Perhaps I should have applied for a patent?

[Ferric Toes]

I had a new idea that shows promise and that is using the wrap-around shield (with gap) as a TX on a RX coil. D=140mm, Lcoil(RX) = 325uH. Wrap-around shield 0.5uH and R = 0.4 ohms. TX pulse 30us, rise time approx 2uS then flat top at 10A. Back emf at switch off 12V. Only the RX coil appears to need damping. Pulse Rep rate is at present 3000pps and current drawn 500mA at 5V TX supply. This rep rate could be reduced to around 1000 so as to be more economical on current, but I am stuck with the 3K at present in order to get the 30uS pulse width.

TX switch is IRFP220N with on resistance of 0.075 ohms and VDss of 200V. It is on a heat sink but barely gets warm. Shield gets slightly warm. I coupled the RX winding to a NE5534 stage just to see what is happening and it looks interesting. Both switch on and switch off decays of a target are plainly seen and sampling could take place after 1uS. A faster front end will be tried next to get more detail.

Modelling the arrangement on Spice would no doubt give more insight if anyone has the time.

Eric.

[dbanner]

Interesting. I'm no expert, so I'll just ask. Would the RX coil act as a core to the TX coil with permeability of copper(assuming the RX coil is wound using copper wire)? would it not absorb some of the energy emitted from the TX coil? Would there be mutual inductance between TX and RX, And finally, would it be a good idea to switch out(isolate from circuit)the RX coil while TX is on? Please forgive me if these questions are nonsensical, like i said I'm no expert.

[Idefix]

Hello Eric,
I have sent a PM with the first results I saw. If I understand everything.
Best regards.

[green]

I had a new idea that shows promise and that is using the wrap-around shield (with gap) as a TX on a RX coil. D=140mm, Lcoil(RX) = 325uH. Wrap-around shield 0.5uH and R = 0.4 ohms. TX pulse 30us, rise time approx 2uS then flat top at 10A. Back emf at switch off 12V. Only the RX coil appears to need damping. Pulse Rep rate is at present 3000pps and current drawn 500mA at 5V TX supply. This rep rate could be reduced to around 1000 so as to be more economical on current, but I am stuck with the 3K at present in order to get the 30uS pulse width.

TX switch is IRFP220N with on resistance of 0.075 ohms and VDss of 200V. It is on a heat sink but barely gets warm. Shield gets slightly warm. I coupled the RX winding to a NE5534 stage just to see what is happening and it looks interesting. Both switch on and switch off decays of a target are plainly seen and sampling could take place after 1uS. A faster front end will be tried next to get more detail.

Modelling the arrangement on Spice would no doubt give more insight if anyone has the time.

Eric.

An attempt at a spice model. Try with series MUR460 diode and with diode shorted. Need Tx damping resistor with spice(resonance 25MHz), didn't test for optimum value. Something to play with.

[Digger429]

>TX 0.5uH
>sampling could take place after 1uS

Dont mean to be critical, but is it simply acting as a ****ty transmitter and just not actually putting out much of an electromagnetic field? Is the reason sampling can take place at 1uS simply because there is so little EM pulse created that has to dissipate?
Any bench tests on distance/performance?

[Ferric Toes]

An attempt at a spice model. Try with series MUR460 diode and with diode shorted. Need Tx damping resistor with spice(resonance 25MHz), didn't test for optimum value. Something to play with.

Yes, I will try with a series diode. I couldn't see any ringing due to the TX and I suspect that at frequencies of upwards of 5MHz the nickel/copper fabric has damping properties of its own. After all, it is advertised as an EMI preventative.

Eric.

[Ferric Toes]

>TX 0.5uH
>sampling could take place after 1uS

Dont mean to be critical, but is it simply acting as a ****ty transmitter and just not actually putting out much of an electromagnetic field? Is the reason sampling can take place at 1uS simply because there is so little EM pulse created that has to dissipate?
Any bench tests on distance/performance?

Many PI metal detectors give good detection ranges where the final peak TX current is 1A or less. With this pulsed shield idea I have a flat top current of 10A and which could be increased to 15 or even 20A, except for the limitation of 2A steady current from my bench power supply. I would have to reduce the pulse rate in proportion to achieve this. These higher levels of pulse current compensate for the lack of turns.
This is all at a very preliminary stage and done within the past week. The idea is to investigate an alternative way of making a coil for a PI that detects much smaller and less conductive objects. It is a bit too early to discuss range or performance.

Eric.

[green]

An attempt at a spice model. Try with series MUR460 diode and with diode shorted. Need Tx damping resistor with spice(resonance 25MHz), didn't test for optimum value. Something to play with.

Used a model I had for an IB coil. Probably needs added coupling between Tx and Rx (K3 L1 L3 .5?). Can't sample at 1usec when added. Maybe something wrong with model.

[Idefix]

Hello,
have drawn on my Spice (Microcap ) what I have understood of the project.
I have used a transformer , in spice , with 0.95 as coupling coef. and your values of coil.
For the moment I used a IRFP22N60, as I have the model in library.

I can only attach jpg files here. (or I don t see the way.. )
So , hereafter some pictures.

I use a sine generator in serial with the coil, 200kHz 2mv to see the OPAmp working at soon as it is possible , according to the spice model.
The pulse begins ot 6µs, 2µS of delay to one level, so 1 level at 8µs
The trailing edge of pulse is at 38µs, the time delay is 2µS to level, making the end of pulse at 40µS
The current flowing in the Mosfet is near 8 amps.
The 200Khz sine is visible at say 45µs, with a 100Mhz opamp.
But in these simulations, i am not sure of accuracy of the setup times. It is very dependant on the quality of the spice model.

I also have put in same schematic, a classic TX, to compare the speeds.
I will try to attach pictures.

[Idefix]

I have many errors while attempting to post , like the "token has expired" ????

here after some pictures






the sine signal is visible.

[Ferric Toes]

Interesting. I'm no expert, so I'll just ask. Would the RX coil act as a core to the TX coil with permeability of copper(assuming the RX coil is wound using copper wire)? would it not absorb some of the energy emitted from the TX coil? Would there be mutual inductance between TX and RX, And finally, would it be a good idea to switch out(isolate from circuit)the RX coil while TX is on? Please forgive me if these questions are nonsensical, like i said I'm no expert.

The RX winding is 0.2mm tinned copper solid wire with PTFE insulation. I doubt that the wire bundle has any permiability effect although one can't be sure until more detailed measurement are taken. If there is any permiablity effect if is more likely to be from the nickel which plates the coil wire and also the shield material.

No significant energy is absorbed by the RX coil, as is the case for a mono coil when used as TX and RX. It is only at the transitions of the pulse that energy could be absorbed and this would show up as a damping effect. It appears that the only major damping needed is that for the RX coil which has a very much lower resonant frequency. With it open circuit I was getting about 650kHz for the ringing waveform. However, I need to look at this again to be sure.
Green suggested a diode to isolate the shield, so I need to try that. It is not possible to switch out the RX as the shield is an integral part of the coil assembly. If you disconnect the coil it will ring badly and if you short the coil you will get a huge decaying signal because you have a tightly coupled continuous LR circuit.

Eric.

[green]

Hi Eric, maybe you could post a scope picture with no target and one with a 1x1 inch piece cut from regular aluminum kitchen foil(TC about 1.6usec).

[Ferric Toes]

I omitted to say that the coil bundle has polythene cable wrap as a spacer and the woven braid tape is wound on top of the cable wrap, which acts as a spacer. The measured capacitance between shield and coil is 80pf.
Eric.

[Ferric Toes]

Hi Eric, maybe you could post a scope picture with no target and one with a 1x1 inch piece cut from regular aluminum kitchen foil(TC about 1.6usec).

Hi Green,
I won't be able to do this for a few days as I have a busy week ahead on other matters. Also, I need to make some improvements to my receiver circuitry and have less of a birdsnest arrangement at these frequencies, otherwise some spurious effects may creep in.

Eric.

[Altra]

Hi Eric
How does it respond to ferrite? Hope you have discovered something new.