Understanding how the sampling integrator works

[brumbarchris]

Hello all,
I am trying to learn from how the Baracuda works.
I have difficulties in understanding how the sampling integrator is operating. I do not currently have a Baracuda itself and it seems to be out of stock in the diyDetector store. I'll place a separate post in the trade section of the forum to try and obtain one.
Until I get one, I am trying to theoretically analyze (LTSpice simulation) and understand the operation of the integrator itself. I was already able to separately build the preamp and TX circuit, so I am able to get a meaningful signal out of the preamp circuit. I measured this signal with a scope, I captured the waveform in digital format and I am able to feed it (in the LTSPice simulation) to the input of the integrator. The picture below indicates the LTSpice simulation circuit. The V7 supply is a PWL source generating the output of the RX preamplifier, as I have captured it with the oscilloscope. I have simulated the Q2 and Q3 transistors with ideal switches.

LTSpice simulation circuit

The picture below shows a period of the RX amplifier output, and the timing relation of the Main and Secondary pulses.

Single period RX Preamplifier signal

I run the simulation by repeating this single period at the input of the integrator a number of times (1kHz repetition rate). The voltage at the output of the integrator gradually increases during each period, until it gets into saturation (see next image below).

Multiple period RX Preamplifier signal and integrator output

From the documentation, however, I understand I should obtain a DC signal at the output of the sampling integrator. Any idea on what I am doing wrong?

Best regards,
Cristian

[green]

Hello all,
I am trying to learn from how the Baracuda works.
I have difficulties in understanding how the sampling integrator is operating. I do not currently have a Baracuda itself and it seems to be out of stock in the diyDetector store. I'll place a separate post in the trade section of the forum to try and obtain one.
Until I get one, I am trying to theoretically analyze (LTSpice simulation) and understand the operation of the integrator itself. I was already able to separately build the preamp and TX circuit, so I am able to get a meaningful signal out of the preamp circuit. I measured this signal with a scope, I captured the waveform in digital format and I am able to feed it (in the LTSPice simulation) to the input of the integrator. The picture below indicates the LTSpice simulation circuit. The V7 supply is a PWL source generating the output of the RX preamplifier, as I have captured it with the oscilloscope. I have simulated the Q2 and Q3 transistors with ideal switches.

LTSpice simulation circuit

The picture below shows a period of the RX amplifier output, and the timing relation of the Main and Secondary pulses.

Single period RX Preamplifier signal

I run the simulation by repeating this single period at the input of the integrator a number of times (1kHz repetition rate). The voltage at the output of the integrator gradually increases during each period, until it gets into saturation (see next image below).

Multiple period RX Preamplifier signal and integrator output

From the documentation, however, I understand I should obtain a DC signal at the output of the sampling integrator. Any idea on what I am doing wrong?

Best regards,
Cristian

Your target simulation looks like what I would expect for no target. Looks like you start sampling when the amplifier is still saturated, to soon.

[waltr]

Attached is a text file of Carl's explanation (copied from a forum post) of how a sampling integrator works. Was very helpful to me.
IntregratorDesign.txt

[Carl-NC]

Looks like you start sampling when the amplifier is still saturated, to soon.

Yes, you're sampling too early. You can do that but will need to also apply an offset.

It is better if you upload attachments directly to the forum instead of using an off-site server. Offsite attachments will eventually disappear.

[green]

From the documentation, however, I understand I should obtain a DC signal at the output of the sampling integrator. Any idea on what I am doing wrong?

Integrator time constant=about 125us. You need to sample over 5times longer to reach steady state.

Integrator gain over 10. Average volts during the sample needs to be .4V or less.

[brumbarchris]

It is better if you upload attachments directly to the forum instead of using an off-site server. Offsite attachments will eventually disappear.

Will do so, but at the time when I made the post I did not have the option to make attachments (possibly because it was my first post). I see that now I can add attachments and I will amend the first post to include pictures and simulation files.

Attached is a text file of Carl's explanation (copied from a forum post) of how a sampling integrator works.

That is great, thank you for sharing this. Do you know where I could find the original post? Carl mentions some component reference designators in his explanations, and I would like to match those to a schematic presumably found in the original post.
As a matter of fact, the explanations, tend to match my observations of the circuit in simulation.

I suppose the following hold true:
- circuit should be so tuned that when no metal target is under the coil, the Main Sample Pulse occurs after the complete decay of the RX preamp output; in such a case, the Main Sample Pulse and the Secondary Sample Pulse should eventually lead to no voltage build-up at the output of the integrator
- as metal comes under the coil, the decay time of the RX pream output increases, and the Main Sample Pulse "catches" some of this decaying slope and feeds it to the input of the integrator, thus building up some voltage at the output of the integrator; subsequent samples under the same conditions gradually increase the voltage at the output of the integrator
- if a metal target is continuously kept under the sensor coil, the the integrator output will eventually saturate at the upper rail

Are these understandings correct?

Best regards,
Cristian

[brumbarchris]

Understanding how the sampling integrator works

Yeah, as per the forum rules, I cannot amend the first post, so I am adding here the pictures and attachments mentioned in the post. In addition to these, I have also added the zip file with the actual LTSpice simulation, if you would be so kind to check it. BR, Cristian

[green]

if a metal target is continuously kept under the sensor coil, the the integrator output will eventually saturate at the upper rail

If it were a true integrator, no feedback resistor. Feedback resistor limits gain, gain=Rfdbk/Rin*sample rate*sample time.

[waltr]

That is great, thank you for sharing this. Do you know where I could find the original post? Carl mentions some component reference designators in his explanations, and I would like to match those to a schematic presumably found in the original post.

Sorry, I have tried looking for the original post but never found it.
When I was first reading the forum to learn about PI detectors I copy/pasted good posts/topics into Text files. Maybe some creative forum searching will find the post.

[green]

Sorry, I have tried looking for the original post but never found it.
When I was first reading the forum to learn about PI detectors I copy/pasted good posts/topics into Text files. Maybe some creative forum searching will find the post.

https://www.geotech1.com/forums/show...4582#post54582

[waltr]

https://www.geotech1.com/forums/show...4582#post54582

Thanks Green.
This does help to put the post into context.

[brumbarchris]

If it were a true integrator, no feedback resistor. Feedback resistor limits gain, gain=Rfdbk/Rin*sample rate*sample time.

Thank you for the explanation and also for finding the original post. I had searched for it also, but without success.

Is it also true that circuit should be so tuned that when no metal target is under the coil, the Main Sample Pulse occurs after the complete decay of the RX preamp output? in such a case, the Main Sample Pulse and the Secondary Sample Pulse should eventually lead to no voltage build-up at the output of the integrator.
On the other hand, as metal comes under the coil, the decay time of the RX pream output increases, and the Main Sample Pulse "catches" some of this decaying slope and feeds it to the input of the integrator, thus building up some voltage at the output of the integrator; subsequent samples under the same conditions gradually increase the voltage at the output of the integrator.

This is my understanding of how the circuit broadly operates, is it correct?

BR,
Cristian

[green]

Thank you for the explanation and also for finding the original post. I had searched for it also, but without success.

Is it also true that circuit should be so tuned that when no metal target is under the coil, the Main Sample Pulse occurs after the complete decay of the RX preamp output? in such a case, the Main Sample Pulse and the Secondary Sample Pulse should eventually lead to no voltage build-up at the output of the integrator.
On the other hand, as metal comes under the coil, the decay time of the RX pream output increases, and the Main Sample Pulse "catches" some of this decaying slope and feeds it to the input of the integrator, thus building up some voltage at the output of the integrator; subsequent samples under the same conditions gradually increase the voltage at the output of the integrator.

This is my understanding of how the circuit broadly operates, is it correct?

BR,
Cristian

Some scope traces I posted in another thread. Think sample could start after 4usec delay in top row, maybe 5us delay bottom row. No target signal doesn't need to be zero when sample starts(maybe less than 1/2 full scale), just needs to repeat. Ground could cause a signal which doesn't matter if it doesn't change. Probably not true about ground signal since it is going to change, would need to take a second sample to ground balance.

[Carl-NC]

Is it also true that circuit should be so tuned that when no metal target is under the coil, the Main Sample Pulse occurs after the complete decay of the RX preamp output? in such a case, the Main Sample Pulse and the Secondary Sample Pulse should eventually lead to no voltage build-up at the output of the integrator.

You can sample into the static decay curve, but the more of the curve you sample the more offset you get at the integrator output. Usually this will reduce the dynamic range of target responses. One way to compensate is to slightly underdamp the coil. Another is to inject an offset current into the integrator. This could even be implemented with an autotune loop, which might have other interesting possibilities.

[green]

You can sample into the static decay curve, but the more of the curve you sample the more offset you get at the integrator output. Usually this will reduce the dynamic range of target responses. One way to compensate is to slightly underdamp the coil. Another is to inject an offset current into the integrator. This could even be implemented with an autotune loop, which might have other interesting possibilities.

Some scope pictures with US quarter for target. If I started the sample while amplifier out was full scale positive the signal would go the wrong polarity in the beginning of the sample. Wondering if something similar happens with Baracuda circuit?