PI limit to early sampling

[mickstv]

You'll have a slow response in your long TC integration and a fast response in yout short TC integration, they wont cancel each other so you'll have a response on the output. I've tried similar intergrator methods like your version before and it doesn't work.

[Phiphi]

Hi Phiphi,

With that sampling method you'll have issues with Earth Field response, when the coil is swept over the ground. The same will happen in areas containing highly magnetic ground like seen in Australia.

Could you explain why ? I suppose that these responses have only slow variations in the sweeping range of the coil so there won't be a significant signal at the output of the differential amplifier.

[moodz]

R9 and R27 are forming a voltage divider so the input to your first amplifier is 1/10 vcoil. The first amp gain is approx 10 ... so your amplifier gain is effectively lost by the input divider R27/R9.

[mickstv]

Hi Phiphi,

With that sampling method you'll have issues with Earth Field response, when the coil is swept over the ground. The same will happen in areas containing highly magnetic ground like seen in Australia.

[Phiphi]

Next time, I'll present my ideas for the receiver following the preamp.

Here is coming what I have imagined for the receiver part. I think it's quite original and I'd like to have your feeling about this idea.

The schematics of the PSpice simulation is below :



Let's me describe this schematics.

On the left you have recognized the Tx I have described in my previous posts. Between 20ms and 21ms, U6 is closed to simulate a target entering below the coil.
The output of the preamp enters in an integrator done with switches S1-D1 and S4-D4 of the ADG442 (a 4 SPST switch with low charge injection). The integration begins 3 us after Tx off for a 20 us duration. After that, the integrator is reseted thanks to S4-D4.
The integrator output is sampled, during 10 us, 35 us after Tx off with two different time constant. The first time constant (S3-D3) is low, 1 us (100 ohms, 10 nF) so the signal sampled reacts quickly to level variations. The second time constant (S2-D2) is high, 100 us (100 ohms, 1 uF) so the signal sampled doesn't react to slow variations as those coming from ground response.
The three opamps group (U3,U4 and U5) acts as an instrumentation amplifier and amplifies the difference between the sampled outputs with a 100 gain. So with no target, the output is near zero even with a ground response varying slowly but when a target pass under the coil, there is a quick variation detected by the fast sampling and the output varies allowing the detection.
Below are some waveforms issued from the simulation :



The green waveform at the bottom is the output of the instrumentation amplifier showing the variation between 20 ms and 21 ms when the target is present.
The two waveforms on top are the two sampled outputs with high RC (in green) and low RC (in red)

The different time constants are chosen approximatively and need likely to be adjusted but I wanted just to present the idea: amplify only the difference between integration with two different time constants.

Don't hesitate to ask me more waveforms or explanations because I am not sure to be very clear !

[Ferric Toes]

Hi Eric,

Did you miss my reply to your request?
http://www.geotech1.com/forums/showt...396#post223396

No, I had not missed it and thank you for the offer which I will duly take up when I have drawn the circuit and described the features I need to look at.

Eric.

[Qiaozhi]

Hi Eric,

Did you miss my reply to your request?

http://www.geotech1.com/forums/showthread.php?23351-PI-limit-to-early-sampling&p=223396#post223396

[geoscash1]

Enjoyed Seasick Steve. I was going to put my Strat and 1968 Twin Reverb (tube amp) on ebay, but now changed my mind.

Eric.

Oh buddy, nice rig.... Don't get rid of it, you'll always kick yourself for it.... Speaking from experience here...

[bbsailor]

Enjoyed Seasick Steve. I was going to put my Strat and 1968 Twin Reverb (tube amp) on ebay, but now changed my mind.

Eric.

Eric,

As a guitar player do a web search on "Low Impedance Pickup Research" to see how I used a current transformer (CSE-187L) and a single hairpin loop of about 5.5 inches of AWG 11 wire with a 2 inch by .25 inch magnet in the center to make a guitar pickup that directly feeds a low impedance microphone input rated at 150 to 250 input impedance ohms with about a 2.4K ohm bridging input impedance load on the pickup.

Have you ever considered using a current transformer to boost the RX signal on a PI machine? This may provide a new way to boost the RX signal with less noise.

Joseph J. Rogowski

[ivconic]

Enjoyed Seasick Steve. I was going to put my Strat and 1968 Twin Reverb (tube amp) on ebay, but now changed my mind.

Eric.

!!!!! You have them !!!!!
You are very rich man now!
Be careful about selling, think twice!

[Ferric Toes]

Enjoyed Seasick Steve. I was going to put my Strat and 1968 Twin Reverb (tube amp) on ebay, but now changed my mind.

Eric.

[mickstv]

What size of coil is used for the detection of the 6X6mm target @ 11 to 14cm? Is the test in air?

Thanks

Homemade 11.5" x 5.5" coil using litz wire.

[ivconic]

Hopefully, Eric won't take offence to that!

That's old saying, not offence! Old saying and very truthful, knowing from very personal own experience.
I am also the same "old dog" from a song.
Meaning that old methods and habits are pretty deep under my skin and i am not very ready to change them.
Especially when they are efficient and giving me results.
From Eric's works and posts is obvious that he is "old school", taking the best from meaning of that term.
My total respect to Eric, by all means!
But coincidently this song, lyrics and characters on video; somehow perfectly matches here, so i couldn't resist...




P.S.
Oi! Are you trying to embarrass me here!
Noty noty!

[Qiaozhi]

John Paul Jones accepted the Seasick Steve's way of making beautiful music; why can't you?


"You can't teach an old dog new tricks."

Hopefully, Eric won't take offence to that!

[ivconic]

Hi Eric,

If you can post the circuit(s) you have in mind, then I'll knock up a simulation.
Perhaps. in the process, we can also get you indoctrinated into the world of SPICE.

John Paul Jones accepted the Seasick Steve's way of making beautiful music; why can't you?