New Concentric Co-planar Coil with Strong MF.

[simonbaker]

Ah, you'd be surprised. It was introduced with digital receivers because of their sampling nature and aliases all over the place. True, there is not much in common with them and MDs, but there is a common behaviour regarding aliases and sampling nature of synchronous detectors. It was on my mind at that moment. I'll try not to use such abbreviations any more.

Thanks for the explanation. I agree IBOC concept is relevant to synchronous detectors. It seems SDs act a little like sample & hold samplers, so maybe the out-of-band detection power is quite low for high-frequency EMI; but is ELF (extremely low frequency) more of a problem (I guess there aren't many 1 Hz sources) ??? We know 50/60 Hz power lines have a major effect; and I know that EMI in my workshop obliterates any depth measurements I try to make.

It seems more secure to have a tuned RX coil, but certainly we should be open to other ideas. I've always wondered about the off-resonant designs which receive maximum power (noise) off to the side -- but they seem to work quite well I must say.

Anyway, we'll be interested in your designs and how they compare.

-SB

[simonbaker]

Hi all,

we don't need impedance matching here. We aren't transferring energy, we are just picking up a RX voltage with very low bias current (ultra tiny energy transfer only).

For best noise performance, just use a low impedance bipolar input front-end. A lot of the bipolar low noise op-amps are doing well.

BTW, the coil will pickup more noise.

My 2 cents
Aziz

Yes, but I think of it as sort of the same idea -- instead of converting a current to a voltage through a noisy resistor, you just directly amplify the current itself with a current amplifier (bipolar junction). At some point you may go to voltage amplification, but eventually you have to return to power to make a beep, so perhaps best to just keep current amplification all the way. Agreeing with you I think.

On the other hand, hi-Q resonant tanks seem to make a high voltage on their own (as Davor says, a kind of impedance matcher), so they seem a good fit for a low-noise JFet front end.

-SB

[Aziz]

Hi all,

we don't need impedance matching here. We aren't transferring energy, we are just picking up a RX voltage with very low bias current (ultra tiny energy transfer only).

For best noise performance, just use a low impedance bipolar input front-end. A lot of the bipolar low noise op-amps are doing well.

BTW, the coil will pickup more noise.

My 2 cents
Aziz

[Davor]

Ah, you'd be surprised. It was introduced with digital receivers because of their sampling nature and aliases all over the place. True, there is not much in common with them and MDs, but there is a common behaviour regarding aliases and sampling nature of synchronous detectors. It was on my mind at that moment. I'll try not to use such abbreviations any more.

[Jerry]

Ok - I don't completely understand your design goals, but maybe your final circuit will make it clearer. Look forward to what you discover.

What is IBOC?

-SB

IBOC pertains to digital broadcasting. Have never seen it used in relation to metal detecting.

http://www.ibiquity.com/hd_radio/iboc_white_papers

Jerry

[Davor]

IBOC= in band on channel, as different from all aliases. RF jargon.

I think I know the goal, sort of. It is "normal" to seek for some angle between Tx and Rx with semi-resonant Rx, and it is usually between 10° and 20°. If I use input impedance ~15 times lower than in a case of a resonant tank I obtain ~20° shift. Well, it is not too low. So instead of common base I can go with a common emitter or a somewhat better opamp instead.
In case I use an Rx coil with more inductance, like for example a Musketeer coil, I'm well within an opamp ball game, and I SHOULD go with higher resonance just to stay within the 20° shift goal.

A short summary: If I connect a coil to a low impedance amplifier input, I'll obtain the same S/N as if I use some kind of impedance transformation and a high input impedance amplifier. Resonant tank IS a way of impedance transformation, and in case of poor component choice it provides you with more pain than joy. Hence, a low impedance method will provide more joy because it will be much easier to set up, and it will tolerate great margins of error in winding of Rx coils.

[simonbaker]

I don't expect to see much of aliases, at least not compared to normal IBOC noise. Sinc. detectors pick very narrow band stripe in VLF, so any additional noise will be of the same nature: not related to Tx or target. Besides, I can rely on feedback capacitor to take care of that. Anyway, I did not decide on optimum configuration yet. Maybe I'll keep a somewhat smaller capacitor in parallel to take care of the Tx harmonics and noise, with resonance at 1.5 Tx or something.

I'll experiment with this setup a bit. I have everything ready for ~6k8 true differential input impedance, and when I put a 10k trimmer in parallel I'll have it ready for testing. My goal will be the lowest impedance that works right, and I'll extrapolate from there. I expect to see some influence on GB, so when I hit the position where GB does not work right - I overdid it.

Optimum power transfer is a kind of goal here because it already happens with standard solution using a semi-resonant tank. Using low impedance is just a way to mimic it by non-resonant means. Real goal will be a bullet-proof solution for multiple frequencies.

Ok - I don't completely understand your design goals, but maybe your final circuit will make it clearer. Look forward to what you discover.

What is IBOC?

-SB

[Davor]

I don't expect to see much of aliases, at least not compared to normal IBOC noise. Sinc. detectors pick very narrow band stripe in VLF, so any additional noise will be of the same nature: not related to Tx or target. Besides, I can rely on feedback capacitor to take care of that. Anyway, I did not decide on optimum configuration yet. Maybe I'll keep a somewhat smaller capacitor in parallel to take care of the Tx harmonics and noise, with resonance at 1.5 Tx or something.

I'll experiment with this setup a bit. I have everything ready for ~6k8 true differential input impedance, and when I put a 10k trimmer in parallel I'll have it ready for testing. My goal will be the lowest impedance that works right, and I'll extrapolate from there. I expect to see some influence on GB, so when I hit the position where GB does not work right - I overdid it.

Optimum power transfer is a kind of goal here because it already happens with standard solution using a semi-resonant tank. Using low impedance is just a way to mimic it by non-resonant means. Real goal will be a bullet-proof solution for multiple frequencies.

[simonbaker]

Actually somewhere in between. Noise is not my primary concern. Typical solution nowadays uses an opamp amplifier with moderate to high input impedance, connected as a typical LF radio frontend with resonant frame antenna, and to avoid phase problems it is detuned. My idea is using lower Zin that would get close to the coil reactance and squeeze as much juice there. So instead of impedance transformation by means of a resonant tank, I'd go directly where juice is produced - at low Z. Also to avoid E field problems I think the best choice would be a true differential amplifier, a low Zin brother of an instrumentation amplifier.

True differential amplifier is neither Zin>>infinity nor Zin>>0 like the solutions in your picture, but Zin = Zin (inv) + Zin (noninv) where Zin (inv) = Zin (noninv)

Picking correct resistor values to set up an opamp to be a true differential is not trivial, but reasonable choice is possible. So instead of R1=R3=6k8 and R2=R4=330k you may use R1 and R2 as they are (inverting branch) while in noninverting branch use 68ohm and 3k3. This combination provides less than 2% Zin (inv) against Zin (noninv).

But would the wide-band coil receiver cause the Synchronous Detector to pick up a lot of aliased wide-band noise?

Also, the appeal of the tuned RX is the potentially high-Q which should boost S/N -- but maybe the SD is so narrow band it doesn't matter (except for the aliased stuff). Have to think on that.

Is your idea basically impedance-matching the amp to the coil for optimum power xfer?

-SB

[Davor]

Ok, sounds like your talking about minimizing noise, different topic from "on-resonant" design phase-shift problems.

I'm guessing that you are comparing these two configurations for noise?

-SB

Actually somewhere in between. Noise is not my primary concern. Typical solution nowadays uses an opamp amplifier with moderate to high input impedance, connected as a typical LF radio frontend with resonant frame antenna, and to avoid phase problems it is detuned. My idea is using lower Zin that would get close to the coil reactance and squeeze as much juice there. So instead of impedance transformation by means of a resonant tank, I'd go directly where juice is produced - at low Z. Also to avoid E field problems I think the best choice would be a true differential amplifier, a low Zin brother of an instrumentation amplifier.

True differential amplifier is neither Zin>>infinity nor Zin>>0 like the solutions in your picture, but Zin = Zin (inv) + Zin (noninv) where Zin (inv) = Zin (noninv)

Picking correct resistor values to set up an opamp to be a true differential is not trivial, but reasonable choice is possible. So instead of R1=R3=6k8 and R2=R4=330k you may use R1 and R2 as they are (inverting branch) while in noninverting branch use 68ohm and 3k3. This combination provides less than 2% Zin (inv) against Zin (noninv).

[Wiltran]

coil

the cable used is a 2x2 pair screend cable, the ground connection for the receiver is made in the coil, this is needed this way for the twisted pair effect, .. the black wire is not used.. prevents earthloops...

If the small coils is picking up a much to large signal... just turn it upside down......if it needs to lift up to get a null.. there are to many turns , if it needs to be moved to the large coils for a null ... some more windings are needed.

Hope it is all ok ..... but if needed corrections are welcome...

perfect, thanks AP!

[ApBerg]

Apberg, would you please draw a schematic of this coil?

the cable used is a 2x2 pair screend cable, the ground connection for the receiver is made in the coil, this is needed this way for the twisted pair effect, .. the black wire is not used.. prevents earthloops...

If the small coils is picking up a much to large signal... just turn it upside down......if it needs to lift up to get a null.. there are to many turns , if it needs to be moved to the large coils for a null ... some more windings are needed.

Hope it is all ok ..... but if needed corrections are welcome...

[Wiltran]

CC coil

Hello Dave,

Made some photo’s from the coilwindig forms I used… they are from late 1980’s …so no computer software then, but the made coils worked fine.

To make a CC coil :

Start with the transmit coil, as seen on the large 22,5 cm form 97 turns give’s 14,5 KHz, (114 turns is 12.5 KHz.) Wire used is 0.25 mm and the coil is connected to the Tesoro transmitter so the cap from the oscillator has also influence at the transmit frequency !


The first receive coil, 18 turns, is added at the 22.5 cm coil, same wire and turn direction . Transmit coil and first receive coil both clockwise !

This 18 turns will , just like in a transformator, pick up a part from the transmit signal .
If the transmit signal is 15 volts then there is 15volt /97turns = ca. 0.15 volt per turn wire so the first receive coils 18 windings pick up 18x0.15 volt = 2,7 volt.

Now the trick is to null out this 2,7 volt … this must be done by the second small receive coil, As the distance to the transmit coil is greater the magnetic field is smaller, so less volts per winding.. so more turns are needed to get that 2,7 volt. AND that voltage needs to be in opposite faze , so the windings for the second receive coil have to be counter clockwise !
For a 10-12 cm coil form some 160-170 turns are needed, also 0.25 mm wire. The two receive coils are connected in series but picking up the volts in opposite faze's so the result is zero.

If you have to move the small receive coil towards the transmit coil (stronger magnetic field) then you have to add windings at the small coil….if you have to lift up the small receive coil (less magnetic field) than you have to reduce windings…

For the receive capacitor look at the first opamps output… select the cap for a nice sinus out.


Good luck !

Ap

And yes... some software to adjust the stuff....hmmm but we do not have that... so...here is the info I have... it works but it needs some experiments to get it right.. winding strength diameters from the coils.... all make’s differents... and I hope I have not made errors...

Apberg, would you please draw a schematic of this coil?

[Carl-NC]

There is a nice application note explaining the subject of input impedance and choice of opamp: http://cds.linear.com/docs/Design%20Note/DN355f.pdf

From the app note:

"For low REQ, the op amp voltage noise dominates (as VN is the remaining term);"

This is true.

"for very high REQ, the op amp current noise dominates (as IN is the coefficient of the highest order REQ term)."

This is true.

"At middle values of REQ, the resistor noise dominates and the op amp contributes little significant noise."

This is not necessarily true, and may be utterly false. Depends on the opamp.

"This is the ROPTIMUM of the amplifier and can be found by taking the quotient of the op amp’s noise specs: VN/IN = ROPT."

Again, possibly utterly false. I think the author looked at a few opamps which met this criterion, and he created a rule-of-thumb. Vn/In is generally meaningless.

- Carl

[simonbaker]

Both parallel or series resonance are the same phenomenon, but the way you take a signal off it makes it series or parallel. In series you see a very low impedance, ideally only parasitic resistances of the tank components, and you are basically measuring current that passes through. Equivalent circuit of an ampere meter is a short circuit. Simultaneously you may measure voltage across a coil and from that point of view it is a parallel resonance tank. In both instances a coil has some voltage across and some current going through. As the voltage is a function of magnetic flux, you are better off if you allow current to flow - thus maximizing power transfer. To ensure current flow you may either connect a capacitor to a coil and achieve resonance, or you may connect it to a low impedance preamp.

Because of the phase problems in resonance, I think the way around it is using a low impedance preamp. If you go too low you'll have some phase shift, but uniformly so, and not a step function as in resonance.

High side of low impedance is also a low noise. Optimum input impedance of an opamp is calculated from voltage and current noise (Vn and In -> Ropt=Vn/In). With NE5534 you get Ropt at ~9kohm, OPA134pa would have Ropt ~ 1800ohm, while with LT1115 you get Ropt at ~750ohm. Hence, LT1115 is better for low Z operation, and NE5534 is best for quasi resonant input. Option B would be using a common base or gate preamp, or some impedance transformer -just like pre-pre for MC phono.

Check out Hiraga pre-pre for ideas on low input impedance
http://www.bonavolta.ch/hobby/en/audio/prepre.htm

You may play with LTspice simulation above for all cases mentioned here. Try terminating a non-resonant branch with, say, 100 ohm instead of 4k7 and see what happens. Your criteria are voltage and phase. As the coil voltage is a function of flux, unless you go with input impedance below the coil reactance your coil voltage will be the same.

Ok, sounds like your talking about minimizing noise, different topic from "on-resonant" design phase-shift problems.

I'm guessing that you are comparing these two configurations for noise?

-SB