minimum sample delay

[green]

TRT schematic

[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.

[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?

[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?

[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]

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

[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]

Playing with minimum delay again. Trying to understand why ferrite beads cause a signal at low delays. Hadn't noticed a signal at longer delays until his morning. Charted signal decay for California ground and some ferrite beads in a plastic bag. To get a positive signal decaying to zero, beads needed to be added to the opposite Rx coil as ground and all other targets I've tried. Both charted a similar log-log decay. Tried a large ferrite bead, decay similar to the bag of smaller beads(orientation seemed to effect decay slope). Why do ferrite beads have a signal and decay with opposite polarity of other targets after Tx off? Slope change below 0v could be measurement error or not.

chart time scale usec

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?

[green]

1 ohm might be a little tough with a 1000uH coil and a practical wire diameter (even 2 might be a challenge... should be able to hit about 3 ohm with 24 awg). Using the same timing methodology as the impulse would put the prt at 3000usec (333.33 Hz prf), if that suits your needs.

Was thinking of trying some AWG19 magnet wire I have for Tx coil. Don't have a need, just trying to learn something and have some fun with the impulse circuit. Would be nice to end with something worth building. Suggestions appreciated if what I'm thinking doesn't make sense.

[KingJL]

Some thoughts for the impulse... Maybe 1000uH coil around one ohm. Around 300usec Tx time.

1 ohm might be a little tough with a 1000uH coil and a practical wire diameter (even 2 might be a challenge... should be able to hit about 3 ohm with 24 awg). Using the same timing methodology as the impulse would put the prt at 3000usec (333.33 Hz prf), if that suits your needs.

[green]

What is the objective

Question asked by KingJL reply #13. Been trying to answer with reference to trying the impulse. Are there targets that the impulse might be a better choice over a normal PI with fast decay? Been playing with spice and I have some thoughts, sometimes wrong. Want to order some parts, having an objective was a good idea. Any thoughts for an objective with an impulse?

Some thoughts for the impulse. Reduced signal for low time constant targets so I'm thinking trying to detect high time constant targets only. Larger coil, maybe 16 to 20 inch diameter. Coil time constant greater than 2 maybe 3 times Tx time. Maybe 1000uH coil around one ohm. Around 300usec Tx time. H bridge with 12 to 18 volt supply. Does what I'm thinking make sense and are there other things I should consider? Think it's interesting that at the same time I'm ordering parts for a detector to detect high time constant targets only I'm ordering parts to try constant current damping again to try to get the sample delay down to around 3usec for very low time constant targets. Don't know if it makes any sense but it could be a challenge.

[green]

What is the objective

Question asked by KingJL reply #13. Been trying to answer with reference to trying the impulse. Are there targets that the impulse might be a better choice over a normal PI with fast decay? Been playing with spice and I have some thoughts, sometimes wrong. Want to order some parts, having an objective was a good idea. Any thoughts for an objective with an impulse?

[KingJL]

What are the pros and cons of using a 4N channel vs a 2N and 2P channel bridge?

With the impulse and the low voltages involved, I am not sure there are any real pro's and con's... dealers choice. I've done both... my personal preference is 4N channel. I have found that I had to use diodes between the H-Bridge MOSFETs and the coil to insure there were not unwanted currents flowing back through the MOSFETS during coil current decay. In the case of the Impulse, they could be very low Vj Schottky diodes (like the ones steering the capacitor currents).

[green]

Thanks for the reply, think I'm starting to figure it out. Still working on a coil for 1usec delay. Attempted to chart signal decay with fast decay and decay I think is similar to the impulse. Wondering if it makes sense. Target distance was adjusted to give a near full scale signal for fast decay with coins and foil(couldn't get full scale with ground). I'll have to try a full bridge circuit.

Thanks KingJL for posting it.

Thinking about Impulse again. Charted ground signal on log-log chart triggered at Tx off, Tx off +46 and 58usec. Ground charts fairly straight line on log-log chart with fast coil decay triggered at Tx off. Doesn't when triggered near zero current with slow coil decay. Probably only matters when charting signal decay? H bridge PWD13F60(N channel) was mentioned in another thread. What are the pros and cons of using a 4N channel vs a 2N and 2P channel bridge?

[green]

Finally got some charts for the AWG24 and 28 solid wire. Used 16 pieces 1inch long for AWG24 and 64 pieces 1inch long for AWG28(pieces laying flat not bundled). Added the TC lines from the projected chart, .8usec for AWG24 and .33usec for AWG28. Looks close. Had a hard time getting enough signal at amplifier out. Log amp not fast enough, charted amplifier out(target-no target)with Excel.

Appears TC chart might be correct. Wondering if TC vs wire size is charted anywhere else to compare.