Selective Frequency

[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]

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]

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?

[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]

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.