signal strength

[Tinkerer]

(With a gain of about 10, the signal of a silver dollar at the center of the coil is about 1.2 volts.)
When I look at the two traces I see about a half division offset. Is this what you are calling 1.2 volts? What would the voltage be if the silver dollar was at the diameter of the coil 260mm?

I can not show you on the scope, directly. The signal undergoes some processing, sampling etc. after the preamp, so there is no more direct proportion. But we can get an approximation by calculating the coil field strength.

I attach the field calculator link and a screenshot of the coil numbers.
I took the medium diameter of the coil at radius 12.5cm and then the distance of the diameter of the coil, x 2, because the signal emitted from the silver dollar target will have to travel that same distance to the RX coil.

The coil, when taking the reading of about 1.2V, as running at 5A peak TX current.

The coil has 26 turns, therefore we input 5x26=130A.

The coil field strength at the coil center is 6.534G

At a distance of 50cm, the coil field strength is 0.093, or about 70 times less.

Therefore, the signal amplitude from the silver dollar at a distance of 25cm from the coil will be about 120mV/70=1.7mV
Magnetic Field of a Current Loop

[mikebg]

(With a gain of about 10, the signal of a silver dollar at the center of the coil is about 1.2 volts.)
When I look at the two traces I see about a half division offset. Is this what you are calling 1.2 volts? What would the voltage be if the silver dollar was at the diameter of the coil 260mm?

I prefer gold dollar, but if the diameter of silver dollar is as diameter of the coil, you can buy with such coin several metal detectors.

The conductive ground (salt water) generates strong GND signal because eddy currents in the halfspace flow in loop with such diameter.

http://www.geotech1.com/forums/showthread.php?19942-X-vs-R&p=163752#post163752

http://www.geotech1.com/forums/showthread.php?14005-Targets-frequency-response&p=104462#post104462

That's why sometimes it's better to use TX coil with a smaller diameter.

[green]

(With a gain of about 10, the signal of a silver dollar at the center of the coil is about 1.2 volts.)
When I look at the two traces I see about a half division offset. Is this what you are calling 1.2 volts? What would the voltage be if the silver dollar was at the diameter of the coil 260mm?

[Tinkerer]

signal amplitude

Maybe this helps:

I was just balancing a coil and have the signal available at the output of the preamp.

With a gain of about 10, the signal of a silver dollar at the center of the coil is about 1.2V.

This means the actual signal is about 120mV

The coil is about 260mm outer diameter.

I add the pictures below:

First picture: output of the preamp, no target.

Second picture: output of the preamp, with a silver dollar at the center of the coil.

Look at the changes produced in the signal shape, by the target.

Enjoy

Tinkerer

[green]

My error. I was reading about the HH 11. Stated gain of 1000 followed by 2 stages with gains of 100 each. Most appears to be low frequency gain. Is the required gain that low because of the ground signal being that high or the Tx pulse.

[Carl-NC]

S, gain 10 to 20dB of preamp is too small value. Note what gain has this preamp:

Depends on the design. My current design has a 2-stage preamp, each 20dB. Any more than that screws things up.

[Carl-NC]

When searching posts I see amplifier gains ranging from less than 10000 to 10 million. Looking for ball park numbers. Signal strength for a one inch and two inch diameter target for a 10 inch diameter coil at 10 inches. Ground signal strength range for a 10 inch coil. What to use to simulate ground when bench testing.
thanks green

Green, there is no answer for a signal strength number, way too many variables. The "amplifier gains" you cite appear to be open-loop gains, not closed-loop. Detectors use anywhere from 10 to 1000 (closed loop) depending on the design. Simulate ground using a ferrite board (plywood with small ferrites glued on).

Another question. With a PI detector what is meant by (going to zero volts)? I calculate about 18 TC to decay a 400 volt pulse to 10 uv with a LR coil. Does a damped LCR coil decay faster?
green

PI flyback decays toward zero volts, but never actually gets there. Adding C always slows down critical damping.

[mikebg]

Often preamps have a gain in the range of 10 and 20dB.

S, gain 10 to 20dB of preamp is too small value. Note what gain has this preamp:

[green]

Another question. With a PI detector what is meant by (going to zero volts)? I calculate about 18 TC to decay a 400 volt pulse to 10 uv with a LR coil. Does a damped LCR coil decay faster?
green

[golfnut]

Often preamps have a gain in the range of 10 and 20dB.

These gains are chosen to fit in with the rest of the system, often 2 subsequent gain stages post demod with a further 50dB gain.

So total system gain approx 60dB.


If you have too much gain, downwind stages will compress and hit the rail havind gone non linear.

Often a log function is incorporated into the last stage to give a greater dynamic range.

Other possibilities are automatic gain control systems, reducing gain for large targets. This can be done by reducing Tx power during large return. Similarly the Tx power could be held the same and the Rx gain reduced.

S

[satdaveuk]

Green, when search head is lifted far from ground, there is an large AIR signal in RX input. It is generated by mutual inductance when search head has separate TX and RX coils, or by self-inductance at Monocoil search head.

When search head is lowered close to ground, a GND signal is added to AIR signal and there is an AIR&GND signal in input. Both signals are very large relative to TGT signal from small and deep targets. The AIR&GND signal limits possible gain of preamp. Gain 1000 or 60dB is the maximum.

To avoid saturation of preamp, some designers use very low gain. Here is an example with gain less than 9:

http://www.geotech1.com/forums/showt...995#post165995

Very well put Mike
Basically the ground is one very very large target that is and always has been the stumbling block of many a designer and manufacture thats why unless your searching for UFOs

[mikebg]

Green, when search head is lifted far from ground, there is an large AIR signal in RX input. It is generated by mutual inductance when search head has separate TX and RX coils, or by self-inductance at Monocoil search head.

When search head is lowered close to ground, a GND signal is added to AIR signal and there is an AIR&GND signal in input. Both signals are very large relative to TGT signal from small and deep targets. The AIR&GND signal limits possible gain of preamp. Gain 1000 or 60dB is the maximum.

To avoid saturation of preamp, some designers use very low gain. Here is an example with gain less than 9:

http://www.geotech1.com/forums/showthread.php?16793-Frequency-Domain-and-Time-Domain-relationships&p=165995#post165995

[green]

When searching posts I see amplifier gains ranging from less than 10000 to 10 million. Looking for ball park numbers. Signal strength for a one inch and two inch diameter target for a 10 inch diameter coil at 10 inches. Ground signal strength range for a 10 inch coil. What to use to simulate ground when bench testing.
thanks green