THE ELUSVE 1us TC TARGET

[ivconic]

[ivconic]

Click the mouse to enlarge the image.
The situation from the previous post is described in several photos.
...
And this is the typical response of a VLF I/B detector on about 85% of such surfaces.
In the video is XP Deus with HF22.5cm coil. (New coil, so I wouldn't scratch it; I wrapped it in nylon).
Keep in mind that the XP Deus is a really impressive detector, with impressive capabilities, especially with HF coils.
At the same time, know that the GEB was carefully performed before this recording.
(Tony I hope this is enough information so far about the type of detector needed?)

[Tinkerer]

any questions you might have about = can you increase gain of U2 and remove U1 a/b ?

Read carefully what Carl wrote in post #6
Try different opamps. For now, I consider as the best ADA4807, but there may exist better ones that I have not tried yet.

[Tinkerer]

Click the mouse to enlarge the image.
The situation from the previous post is described in several photos.
...
And this is the typical response of a VLF I/B detector on about 85% of such surfaces.
In the video is XP Deus with HF22.5cm coil. (New coil, so I wouldn't scratch it; I wrapped it in nylon).
Keep in mind that the XP Deus is a really impressive detector, with impressive capabilities, especially with HF coils.
At the same time, know that the GEB was carefully performed before this recording.
(Tony I hope this is enough information so far about the type of detector needed?)

OK, I understand.

[Carl-NC]

Hi Carl,

Please, comment this text from the book for engineers:
So, Schottky diodes do not have reverse recovery time because they do not have anything to recover from. However, the vacuum is effectively acting as a dielectric in one direction, so there is some small amount of parasitic capacitance. The reverse current seen in Schottky diodes is not actually reverse conduction, but merely a capacitive discharge. This is why Schottky's are said to have 'soft' recovery, as the curve is really just a capacitor discharge curve, and that takes time. But it is not 'on' and allowing reverse current flow. All the current flowing in reverse is due to energy stored capacitively from the diode itself.

When it comes to the clamp diodes, the TX FET, or the preamp, I have found data sheets to be minimally useful. The only way to know if something will work well is to try it. For the clamp diodes, I have tried all sorts of Schottky and "ultrafast" diodes but I always come back to the 1N4148.

[Carl-NC]

any questions you might have about = can you increase gain of U2 and remove U1 a/b ?

You can, but often a 2-stage preamp settles faster than a 1-stage preamp having the same overall gain. Also, in this case, the 1st stage is fully differential. See the discussion in the AMX RX thread.

[ivconic]

Read carefully what Carl wrote in post #6
Try different opamps. For now, I consider as the best ADA4807, but there may exist better ones that I have not tried yet.

I just checked in JLPcb database, it is affordable, providing that they do the assembly:



Mouser is more expensive:

[ivconic]

When it comes to the clamp diodes, the TX FET, or the preamp, I have found data sheets to be minimally useful. The only way to know if something will work well is to try it. For the clamp diodes, I have tried all sorts of Schottky and "ultrafast" diodes but I always come back to the 1N4148.

Well done!
The first serious engineer who has the courage to publicly declare such a thing!
I have been criticizing and mentioning similar cases for years.

[Tinkerer]

OK, I understand.

Lets see if I understand right.

You want a small coil that gives you over 20cm depth on a small silver or gold coin.
You need iron or magnetic differentiation for ceramics and small rusty iron.
You prefer a non-motion detector because the rocks and vegetation hinder a normal swing.
Is that it?

[ivconic]

Lets see if I understand right.

You want a small coil that gives you over 20cm depth on a small silver or gold coin.
You need iron or magnetic differentiation for ceramics and small rusty iron.
You prefer a non-motion detector because the rocks and vegetation hinder a normal swing.
Is that it?

Yes, that's it in short.

[Auto-Mation-Assist]

When it comes to the clamp diodes, the TX FET, or the preamp, I have found data sheets to be minimally useful. The only way to know if something will work well is to try it. For the clamp diodes, I have tried all sorts of Schottky and "ultrafast" diodes but I always come back to the 1N4148.

I have done the same thing over the years and mostly ended up using ultrafast diodes. But these are not always the best for all circuit designs because each design has different decay curve currents controlled by a series resistor. The value of the series resistor is often based by the type of input opamp used and its input impedance. A low input impedance opamp will require higher clamping diode currents if low signal loss is required due to the series resistance value and input impedance of the opamp. If you have a lot of current through the diode due to low input impedance of the 1st stage the clamping voltage will be high and you will need a very fast input stage to recover from its overload before any signal gating is done because the opamp will be overloaded for a longer period of time.

In my own designs I strictly use FET opamps which allow for large value series resistors at the expense of a slight increase in noise which is in part overcome by the choice of resistor type and then using parallel sets of resistors. My own detector presently uses a series resistor value of 6.6K and uses two MMBD452LT1G Schottky diode arrays in parallel for clamping. My coil decay voltage runs just under 700 volts and is not flat topped and thus I will have about 106 ma being dumped into the two clamping diodes. This combination is working extremely well for me even though my present input stage is a little slower than I like. Overload recovery occurs at 0.7us before the first signal gate directly after the first stage enabled. This is more than sufficient time but I do plan on increasing the 1stage bandwidth a bit in the future. Designs that pick up the coils receive signal using the positive input of 1st stage have the most the most flexibility in regards to input impedance since the opamps negative feedback path is not involved.

One of the issue with clamping diodes is that when they abruptly stop conducting as the decay waveform decays the load presented by the series resistor which in realty disconnects itself as for as the load of the coil is concerned with the diodes stop conducting. This is really noticeable when using the positive input of the opamp and the opamp has a very high impedance front section like FET's. The sudden loss of part of the load on the coil results in a under damped decay waveform due to it no longer being properly terminated, (having the proper load value) at that moment. This typically results in a slight under damped condition during that time. In my case I found that about 90% of this overshoot to be caused by the presence of the diodes and them no longer conducting. So under this condition there is never a condition that allows the 100% correct resistive load placed on the coil. If my normal coil load is say 632 plus a parallel 6.6K clap series resistor then then load my load during this time will change from about 576.8 to 632 ohms or about 9% difference. This would be greater than 9% with a lower value series resistor. Without attempting to compensate for this the best thing to do for a first stage is to minimize its gain so fast response bandwidth can be obtained but this must be without leading edge overshoot as shown with square wave tests of 100kHz or above.

Lets also consider how IF signals in radio receivers are generated and that metal detectors are basically direct conversion receivers that typically do not use intermediate frequencies. In most radio receivers intermediate frequencies are generated by mixers and a local oscillator which generate plus and minus frequencies for later amplification. These IF frequencies are generated by nonlinear devices when they are stimulated by a local oscillator which can be a any type of waveform and basically turn these none linear devices on and off at a fixed rate. Many metal PI metal detectors use switching circuits at the first stage input to block the coils decay voltage and all these have regions of none linearity and will act as mixers under certain conditions such as been turned on and off. Switching FETs or diodes in series with the receive signal in front ends can cause higher susceptibility to interference since these have the ability to become mixers while switching on and off by coils pulse which acts as a local oscillator. Utilizing such parts in the front end where fast switching is being used make these workable solutions but under some conditions can present a real problems. Use these types of solutions with caution and keep them as far away as possible from the coil.

Recently I ran a number of simulations while considering adding a 24 Bit ADC to my design. One of the requirements for obtaining 24 bit accuracy is low harmonic distortion. While working on my simulations I found that the clamping diodes generated harmonic distortion levels that would never allow 24 bit accuracy. The solution I found for this was to add reversed diodes in series with the normal clamping diodes and reverse bias these all the time. But this increased the clamping voltage which is not good. It is likely some diodes may produce higher harmonic distortion that others while simply placed in a circuit it can effect the accuracy ADC converted voltage levels significantly with higher bit counts.

[Detectorist#1]

Hi Carl,

For the clamp diodes, I have been surprised from your statement that the low power schottky diodes are slower than 1N4148.
Today, I soldered simple tester ( schematic is attached). The results are very clear. You are right!
Thank you - you pay attention for this phenomena for me. In popular books, the authors claim that NO diffusion capacitance in forward biased schottky diode.
I just found scientific paper with explanation of diffusion capacitance in forward biased schottky diodes. (Abstract of the paper is attached)

[Tinkerer]

[Detectorist#1]

Hi Tinkerer,

These diagrams can be found in every popular book for diodes.
But, in PI MD, the situation is different - clamp diode at input haves not reverse voltage at the end of the TX pulse.
In this case, for discharge of diffused capacitance helps only safety resistor and timing diagrams are totally different!
See post #27

[Tinkerer]

Hi Tinkerer,

These diagrams can be found in every popular book for diodes.
But, in PI MD, the situation is different - clamp diode at input haves not reverse voltage at the end of the TX pulse.
In this case, for discharge of diffused capacitance helps only safety resistor and timing diagrams are totally different!
See post #27

Indeed, the diode reverse recovery graphs and explanations can be found everywhere on the net. What matters for the PI, is what we are interested in.
The way I understand it is: the clamping diodes at the input of the opamp discharge a part of the current in the coil. It is the
IF, the forward current that matters. The higher the forward current, the stronger is the reverse recovery when the current stops.
We use the input resistor to the opamp as part of the damping. This input resistor also limits the current that discharges through the clamping diode.
A soft recovery, like the Schottky diode, takes longer but is softer. It does not matter what the actual physical process is.
A very fast recovery often causes oscillations until the final settling.
Some datasheets give the recovery time.

In the end, there so many variables involved, that only actual real components defined by testing will show which is the best frontend for a specific purpose.

That is the way I see it. wrong? or right? I am always willing to learn.