Coil calculations, finding RX value

[kliner]

I need a liitle help from the experts here. I have been playing around making coils for different projects on this forum, there is no problem choosing the Tx frequency or the number of turns required for a given inductance .
The problem for me comes with choosing the Rx frequency. Without thinking too long it would seem to be a good idea to have it resonate with the Tx, but this seems never to be the case. So it resonates at another frequency but which, is it a harmonic?. Some makers choose 1.19 times the Tx (Tesoro) some even choose less than the Tx, are these values plucked out of the air or is there some scientific way of finding them.

[Carl-NC]

There are 2 reasons for not resonating the RX coil at the TX frequency. First is that the phase response is very non-linear at resonance, making target ID more difficult. Second is that at resonance slight variations in L or C make a big difference in response. So most manufacturers resonate the RX off of the TX frequency, usually higher, usually 2x to 5x.

[mikebg]

There are 2 reasons for not resonating the RX coil at the TX frequency. First is that the phase response is very non-linear at resonance, making target ID more difficult. Second is that at resonance slight variations in L or C make a big difference in response. So most manufacturers resonate the RX off of the TX frequency, usually higher, usually 2x to 5x.

Kliner, please do not use the misleading information given by Carl Moreland (because he hates to receive information from this frorum and wants to keep secret the design processes of White's :-).
The phase response is linear in the region of resonace frequency, but it is very steep, which makes phase instability. For best results, White's uses LOWER frequency for RX tank circuit. To determine resonance frequency of RX tank, simply multiply TX frequency to 0,75:
http://www.geotech1.com/forums/showthread.php?t=17949

[Geo]

Before years i checked the coil of MD5002 and it was as Carl says. Rx coil was tuned at 3x the Tx frequency.
From the other side the Bluemax coils are as Mike wrote....

So, who is the best solution????
I believe the Bluemax

[golfnut]

these numbers dont sit accurately with the IGSL. That case ~14kHx Tx ~16kHz Rx.

many ways of skinning a cat, none of them wrong.

[kliner]

Well thanks for all the information guys, a very interesting thread that you posted the link to mikebg, it seems that there is a bit of poetic license involved but 0.75 x the TX value seems like a good place to start!

[Carl-NC]

Kliner, please do not use the misleading information given by Carl Moreland (because he hates to receive information from this frorum and wants to keep secret the design processes of White's :-).
The phase response is linear in the region of resonace frequency, but it is very steep, which makes phase instability. For best results, White's uses LOWER frequency for RX tank circuit. To determine resonance frequency of RX tank, simply multiply TX frequency to 0,75:
http://www.geotech1.com/forums/showthread.php?t=17949

You're right, "nonlinear" isn't the best word here; "unstable", "highly variable", etc. On some White's models the RX coils are tuned to the high side (Classics, Prizms) and on some models they are tuned to the low side (MXT, GMT). I doubt it makes any difference.

[simonbaker]

I need a liitle help from the experts here. I have been playing around making coils for different projects on this forum, there is no problem choosing the Tx frequency or the number of turns required for a given inductance .
The problem for me comes with choosing the Rx frequency. Without thinking too long it would seem to be a good idea to have it resonate with the Tx, but this seems never to be the case. So it resonates at another frequency but which, is it a harmonic?. Some makers choose 1.19 times the Tx (Tesoro) some even choose less than the Tx, are these values plucked out of the air or is there some scientific way of finding them.

It will depend completely on the circuit the coil is connected to. It does seem that "on-resonance" designs are probably the devil to calibrate, so the off-resonant designs seem to be favored.

But basically each project will specify the RX inductance or desired resonant frequency, because the rest of the circuit will depend on a certain phase relationship (for VLF designs).

If you are designing your own MD from scratch, then you can choose any relationship you want as long as you know what you are doing, as Carl indicated.

One way to get a feel for it is to use LTSpice or a calculator to plot the frequency/phase response of an RLC tank (resistor, coil, capacitor) connected as in your MD circuit. The "R" will be the equivalent resistance of your coil, which actually could vary with frequency). You will see how the phase of the response relates to the TX frequency (and the Q of your tank circuit). You will also see how the gain of the response relates to how far you are off resonance. That "1.19" factor is a reasonable compromise where you have very little phase shift due to resonance and still enough gain. But that does not preclude other designs.

Regards,

-SB

[mikebg]

SPICE of RX input
RX coil operates as Second Order Lowpass Filter.
The old metal detectors of White's operate at relative frequency almost 1.3, but the new one operate at almost 0.5. The incompetent designed metal detectors operate at relative frequency 1, where the ground causes significant parametric amplitude and angle modulation of received signals.

[golfnut]

confused here still, let me think aloud..apologies.. (IGSL)



1) The target signal to be received is the same frequency as the transmited signal ?


(I think it is.... we dont get a spectrum from a target ? ?? Do we??)


---------------------------------
If Ans is yes from 1), then..

a)Rx input LC tank - gets the Tx 'frequency' re-radiated from a target - but very small amplitude with a phase shift relative to outgoing Tx signal. (We also add to the phase shift with motion.)?




b)Tuning the LC on the RX front end away from the Tx frequency will...

1)Stop the Rx tuned LC front-end sucking energy direcly from Tx LC circuit (if both were exact same frequency Tx would ring the Rx tank)

In this case, the leaked Tx signal would swamp the smaller return signal.
(not an issue if the Rx had large dynamic range) - but there would be two phase components to deal with - the high amplitude leakage signal and the target return - with its small amplitde and different phase.

2)Off channel Rx LC front end attenuates the received taget signal - as signal is down the skirt of the simple filter.

But gets away from the mutual resonance coupling.

Apologies for the rant - I still dont get it!

Steve

[simonbaker]

confused here still, let me think aloud..apologies.. (IGSL)



1) The target signal to be received is the same frequency as the transmited signal ?


(I think it is.... we dont get a spectrum from a target ? ?? Do we??)


---------------------------------
If Ans is yes from 1), then..

a)Rx input LC tank - gets the Tx 'frequency' re-radiated from a target - but very small amplitude with a phase shift relative to outgoing Tx signal. (We also add to the phase shift with motion.)?

Yes. Motion does not make additional phase shift I'm pretty sure. The phase shift of the target is partly due to the target and partly due to the RX resonant circuit. The LF353 pre-amp seems to shift another 20 degrees typically in the TGSL.

b)Tuning the LC on the RX front end away from the Tx frequency will...

1)Stop the Rx tuned LC front-end sucking energy direcly from Tx LC circuit (if both were exact same frequency Tx would ring the Rx tank)

That's something to think about. Of course "nulling" is supposed to minimize the interaction, but the null signal is good and strong as we know. Not sure if in the end it would help or hurt, since the target signal would also ring the RX tank, but at a different phase. Definitely a good point and a simulation would be useful.

In this case, the leaked Tx signal would swamp the smaller return signal.
(not an issue if the Rx had large dynamic range) - but there would be two phase components to deal with - the high amplitude leakage signal and the target return - with its small amplitde and different phase.

Not sure if really different from the off-resonant design, where the "null" signal swamps the target signal anyway. I always assumed that the ratio of "null" signal to target signal doesn't change much on or off-resonance, but that maybe you are knocking down the noise better in the nearby frequencies with on-resonance design. However, need to do the math to see.

2)Off channel Rx LC front end attenuates the received taget signal - as signal is down the skirt of the simple filter.

But gets away from the mutual resonance coupling.

Apologies for the rant - I still dont get it!

Steve

It is a good issue, since some of us are curious about experimenting with on-resonant designs. Phase stability is the real nasty part of on-resonant designs. Tweak a component value and the phase can change a lot.

Cheers,

-SB

[golfnut]

I hazard a guess that the Rx coil does not have to resonate with a cap 1x, 2x, or Nx the Tx frequency.

The fact that its 'receiving' should be good enough. Anyone up for taking their cap off to see impact.

S

[simonbaker]

I hazard a guess that the Rx coil does not have to resonate with a cap 1x, 2x, or Nx the Tx frequency.

The fact that its 'receiving' should be good enough. Anyone up for taking their cap off to see impact.

S

I agree. Taking cap off will drop gain some I think, you'll probably will pick up more low freq noise, may or may not matter, and you'll lose a few degrees phase shift. But could still function.

-SB

[kliner]

If you change the RX capa value significantly the voltage on the RX side goes right up and the detection depth is reduced.
When you look closely the TX is supposed to be at resonance, but I'll bet by the time you take into account the tolerance of the components and the hand wound coils not strictly at the right inductance, (not talking about temperature changes) the frequency is probably a fair way away. Does this matter? it surely doesn't help matters. Perhaps we should look more closely at tuning and stabilising our TX circuits. Perhaps this is one of the reasons we often fall short of the professionally built detectors.
One of the advantages we have with respect to the pros is that we can afford to spend a lot of time tuning and matching our coils to our detectors.

[simonbaker]

When you look closely the TX is supposed to be at resonance,...

Hi kliner:

Would you clarify what you mean by "TX is supposed to be at resonance"?

Regards,

-SB