New Concentric Co-planar Coil with Strong MF.

[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!

[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).

[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]

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]

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]

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.

[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]

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.

[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

[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

[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