minimum sample delay

[chemelec]

I have lots of Soft Ferrite Rods that I have used in Pin Pointers.
They work pretty good.

[KingJL]

This has me puzzled as I can easily detect this type of ferrite, would it constantly be detecting itself?

From my experience, yes. I made two probes for my Vallon VMH3CS, one made from a cheap ferrite rod found on ebay (the rod had no ferrite type # and had two flats on opposite sides of the rod) and one from a fairly expensive softer rod (I forget the ferrite type #, maybe type 33). The one made from the cheap ferrite would give a constant alarm with a very significant target strength indication. The one with the softer ferrite was as quiet as a mouse. For a guide, if the offering of the ferrite is without the type #, do not use it for a probe.

[Polymer]

That is correct. Ferrite rods from radios have a high magnetic viscosity, whereas ferrite used in power converters and line transformers driving scanning coils for CRT's have low viscosity. The ferrite makeup is different for the two types and I remember that there are nickel zinc and nickel manganese ferrites. Which is which I don't remember. A good book I used to refer to is 'Soft Ferrites' by E C Snelling (1969) but unfortunately it has vanished at some point in my timeline. Copies are still available from Amazon, but are expensive.

Eric.

Thank You Eric!

I would like to implement a pinpointer on my MMP vers E and will probably
have to test both soft/hard ferrite variants for decay times as not much info available here ... or I didn't find it.
I have seen one ferrite pinpointer on the MPP here, but apparently made with a "hard" ferrite from an AM radio.

This has me puzzled as I can easily detect this type of ferrite, would it constantly be detecting itself?

*** just found the PDF online, Snelling, if of interest *** 42MB

https://ia802300.us.archive.org/22/items/SNELLING__SOFT-FERRITES__1969/SNELLING__SOFT-FERRITES__1969.pdf

[Ferric Toes]

I'm curious about the ferrites you are mentioning, what kind are you using?

Thought I'd post since I found a huge difference in my MPP vers. E when waving different ferrites in front of the coil.
I think Eric has mentioned this somewhere but I can't find it.

The ferrites that do not cause a reaction (unless being placed in the coil plane) are quite big and have a DC resistance of around 10k Ohm.
The ferrites that cause the detector to squeal from quite a distance are from am radios and have a basically infinite resistance on my DMM.

That is correct. Ferrite rods from radios have a high magnetic viscosity, whereas ferrite used in power converters and line transformers driving scanning coils for CRT's have low viscosity. The ferrite makeup is different for the two types and I remember that there are nickel zinc and nickel manganese ferrites. Which is which I don't remember. A good book I used to refer to is 'Soft Ferrites' by E C Snelling (1969) but unfortunately it has vanished at some point in my timeline. Copies are still available from Amazon, but are expensive.

Eric.

[Polymer]

I'm curious about the ferrites you are mentioning, what kind are you using?

Thought I'd post since I found a huge difference in my MPP vers. E when waving different ferrites in front of the coil.
I think Eric has mentioned this somewhere but I can't find it.

The ferrites that do not cause a reaction (unless being placed in the coil plane) are quite big and have a DC resistance of around 10k Ohm.
The ferrites that cause the detector to squeal from quite a distance are from am radios and have a basically infinite resistance on my DMM.

[green]

Think I know why. Added a 2000 to 1 divider between the coil and diff amp. Amplifier still goes full scale when ferrite is inserted in the coil(greater than 12 Rx coil volts)when Tx coil current is decaying. Might have to try switches between coil and amplifier(switches open for about 2usec after Tx off).

[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?

I suspect you are correct. Connected mono amplifier to two different mono coils and oval Tx coil that I've been using. Doesn't go -, looks correct. Looks like something with my differential amplifier. Don't know what the problem is yet. If the beads are inserted in coil A, output goes -(not correct), adding beads to coil B reduces the signal and output looks normal. Figure8 Rx solves the problem. To large a signal from the ferrite causes the bad signal. Why I don't know.

[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.

[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]

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]

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]

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.

[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.

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.

[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.