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Offline GoldDigger1950
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« Reply #20 on: August 05, 2012, 08:39:14 pm »
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You guys can believe what you want but I used to teach this stuff many moons ago. Current in a tuned circuit is minimal. If you want a field in the coil, don't design it for resonance. Make the coil match the frequency dut de-tune the rest for 3db down from resonant.

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« Reply #21 on: August 05, 2012, 09:28:09 pm »
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I know for a Fact you have to detune the coils by 500 or more HZ in order to get the phase difference to do any Discrimination.

Through it out of phase.  Make  Lump in the Voltage that can be detected.

like you said, this is the place it will show up.

I guess being a RF Wiener, I'm use to the tickle of the Tuned Circuit that We Used.

Heterodyne is one thing and Detecting Magnetic Response is another.   Huh?Huh?Huh??





 Huh? Huh? Huh?

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« Reply #22 on: August 06, 2012, 03:24:08 am »
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It depends what you mean by "current in a tuned circuit".  At resonance the current supplied to the circuit falls to a minimum, and ideally would fall to zero.  However this is not the same as the current circulating in the coil/capacitor loop.  This can be, and usually is, many times larger.  The multiplication factor is called "Q".  That's the textbook account, and it's good enough for me.  (See

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http://www.insula.com.au/physics/1221/L16.html
for example)

I like discussions like this, they really make you think...


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Offline GoldDigger1950
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« Reply #23 on: August 06, 2012, 11:22:32 am »
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Quote:Posted by Gwil
It depends what you mean by "current in a tuned circuit".  At resonance the current supplied to the circuit falls to a minimum, and ideally would fall to zero.  However this is not the same as the current circulating in the coil/capacitor loop.

Yes it is because when you put an AC signal into the circuit, a resistive component comes into play making the circuit unbalanced and allowing a field. My statement is absolutely correct. IF YOU TUNE AN RCL CIRCUIT TO RESONANCE, THERE IS NO MAGNETIC FIELD DEVELOPED IN THE COIL. Don't get all emotional over this issue. It's a fact of engineering design. When a circuit for a metal detector is built, you DO NOT WANT resonance. What you want is a coil tuned to the frequency for both transmitter and receiver. The transmit circuit must then be unbalanced at 3db down from center frequency while the receiver is a perfect frequency balance. If they happen to be the same circuit (as in a PI detector), your gain on the RX side is critical to proper use.

Xavier can tell you all about this.

Posted on: August 06, 2012, 07:15:16 PM
Quote:Posted by Gwil
I like discussions like this, they really make you think...

I do too but to argue for the sake of arguing can be taxing. Just assume I am correct for a moment. The perfectly tuned circuit (as the one in the Smithsonian proves) emits no outside energy or it would stop. In order to emit a magnetic field, it needs something to hold the electrons in the coil and that something is AC resistance. This is something you can calculate but cannot measure unfortunately. Once the electrons are delayed inside the coil, a field builds up and current flow is stalled (on purpose). This field then must e allowed to collapse or the RX circuit will be over-driven. There is a blanking pulse required in the PI circuit for this - typically between 10 and 30 milliseconds. The RX is shut off from the time of the TX pulse plus the blanking time to allow the coil to be nearly devoid of a field.

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« Reply #24 on: August 06, 2012, 12:00:58 pm »
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I'm starting to like this more and more as I read do go on guys  Smiley and could one of you nice gentlemen please tell me how must I work out the 3db down?  I know PI not VLF.

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« Reply #25 on: August 06, 2012, 12:40:03 pm »
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Quote:Posted by homefire
I guess being a RF Wiener, I'm use to the tickle of the Tuned Circuit that We Used.

Yes, indeed. This is where the misconception comes in regarding resonance versus de-tuning for purely magnetic fields versus radio frequency electromagnetic fields.

Posted on: August 06, 2012, 08:26:37 PM
Quote:Posted by xavier
I'm starting to like this more and more as I read do go on guys  Smiley and could one of you nice gentlemen please tell me how must I work out the 3db down?  I know PI not VLF.

You're part way there if you just build it to be in resonance. You see, there's a difficulty in determining the effect of the circuit regarding the resistance that develops from the coil wire. Always has been. You can measure the resistance with a meter and get a value that increases ever so slowly as time passes. This is because the wire itself heats up a bit during the measurement. When you apply a very fast AC pulse, the wire heats and cools rapidly causing a varying resistance. Also, the capacitor offers an additive series resistance to the circuit. Thus, the resonant point is a dynamic, ever changing point that is very hard to calculate.

To get the perfect 3db point, set up a circuit to measure the frequency response across a receiver coil which is tuned to the frequency ou wish to generate. Then, using your sound card analyzer, set it up for a FR curve and look at it. The bell curve will have a midpoint at the frequency you desire on the receiver side. Compare that to the same curve on the TX side and note where that same RX frequency occurs on the TX curve. If it is 3db or greater without exceeding 6db, you're golden.

Good luck with this one, Xavier. This is the most complex tasking you have had yet. And, surprisingly, the least consequential. This is fine tuning stuff here, not make or break theory. You see, normally the internal AC resistance (not at all the same as DC resistance) within your tuned circuit is just about right for the 3db point. A serendipitous happenstance. In spite of all this discussion of theory and etcetera, a field will be developed regardless. My comments were mostly to do with optimizing that field, not simply in generating one.

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« Reply #26 on: August 06, 2012, 01:26:41 pm »
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OK well all this discussion led me to go to Google and do some research and I found some nifty stuff about coils and measuring db's  I'd love to post it but it's 980Kb and even if I zip it, it will be too big. But your explanation is great and I will make some sort of set-up to do this and remember I did buy a scope so now I have the best of both worlds LOL I actually think that this is going to be quite an exciting experience. Thanks now I know what I'll be doing this long weekend .   

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« Reply #27 on: August 06, 2012, 01:46:08 pm »
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Save your self a Headache and remember that DB is just a way to  measure Power using Ratio's.

3db is about  twice or 1/2 power.   



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Offline GoldDigger1950
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« Reply #28 on: August 06, 2012, 04:30:34 pm »
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Quote:Posted by homefire
Save your self a Headache and remember that DB is just a way to  measure Power using Ratio's.

3db is about  twice or 1/2 power.

In the use of decibels in frequency response, it's also an offset and it is quite more than half of the slope.

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« Reply #29 on: August 07, 2012, 02:47:47 am »
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Quote:Posted by homefire
Save your self a Headache and remember that DB is just a way to  measure Power using Ratio's.

3db is about  twice or 1/2 power.


There are a few reason's why I want to do this. Firstly to prove to myself that I can, secondly to gain a better understanding on the Smith chart. I also want to see if there will be a significant improvement on the functionality of the machine, will there be a power saving? will sensitivity increase and if so by how much? how will it affect the stability of the machine? will discrimination be improved?  If you take a bad machine and place an excellent coil on it the chances are that you will end up with a fair machine or even a good machine. 

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