minimum sample delay

[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]

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).

[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.

[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]

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/i...ITES__1969.pdf

[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.

[chemelec]

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

[bbsailor]

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

Here is a link that will explain soft ferrites:http://www.magneticsgroup.com/pdf/mmpa%20SFG-98.pdf

Basically, soft ferrites do not hold much energy after the TX pulse turns off.

[Polymer]

Thanks for all your replies. It looks like HARD ferrites are used only for permanent magnets. I had a misconception of this.
The ones we are interested in are SOFT ferrites and there seem to be two main categories of these:
NiZn and MnZn.

@KingJL -
Thank You for the confirmation. The ones that don't work well with PI are apparently the NiZn ones (e.g. am radio antenna)
Am lucky enough to have many MnZn rods - they are not as cheap or as easy to get as you say.

@bbsailor -
I get "server not found" error with the link, even with the base address.
Sometimes this happens with US websites whwn accessed from Europe.
Could you please test from your location!?

[6666]

@bbsailor -
I get "server not found" error with the link, even with the base address.
Sometimes this happens with US websites whwn accessed from Europe.
Could you please test from your location!?

Works ok from Australia

[bklein]

Thanks Kingjl, is it this circuit ? always been curious about it, DaveJ has said a few times its worth investigating.

is there a schematic error with Q5/6 bases?

[6666]

Carl's method of measuring resonance is most accurate, as the result will have included ALL of the capacitance (intended and unintended) of the TX circuit (i.e. coil, MOSFET, cable, internal wiring/routing, etc.). Also the correct damp resistance is not just a function of resonant frequency, but more importantly a function of the ratio of inductive reactance relative to capacitive reactance (which corresponds to the ratio of L to C as well as frequency). In other words if 2 coils have the same resonant frequency but different ratios of L to C, they require different damp R values.

Where is Carl's method described ? thanks

[6666]

is there a schematic error with Q5/6 bases?

Maybe DaveJ can answer that, but does look a little unusual .

[KingJL]

... Also the correct damp resistance is not just a function of resonant frequency, but more importantly a function of the ratio of inductive reactance relative to capacitive reactance (which corresponds to the ratio of L to C as well as frequency). In other words if 2 coils have the same resonant frequency but different ratios of L to C, they require different damp R values.

Amen!!!

[KingJL]

Where is Carl's method described ? thanks

"... Carl mentioned in another thread to disconnect Rd with an operating PI to check resonance..."