minimum sample delay

[green]

Used AWG34 solid magnet wire to make the last coil I built for minimum delay. Rx_(2)43mm round fig8, Tx oval around Rx. Tx around 10 ohms limited how fast I could charge the coil at constant rate. Added a second winding over the first(both, single layer solenoid)connected in parallel. Seemed to make the coil slower, don't know why. Added 1k resistors between Rx coil leads and differential amplifier input with a 100 ohm across amplifier input(attenuate 20 times) to see if I could learn something. Was faster than original coil but lower amplitude. Including scope pictures with added resistors(copper wire_5). (copper wire_4) was taken with 10 ohm Tx coil, log out calculated with Excel. Log out amplifier not fast enough for AWG28. Wire TC sees to be a little shorter with _5png. Wondering if there is something I could try to get delay time aroud 2usec with coil connected direct to amplifier.

Increased number of wire pieces to increase signal amplitude, doesn't effect time constant.

25mv in=0v out, not 25mv in-0v out(first picture)

[green]

Used AWG34 solid magnet wire to make the last coil I built for minimum delay. Rx_(2)43mm round fig8, Tx oval around Rx. Tx around 10 ohms limited how fast I could charge the coil at constant rate. Added a second winding over the first(both, single layer solenoid)connected in parallel. Seemed to make the coil slower, don't know why. Added 1k resistors between Rx coil leads and differential amplifier input with a 100 ohm across amplifier input(attenuate 20 times) to see if I could learn something. Was faster than original coil but lower amplitude. Including scope pictures with added resistors(copper wire_5). (copper wire_4) was taken with 10 ohm Tx coil, log out calculated with Excel. Log out amplifier not fast enough for AWG28. Wire TC sees to be a little shorter with _5png. Wondering if there is something I could try to get delay time aroud 2usec with coil connected direct to amplifier.

Increased number of wire pieces to increase signal amplitude, doesn't effect time constant.

25mv in=0v out, not 25mv in-0v out(first picture)

Any suggestions? Wondering if adding switches between Rx coil and amplifier input would help, switches open for 2usec after Tx off. Should prevent amplifier from saturating during Tx coil decay.

More scope pictures with 20 to 1 attenuation resistors. Tx_constant current(cc).5A peak and constant rate(cr)1A peak. Damping didn't make a big difference, don't know why underdamped was better with .5A peak constant current and over damped was better with 1A peak constant rate.

target_AWG28 solid wire

[Ferric Toes]

Why do ferrite beads have a signal and decay with opposite polarity of other targets after Tx off?

I've seen in other threads suggesting using a bed of ferrite beads to simulate ground. If beads decay from an opposite polarity is that a problem? Am I missing something?

If you test any type of ferrite rod, or bead, they will have the same polarity decay; that is if tested with a mono coil. Some ferrites have minimal viscous decay while others give a huge signal. Only those with minimal viscous decay (so called 'soft' ferrites) should be used. A TX coil with a fig8 RX, or a balanced concentric TX/RX, will give both polarity signals depending where the target is in relation to the coil geometry. Ground is not best simulated by ferrite beads as the magnetic susceptibility will be much higher than typical ground. Better to use fired red housbricks or fired clay tiles.

Eric.

[green]

If you test any type of ferrite rod, or bead, they will have the same polarity decay; that is if tested with a mono coil. Some ferrites have minimal viscous decay while others give a huge signal. Only those with minimal viscous decay (so called 'soft' ferrites) should be used. A TX coil with a fig8 RX, or a balanced concentric TX/RX, will give both polarity signals depending where the target is in relation to the coil geometry. Ground is not best simulated by ferrite beads as the magnetic susceptibility will be much higher than typical ground. Better to use fired red housbricks or fired clay tiles.

Eric.

Thanks for the reply. Tried repeating the test with ferrite beads. Targets positioned in or over coil A should give a positive signal. The beads test the same polarity as other targets until they are positioned inside the coil giving a higher strength signal. I'm thinking either something happens to the ferrite or the amplifier is giving a bad reading. Any thoughts on what might be causing the signal to reverse polarity?

[Qiaozhi]

Any thoughts on what might be causing the signal to reverse polarity?

The signal appears to be under-damped. Either your damping resistor is too high a value, or the MOSFET is not switching off properly. Can you post the schematic of your TX circuit?

[Ferric Toes]

Thanks for the reply. Tried repeating the test with ferrite beads. Targets positioned in or over coil A should give a positive signal. The beads test the same polarity as other targets until they are positioned inside the coil giving a higher strength signal. I'm thinking either something happens to the ferrite or the amplifier is giving a bad reading. Any thoughts on what might be causing the signal to reverse polarity?

It is hard to see what is happening, but what is the source of the scope trigger? Sometimes waveforms flip to a different part as signal amplitudes change; that is when internal triggering. Presumably your total waveform comprises both after TXon and TXoff where different polarities will occur.

Eric.

[green]

TRT schematic

[Qiaozhi]

TRT schematic

OK - since you're driving the MOSFET directly with a rail-to-rail opamp, the ringing is probably due to underdamping. Try adjusting the damping resistor to provide critical damping, and see if the polarity issue goes away.

[green]

OK - since you're driving the MOSFET directly with a rail-to-rail opamp, the ringing is probably due to underdamping. Try adjusting the damping resistor to provide critical damping, and see if the polarity issue goes away.

Scope pictures with coil over damped and under damped. Damping has little or no effect. Still makes sense when beads aren't in coil, amplifier output reverses polarity when beads are in coil(higher signal). Subject I know nothing about, ferrite can saturate. Can Tx saturate the ferrite and if it can what would you expect the decay to look like?

TRT schematic may not have the same chip numbers I ended up with. If anyone is interested I can up date the schematic with chip numbers I used.

[dbanner]

Rip open an old desktop PSU, try the "yellow" toroidal ferrite cores within.
Could be interesting. I think these are closer to the redbrick.

[green]

OK - since you're driving the MOSFET directly with a rail-to-rail opamp, the ringing is probably due to underdamping. Try adjusting the damping resistor to provide critical damping, and see if the polarity issue goes away.

Thinking maybe you wanted me to change damping on Rx coil instead of Tx coil like in above reply. Rx damping, ferrite_3. Tx damping, ferrite_2.

[Qiaozhi]

Thinking maybe you wanted me to change damping on Rx coil instead of Tx coil like in above reply. Rx damping, ferrite_3. Tx damping, ferrite_2.

Could this simply be the result of the ferrite changing the coil inductance once it gets close to the coil, and hence upsetting the coil damping which causes ringing?

[green]

Could this simply be the result of the ferrite changing the coil inductance once it gets close to the coil, and hence upsetting the coil damping which causes ringing?

Not sure where you are seeing ringing. Looking at pictures in ferrite_1. (picture 2)with ferrite on coil A signal goes +, same as other targets. (picture 3)when ferrite is inserted in coil A causing more signal, signal goes + for about 10usec and then goes -, shouldn't go -? (picture 4)when ferrite is inserted in coil B signal starts - which it should but then goes + at about 10usec which it shouldn't and stays + for more than 200usec(picture 5). First, my concern is the tester lying to me. Second, if it isn't it would be nice to know why the ferrite when inserted in the coil is acting different than other targets.

Maybe Eric could try some ferrite beads in his ground tester. Not the same, but if decay did go - it would show something weird is happening.

[Qiaozhi]

Not sure where you are seeing ringing. Looking at pictures in ferrite_1. (picture 2)with ferrite on coil A signal goes +, same as other targets. (picture 3)when ferrite is inserted in coil A causing more signal, signal goes + for about 10usec and then goes -, shouldn't go -? (picture 4)when ferrite is inserted in coil B signal starts - which it should but then goes + at about 10usec which it shouldn't and stays + for more than 200usec(picture 5). First, my concern is the tester lying to me. Second, if it isn't it would be nice to know why the ferrite when inserted in the coil is acting different than other targets.

Maybe Eric could try some ferrite beads in his ground tester. Not the same, but if decay did go - it would show something weird is happening.

Now I can see what you're getting at, and I suspect it's something to do with your setup. If you examine a VLF coil with TX and RX windings, the RX signal can shift either left or right depending on the coil configuration and the type of target. Maybe that's what's going on here. What happens if you do the same test with a DD rather than your unusual figure 8 coil?

[green]

Now I can see what you're getting at, and I suspect it's something to do with your setup. If you examine a VLF coil with TX and RX windings, the RX signal can shift either left or right depending on the coil configuration and the type of target. Maybe that's what's going on here. What happens if you do the same test with a DD rather than your unusual figure 8 coil?

Thanks for the reply. Don't have a small DD coil to try. Tried with a small mono coil this morning, didn't go -. Coil not shielded and acted a little funny. Think I'll make another mono coil, more turns and shielded.


Curious what you mean by unusual figure 8 coil. Figure 5-22 page 57 ITMD shows a similar coil(rectangle instead of round). I use a figure8 coil because of less noise, really important when trying to chart log decay direct from amplifier.