as the target is concerned.

As usual, need to do the equations and simulations to get exact answer (Aziz probably already knows well).

2. Now to question related to Tinkerer's (double-strike) question -- fundamental difference between behavior of center-tapped coil and simple coil during TX turn-off.

This question depends on understanding of part (1) above. Does magnetic field suddenly grow before it declines, or is it a steady decline?

I think we agree for a simple (non-center-tapped) coil the magnetic field is a pure decline, giving some kind of di/dt pulse that does not cross zero.

But for center-tapped, if the field actually grows before it shrinks, then the di/dt changes polarity at some point, essentially giving two pulses of opposite polarity. Well, does it or doesn't it? Need answer to part (1).

If it does change polarity, is that good or bad? Sounds bad to me, sort of self-canceling, but that would depend on natural frequency of the "bell" you are ringing.

3. More specific to Tinkerer's double pulse question: is this an important reason to prevent MOSFET breakdown, which I think can cause a double di/dt pulse? I do still believe in the "bell" analogy, that for some specific targets a double pulse can be a good thing, where for others maybe bad, depending on natural frequency.

But we need to be careful about "bell" analogy because I think probably that most targets are underdamped, so they are more like a pendulum dragging in water.

However, natural response of target is still important to how it reacts to pulses of different duration (but same energy). As long as pulses are fairly short compared to time constant, I think they are all fairly equivalent. When longer compared to time constant, we lose efficiency. A double pulse is similar to a longer pulse in this regard, and may not be much different from a single pulse of same total energy and approximate duration.

Decorum prevents me from blabbing further in this message.

Cheers,

-SB

[moodz]

Hi Paul,

I made a quick & dirty balanced coil drive and had a look into the flyback voltages (no differential frontend yet).
The differential coil damping needs to be splitted or compensated to the common (ground) due to the inbalance of the coil halves due to the production tolerances (coil to coil capacitance, coil capacitances, inductivities). Particularly, the coil capacitances may differ and needs individual coil damping.

So I can confirm, what Carl has already found this earlier (month ago).

There will be slightly inductivity inbalance of the coils too. So probably a mixture of compensation adjustment of damping resistors and capacitors would be convenient.

Regards,
Aziz

Hi Aziz ...

hmmm ... I get very good balance ... my coil is very rough .... at the limits of resolution of CRO. My first diff amp has a gain thousand plus ... and can produce a 350 mv response for 5 cent piece at centre of coil ( ie very small target ). .... as per my analysis for simon the recieve phase is not a split coil .... it is one coil of 2N turns ... it does not matter for receive about coil balance etc. It is the diff amp which must be correctly matched to the coil and DC isolated by caps .... I found out through much pain thinking damping was incorrect when all the time the diff amp input impedance was wrong. The philosphy of the front end is for the coil to feed the amp differentially not single ended . ie coil is not referenced to ground during recieve .... the test bed results are now so good I am constructing the field unit for ground tests.

moodz

[Aziz]

Hi Paul,

I made a quick & dirty balanced coil drive and had a look into the flyback voltages (no differential frontend yet).
The differential coil damping needs to be splitted or compensated to the common (ground) due to the inbalance of the coil halves due to the production tolerances (coil to coil capacitance, coil capacitances, inductivities). Particularly, the coil capacitances may differ and needs individual coil damping.

So I can confirm, what Carl has already found this earlier (month ago).

There will be slightly inductivity inbalance of the coils too. So probably a mixture of compensation adjustment of damping resistors and capacitors would be convenient.

Regards,
Aziz

[Tinkerer]

Simonbaker & moods,

the idea of the fully differential front end is intriguing. Experimenting only with the differential input preamp already got my head spinning. Things are just not the same anymore.

About stimulating the target:
What counts are ampere turns. Trade turns for amperes or the other way around, it is still the same.
Now, when we have a finite amount of energy stored in the inductor, we need to apply this energy in a way that is most efficient in stimulating the target.

Simonbaker has compared the target with a bell before.

What makes the bell ring better, a very short, hard blow, or a softer longer blow? (considering the same amount of energy)
I think it will depend on the size of the bell. Large bells ring at a lower frequency (TC?) The ideal bell stroke has to be adjusted to that frequency.

Now, what happens when you hit the large bell in quick succession twice?
You kill the ringing.

Analyse the Flyback. Does it not go in one direction first and then in the other direction? Which way stimulates and which way kills the eddy currents?

Tinkerer

[simonbaker]

Hi Simon ... at the risk of kicking off a bun fight .. here goes.

1. The coil is an N turn bifilar wound coil. When the the mosfet turns on the current starts increasing in the coil at a rate Ldi/dt depending on the inductance and peaks if the mosfet is on long enough at a value that is only dependant on the resistance of the coil , the feed wire and mosfet ON resistance.This current only flows in one of the bifilar conductors ( there is a small current due to the damping resistance across the other bifilar conductor )

Proposition 1. During TX the differential coil behaves the same as a 'standard' mono coil of N turns.

2. When the mosfet turns OFF however the magnet flux collapses through BOTH coils making up the bifilar coil and because of the series connection of the two bifilar conductors the damping resistor sees a 2N coil. ( twice the number of windings ). The voltage is also doubled as the flux to windings for the same current is directly proportional. The mosfet however is only reference to the centre connection to the series connected coil so it only sees half of the fly back voltage.

I agree with that, but it doesn't say anything about what the target sees. The target only sees the magnetic field from N turns. If we put the current through 2N turns (non-differential design), we'd have twice the magnetic field if we had the same current. Now, the resistance of 2N turns is twice that of N turns, so maybe we have half the current. If we accept the premise that our TX pulse is long enough and our battery strong enough to reach steady state, then I agree the differential design seems advantageous. If we cannot achieve twice the current through the N turns compared to 2N turns, then the advantage seems to be lost. (Well... see below as I begin to maybe see your point...)

Proposition 2. The peak current in a differential coil at mosfet turn off is the same as a mono coil of N turns however the peak fly back flux is twice as great because the current in the differential coil flows through 2N turns. Target is stimulated with twice the flux at peak flyback.

That one I'm not sure about and had not considered. Maybe that is what I was missing. It implies that during TX pulse turn-off, the magnetic field actually grows before it diminishes. In other words, although the current starts decreasing in the first N turns, the second N turns has current start flowing so that the total current * turns increases before it decreases. I guess I need to study that.

3. When the fly back starts to decay the diodes ( now located at coil ) connected to mosfet and battery isolate the coil as they reverse bias and the target response is picked up by a 2N coil.

Yes that's good, but no better than a 2N TX coil.

Proposition 3. The target is stimulated by a back EMF flux that is twice that of an N turn monocoil and the target eddy current flux is picked up by a coil that is twice the number of turns of a standard monocoil. Two by two is four ....

That is the crux I have to accept, that the magnetic field doubles the instant the current is turned off. I guess it makes sense if the "flyback voltage" drives the current (which does not change instantaneously) through the second set of N windings.

The short answer is that the 'charging' flux ie the 100 odd uS MOSFET ON is the SAME as a monocoil.However when the mosfet turns off the flyback flux is double. The RX coil response is double.
Regards,

moodz.

Ok, thanks for going over that for me.

Now -- assuming the above is correct, what is the optimum ratio for the "center tap"? Should we have N:3N or N:10N?

Regards,

-SB

[moodz]

I think this has been asked before, but not sure...

With differential design, we put current through half the coil, but use whole coil to detect target signal. Advantage is less flyback voltage across MOSFET, right?

But.... what about magnetic field? Is it half of that with non-differential coil? Or do we try to pump twice the current into it?

What I'm asking is: is our magnetic field weaker with differential design, so we get less target stimulus?

You might say: yes it is weaker, but with non-differential design it was wasted anyway because MOSFET breakdown.

Reply to that is -- use slower cut-off to avoid MOSFET breakdown, and we'll get longer (therefore stronger) target stimulus -- at least for targets that are not extremely "fast".

So does differential design have weaker transmitted magnetic field pulse?

Regards,

-SB

Hi Simon ... at the risk of kicking off a bun fight .. here goes.

1. The coil is an N turn bifilar wound coil. When the the mosfet turns on the current starts increasing in the coil at a rate Ldi/dt depending on the inductance and peaks if the mosfet is on long enough at a value that is only dependant on the resistance of the coil , the feed wire and mosfet ON resistance.This current only flows in one of the bifilar conductors ( there is a small current due to the damping resistance across the other bifilar conductor )

Proposition 1. During TX the differential coil behaves the same as a 'standard' mono coil of N turns.

2. When the mosfet turns OFF however the magnet flux collapses through BOTH coils making up the bifilar coil and because of the series connection of the two bifilar conductors the damping resistor sees a 2N coil. ( twice the number of windings ). The voltage is also doubled as the flux to windings for the same current is directly proportional. The mosfet however is only reference to the centre connection to the series connected coil so it only sees half of the fly back voltage.

Proposition 2. The peak current in a differential coil at mosfet turn off is the same as a mono coil of N turns however the peak fly back flux is twice as great because the current in the differential coil flows through 2N turns. Target is stimulated with twice the flux at peak flyback.

3. When the fly back starts to decay the diodes ( now located at coil ) connected to mosfet and battery isolate the coil as they reverse bias and the target response is picked up by a 2N coil.

Proposition 3. The target is stimulated by a back EMF flux that is twice that of an N turn monocoil and the target eddy current flux is picked up by a coil that is twice the number of turns of a standard monocoil. Two by two is four ....

The short answer is that the 'charging' flux ie the 100 odd uS MOSFET ON is the SAME as a monocoil.However when the mosfet turns off the flyback flux is double. The RX coil response is double.

Regards,

moodz.

[Carl-NC]

With previous layouts the first amp was very close to the switching mosfet ... and it's the high voltage transient spikes that were getting into the front end of the opamp causing the protective diodes to avalanche and basically conduct to V+ and charge the cap thus provide a sort of 'switched' bias.

I would not have guessed that was happening... at least you found the cause. Good to know!

- Carl