Selective Frequency

[Daves]

Hi guys,
Suppose a detector operating with a selectable single frequency. Let's assume the frequency alternatives are 5, 10, 15kHz

-Do I need to change the resonance capacitances of both TX and RX?
-What should be the frequency difference between TX and RX?
-What should be the phase difference when the Rx coil is trying to nullling?
-Should drive at selectable frequency be with sine or square wave? -How should the frequency switching circuit be? A example schematic or drawing would be great.

[waltr]

Assuming this is a VLF type detector like the TGS or IDX....

Hi guys,
Suppose a detector operating with a selectable single frequency. Let's assume the frequency alternatives are 5, 10, 15kHz

-Do I need to change the resonance capacitances of both TX and RX?
Yes
-What should be the frequency difference between TX and RX?
Enough so it is stable through a range of temperatures
-What should be the phase difference when the Rx coil is trying to nullling?
? Not sure.
-Should drive at selectable frequency be with sine or square wave? -How should the frequency switching circuit be? A example schematic or drawing would be great.

Typically Sine but it can work with square.

I have though of trying to do this with the TGSL which runs at 14.5kHz. This is a good frequency for low conductors like US nickles but not so great on silver coins.
The IDX (another project here on Geotech) runs at ~7.5kHz and is better on silver coins.

[Daves]

Assuming this is a VLF type detector like the TGS or IDX....


Typically Sine but it can work with square.

I have though of trying to do this with the TGSL which runs at 14.5kHz. This is a good frequency for low conductors like US nickles but not so great on silver coins.
The IDX (another project here on Geotech) runs at ~7.5kHz and is better on silver coins.

First of all thank you for the reply.

Yes I asked for a VLF.

There was a question I forgot to add. I will use graphite spray for the shield, but how much resistance should it be?

[Carl-NC]

The easiest way to do this is with a square wave TX. Use an H-bridge driver for the TX coil. Use a wide band RX preamp and demods and do time domain sampling. Results will be similar to a sinusoidal narrow band system.

[Daves]

The easiest way to do this is with a square wave TX. Use an H-bridge driver for the TX coil. Use a wide band RX preamp and demods and do time domain sampling. Results will be similar to a sinusoidal narrow band system.

*I did not put resonance capacitance on the rx coil for single frequency and it was not a problem. It did rx detection when I passed a metal over the coil.
*I chose 3 different frequencies.
*I did not put resonance capacitance on the RX side. On one of the frequency I selected the rx was overloaded and never recovered. I added resonance capacity to Rx and it was fine.
*If I'm going to add 1 resonance capacity to Rx, which frequency should I adjust it to?
*The frequencies I chose for TX are 5-10-15kHz. Should I tune the RX resonance like 13 or 14 just below the highest frequency or like 4 or 3 smaller than the lowest frequency 5?
* As the TX frequency changes, the TX driving power changes to Vpp. What should be the minimum acceptable value?
*Which frequency should I work while adjusting RX phase and Vrms? Is it the highest or the lowest?
*Is there anything I should pay attention to when I buy ferrite for RX? Can I do it with any ferrite?

[Carl-NC]

On one of the frequency I selected the rx was overloaded and never recovered.

What frequency, and why?

The frequencies I chose for TX are 5-10-15kHz. If I'm going to add 1 resonance capacity to Rx, which frequency should I adjust it to?

You should not use a resonant cap on the RX at all. Find out why it overloads and fix that.

As the TX frequency changes, the TX driving power changes to Vpp. What should be the minimum acceptable value?

Yes, inductor current is V/(2πfL) so higher frequencies will have a lower TX current. However, induced RX voltage increases with frequency so it sort of washes. Acceptable value is totally up to you and how well you can balance a coil.

Is there anything I should pay attention to when I buy ferrite for RX? Can I do it with any ferrite?

What do you plan to do with the ferrite?

[Daves]

On one of the frequency I selected the rx was overloaded and never recovered.

What frequency, and why?

The frequencies I chose for TX are 5-10-15kHz. If I'm going to add 1 resonance capacity to Rx, which frequency should I adjust it to?

You should not use a resonant cap on the RX at all. Find out why it overloads and fix that.

As the TX frequency changes, the TX driving power changes to Vpp. What should be the minimum acceptable value?

Yes, inductor current is V/(2πfL) so higher frequencies will have a lower TX current. However, induced RX voltage increases with frequency so it sort of washes. Acceptable value is totally up to you and how well you can balance a coil.

Is there anything I should pay attention to when I buy ferrite for RX? Can I do it with any ferrite?

What do you plan to do with the ferrite?

Carl:What do you plan to do with the ferrite?
Answer: I quoted from your comment. For use when balancing
"When I design detectors I arrange the coils so that a ferrite target produces an in-phase voltage on the RX coil. I can't tell from that project what needs to be done, so just wind the coils in any fashion but before you permanently affix them test the detector. If it doesn't work right, simply flip one of the coils upside-down."

Is it just a low Vrms value that matters when balancing the coils? Or does the TX-RX phase difference matter?
While the coil is balancing;
-find minimum Vrms
-react in phase to ferrite
are these enough?
Question: Let's say the ferrite reacted on the negative side, does it change anything if the phase difference between tx and rx is -100, -120 -180

Carl: What frequency, and why?
Answer: I can't remember which one it's for right now, I'll try and let you know.

Carl: You should not use a resonant cap on the RX at all. Find out why it overloads and fix that.
Answer: Ok, i'll try again and let you know.

[Carl-NC]

Oh, OK, any ferrite should work. The raw phase shift from TX-to-RX (monitoring the voltages on both coils) should be either 0° or 180° for large imbalances or strong ferrite. I like to achieve 0° just for consistency. As you get down to a minuscule level of imbalance you may see the phase shift move around due to parasitic capacitance. It doesn't matter, just get the IB amplitude as small as you can. Ferrite should still produce a 0° response.

[Daves]

Oh, OK, any ferrite should work. The raw phase shift from TX-to-RX (monitoring the voltages on both coils) should be either 0° or 180° for large imbalances or strong ferrite. I like to achieve 0° just for consistency. As you get down to a minuscule level of imbalance you may see the phase shift move around due to parasitic capacitance. It doesn't matter, just get the IB amplitude as small as you can. Ferrite should still produce a 0° response.

Ok,
I will balance the RX phase to be in phase (0) or 180 degrees difference from the TX.

I'll try and let you know, thank you Carl

[Skippy]

I recommend using an existing coil that you know is suitable. Then when you have a working detector, consider making your own speciality coil ( example extra-large, or long 'clean-sweep' style.)
I think coils for the Fisher CZ range ( CZ-3 / -5 / -7 / -20 / -21 ) would be a good choice, as they are made to work at 5 kHz and 15 kHz simultaneously. The CZ machines drive the coil with a 5 kHz square wave using some variation of a H-bridge circuit. There is a hand-drawn schematic for the CZ-5 posted on the forum, somewhere, which may give you a start.
If you are planning on sine wave drive, the coils for the Fisher GoldBug Pro, F19 , F5, Teknetics G2 , Delta, Gamma + others, will work over a 7.8 kHz to 19 kHz range, and could probably go a bit lower. Possibly 6 kHz to 20+ kHz. There is a huge range of coils available for these machines.

[Skippy]

Some relevant recent CZ threads:
https://www.geotech1.com/forums/foru...d-cz-5-project
https://www.geotech1.com/forums/foru...d-it-s-secrets
There are many more discussion threads.

[Daves]

Some relevant recent CZ threads:
https://www.geotech1.com/forums/foru...d-cz-5-project
https://www.geotech1.com/forums/foru...d-it-s-secrets
There are many more discussion threads.

Thanks for the comment, I'll look into the links.

What I want to ask you: Q, L, R, Inductance etc for the coil. I am searching for information.
*What does being high and low affect?
*Which means what

[Daves]

Oh, OK, any ferrite should work. The raw phase shift from TX-to-RX (monitoring the voltages on both coils) should be either 0° or 180° for large imbalances or strong ferrite. I like to achieve 0° just for consistency. As you get down to a minuscule level of imbalance you may see the phase shift move around due to parasitic capacitance. It doesn't matter, just get the IB amplitude as small as you can. Ferrite should still produce a 0° response.

Hi again Carl,

I bought 9 different ferrites in total.
When TX and RX are as in the "picture 1", when I bring Ferrite closer to rx, the reactions are;

In-Phase Response + Amplitude Increase
CF139T1607C Toroid 1500nh
CF139T2815C Toroid 4300nh
KT130-26A Toroid
CF139T2515C Toroid 3200nh
MS-184026-2 Toroid 59nh
CF199T3112C Toroid

Phase change + amplitude increase;
CF193T3829C Toroid 6600nh
CF139R0620 Toroid (little rod black)
No Code - Long Rod black

When I pass the "ferrous object" while the RX and TX phases are as in the picture, there is a phase change + amplitude increase.

Which side should I choose?

When I set it according to "picture 2", if I hold "iron metal" to the coil, it gives an in-phase response + amplitude.
CF193T3829C, CF139R0620, Long Rod Black = in-phase response + amplitude boost.

[Skippy]

Carl's suggestion that 'any ferrite should be OK' is not 100% accurate. Some very high mu ferrite materials, usually ones designed to absorb high frequency energy, give unusual results. They are commonly painted green, and are found on mains ( 50 Hz / 60 Hz ) power filters, such as common-mode filters. The toroid "CF139T3829C Toroid 6600nh" is likely this type. I suggest using the lower A[sub]L[/sub} value toroids.
Also, type "KT130-26A Toroid" is not ferrite, it is powdered iron. This would be my choice for you to use as a "test ferrite"

[Daves]

Carl's suggestion that 'any ferrite should be OK' is not 100% accurate. Some very high mu ferrite materials, usually ones designed to absorb high frequency energy, give unusual results. They are commonly painted green, and are found on mains ( 50 Hz / 60 Hz ) power filters, such as common-mode filters. The toroid "CF139T3829C Toroid 6600nh" is likely this type. I suggest using the lower A[sub]L[/sub} value toroids.
Also, type "KT130-26A Toroid" is not ferrite, it is powdered iron. This would be my choice for you to use as a "test ferrite"

Let's make it clear if Carl replies when he's available.