Is this a major advance in PI sensitivity ??

[moodz]

More experimentation has revealed the circuit configuration that gives high level signal changes for small targets in the coil field.

The signal is stable in time and amplitude and the configuration is a actually a simplified version of my original circuit which was "over protected" with diodes. The coil can be moved around etc and returned to same position so it is not some weird resonance effect etc.

There is now only a single clamping diode which has been beefed up to a SF18G ... see specs. The input balance resistors have been dropped to 500 ohms. A dollar coin placed in the centre of the coil generates a volt of wave form amplitude displacement in addition to an obvious phase / freq delta. The gain of the amp is 100.

I urge those with suitable test / measure equipment to build this circuit and confirm / repeat the results.

The GOOD news is that this is a significant signal to noise improvement for PI technology.

The BAD news is that this effect is in a 500 ns window at the base of the flyback response at the amp output so a dspic will probably not be able to extract this artifact and I will probably be looking at developing this further in my FPGA project with a fast ADC. Although you could probably do something with a peal hold circuit etc. Those with CROs should be able to verify on screen though.

Some very quick tests show that the 22cm 32 turn diff coil is much more sensitive .... it shows nearly 100 mv of signal at 1 meter for large targets like the metal side cover off my computer.

We need now for others to confirm the results, find the optimal circuit ( I only spent 10 minutes trying this ) and investigate if target descrimination info is also present.

happy new year from moodz.

The circuit .... simpler yet better. I have removed all the protective diodes except one and changed the balance resistors to 500 ohms.

The flyback pulse at ouput of amp. Note the stepped response now ....

Purple = NO target .... White = Australian $1 dollar coin placed in centre of coil. 1 volt of signal + phase shifts

[moodz]

Forgot to mention ...

My bad ... forgot to mention that I changed the switching mosfet to an FQP1N60 device to limit current draw during experiments .... so I am getting these results with less peak currents in the TX coil than I normally get with the IRF device.

Regards

Moodz.

[WM6]

The BAD news is that this effect is in a 500 ns window at the base of the flyback response at the amp output

]

What mean this limitations regarding to use your front end in connection with classical PI detectors?

Happy and successful New Year to you.

[moodz]

What mean this limitations regarding to use your front end in connection with classical PI detectors?

Happy and successful New Year to you.

The aquisition window of a conventional sample and hold in classical design would be too wide and timing accuracy might be an issue also. So it would either swamp the signal or miss it timing wise.

I am sure this is not an impossible problem ... if the CRO can capture it ...so can we.

The timing can be generated from the signal itself ... this will provide the capture pulse and phase shift information ... a circuit begins to form in my head now ....

best wishes.

moodz.

[WM6]

I urge those with suitable test / measure equipment to build this circuit and confirm / repeat the results.

I have measurement equipment, but those parts (as THAT1510, TC4421, FQPIN60, SF18G) are totaly unknown on my components stock list. I have to see first where to purchase those parts - if possible.

What are adequate FQPIN60 replacement? From memory I have IRFs 740, 640, 840 in my lab stock.

[moodz]

I have measurement equipment, but those parts (as THAT1510, TC4421, FQPIN60, SF18G) are totaly unknown on my components stock list. I have to see first where to purchase those parts - if possible.

What are adequate FQPIN60 replacement? From memory I have IRFs 740, 640, 840 in my lab stock.

I did some tests ...
IRF740 is fine .... I just use the FQPIN60 because it has 10 ohm on resistance to limit power.
A 1N4148 works OK for the damping diode ... it can handle 4 amp spikes. The SF is more rugged.
A 1n4004 works fine on the Mosfet switch.
You could try a fast diffamp based on an op-amp instead of the THAT1510 ... I will look into it.
The TC4421 could be replaced with discrete component gate driver.

moodz.

[WM6]

You could try a fast diffamp based on an op-amp instead of the THAT1510 ... I will look into it.

THAT1510 is not obtainable here.

I found by google this:

The THAT 1510 is a high performance audio preamplifier suitable for use in microphone preamp application. It is pin compatible with the now discontinued, Analog Devices SSM2019 and SSM2017 IC chips, and the Texas Instruments INA217 and INA163 mic preamp chips.

The SSL 9K probably has a very simular topology and Standard NE5532 opamps too.

[KingJL]

The input balance resistors have been dropped to 500 ohms.

The change of your balance resistors may give less noise, but they extend the flyback /decay appreciably. During the time that the flyback voltage is above the diode threshold, 1000 ohms is put in parallel with the damping resistor causing it to be significantly overdamped. I have been using 2.2K. Noise may be higher, but it still below any valid target signal. The noise with a 2.2k and a THAT1500(or INA217) is still below the typical single ended PI front-end. I guess the trade-off you accept is what suits your design goal.

There have been several enlightening discussions on this subject by bbsailor and others.

Regards,
J. L. King

[WM6]

The noise with a 2.2k and a THAT1500(or INA217) is still below the typical single ended PI front-end.

I ordered some parts (TC4421, INA217) over eBay. Two weeks of delivery in best case.

[J_Player]

The change of your balance resistors may give less noise, but they extend the flyback /decay appreciably. During the time that the flyback voltage is above the diode threshold, 1000 ohms is put in parallel with the damping resistor causing it to be significantly overdamped. I have been using 2.2K. Noise may be higher, but it still below any valid target signal. The noise with a 2.2k and a THAT1500(or INA217) is still below the typical single ended PI front-end. I guess the trade-off you accept is what suits your design goal.

There have been several enlightening discussions on this subject by bbsailor and others.

Regards,
J. L. King

Hi KingJL,
I have been wondering about what you mean when you say "design goal".
The majority of PI detector users are looking to find metal items that may weigh between 1 gram and 1 ounce.
But there are other PI users who are looking to find larger buried objects, such as large meteorites or large caches of metal treasures.
When you say "design goal", I am thinking you may be focused on the more common goal to find the smaller targets. Am I correct?

Best wishes,
J_P

[moodz]

The change of your balance resistors may give less noise, but they extend the flyback /decay appreciably. During the time that the flyback voltage is above the diode threshold, 1000 ohms is put in parallel with the damping resistor causing it to be significantly overdamped. I have been using 2.2K. Noise may be higher, but it still below any valid target signal. The noise with a 2.2k and a THAT1500(or INA217) is still below the typical single ended PI front-end. I guess the trade-off you accept is what suits your design goal.

There have been several enlightening discussions on this subject by bbsailor and others.

Regards,
J. L. King

Actually my design goal with this version of the circuit is to investigate the target signal at the base of the flyback. I have examined the input signal to the diff amp on a CRO and it is quite different from normal target decays. I am not sure how it is produced as simulations cannot reproduce it ...

regards,

moodz

[Tinkerer]

Driving the capacitors C6 +C7 with the TC4421, slows down the switch OFF speed considerably. This has a lot of influence on the Flyback and damping time.

I have been using a differential input preamp for some time now and am very satisfied with the results. However, it is not only differential, it also gives different results. It did take me quite a bit of experimenting with different input impedance, damping, and gains to find the mode that I am using now.

I don't think I will ever go back to a single ended preamp.

There are still many more things to try, for example I am looking for a good differential preamp that has not only differential input, but also differential output.

Can anybody recommend a low noise one?

Tinkerer

[WM6]

Driving the capacitors C6 +C7 with the TC4421, slows down the switch OFF speed considerably. This has a lot of influence on the Flyback and damping time.

Maybe we need to experiment with different lower value of C6 + C7.

[moodz]

Driving the capacitors C6 +C7 with the TC4421, slows down the switch OFF speed considerably. This has a lot of influence on the Flyback and damping time.

I have been using a differential input preamp for some time now and am very satisfied with the results. However, it is not only differential, it also gives different results. It did take me quite a bit of experimenting with different input impedance, damping, and gains to find the mode that I am using now.

I don't think I will ever go back to a single ended preamp.

There are still many more things to try, for example I am looking for a good differential preamp that has not only differential input, but also differential output.

Can anybody recommend a low noise one?

Tinkerer

I have to respectfully disagree with you tinkerer .... I did originally drive the bias generator off a separate 4421 dedicated for that purpose however in order to reduce the component count I measured the impact on the fall time to the gate and it is approximately 5 ns ..... given the modest design goals of the Model T .... 5 ns is neither here nor there. The 4421 is a surprising beast in terms of drive capability .... if it had a higher voltage rating it would make an excellent coil driver in its own right.

Here is a nice diff amp for you.
http://www.thatcorp.com/1570_Low-Noi...fier_ICs.shtml

regards moodz,

[Tinkerer]

moodz,

thanks for the link. That seems to be just what I was looking for.

About C6 and C7, I dont't see any values on your schematics, just guessed at a few tens of nF and looked up the datasheet. The TC4421 indeed has lots of power and you have paralleled the 2 outputs. 5ns is nothing. Good show!!!!

I found that in a situation like you show on the scope, it helps to add a small feedback capacitor. It reduces the band width and the signal amplitude, but it also integrates the signal and gives more time to sample it.

For the discrimination you must look for THE PIVOT. This is what gives the information about the characteristics, FE, foil and volume of the targets.

All the best

Tinkerer