Target response tester

[Skippy]

Green: Your log-amp calibration looks OK ( I assume -0.801mV is actually intended to read -0.801 V ).
Re:"What's different about the circuit?
Same circuit, Tx current in a control loop. Constant rate, 2780A/sec. It's all one recording, Tx damped and Rx damped separate."
I meant things like "a transistor is turned on , switching in a resistor" vs. "open-circuit".
I've never seen the circuit, and having trawled through this entire thread, I can't find it here either, so I can't say much at the moment.
There's a damping resistor on the RX coil all the time? There's a damping R on the TX all the time? And when the transmit fet(?) turns on, it's a perfect current source?
Is the circuit one of the standard 'project' ones on here?

Also: I too thought Eric's US 5c 'nickel' decay curve was odd. If that best-fit figure of 0.127 is reciprocated you get TC = 7.87 microsecs, ( f_c = 20.2kHz ) which is certainly different to what is generally measured ( about 9.5 usec ).

[green]

Yes, I can do linear, log or semi-log plots' but opted for linear at the moment as the log compression limits the detail that can be seen at the 10 - 20uS start of the decay where signal levels are high and deviations are most likely to occur. I have included the data values so that others can re-plot if preferred.

I will plot a clad quarter, but I have a MK2 version of my equipment that would be better to use. The minimum delay on this is 20uS, but it will measure out to 1mS, which is good for larger and more highly conductive targets. Also, I can set the delay time spacings to give more resolution at the start of the decay as you suggest.

I will have a look at the noise on this MK2 unit which uses a single NE5534A as the first amplifier and followed by a NE5532 as a cross coupled phase inverter. This then feeds a single time constant integrator which give better low frequency noise rejection than a differential integrator with it's TC matching problems. Maybe my high front end noise is due to rf pickup from outside sources and the whole unit needs to be in a screened box, or at least the preamplifier. However, the noise I have seems to vanish in the subsequent filtering and dc amplification.

Eric.

Thanks, what ever you can do to test for the increase in slope at beginning of decay.
I'm just trying to understand the noise part. Was thinking shorting the 1k input to common would eliminate emi pickup at the input and allow testing for rti noise. A stable reading at integrator out is what matters. I'm recording amplifier out log scale, no integrator. Trying to minimize noise.

[green]

Green: Your log-amp calibration looks OK ( I assume -0.801mV is actually intended to read -0.801 V ).
Re:"What's different about the circuit?
Same circuit, Tx current in a control loop. Constant rate, 2780A/sec. It's all one recording, Tx damped and Rx damped separate."
I meant things like "a transistor is turned on , switching in a resistor" vs. "open-circuit".
I've never seen the circuit, and having trawled through this entire thread, I can't find it here either, so I can't say much at the moment.
There's a damping resistor on the RX coil all the time? There's a damping R on the TX all the time? And when the transmit fet(?) turns on, it's a perfect current source?
Is the circuit one of the standard 'project' ones on here?

Also: I too thought Eric's US 5c 'nickel' decay curve was odd. If that best-fit figure of 0.127 is reciprocated you get TC = 7.87 microsecs, ( f_c = 20.2kHz ) which is certainly different to what is generally measured ( about 9.5 usec ).

Yes, -.801V

Including the schematic of the TRT tester that was posted in another thread. I labeled the output off pin 14, am using output off pin 12. Schematic doesn't show Rx(2 round coils connected figure eight) and differential amplifier(Rx damping resistor connected across input)

Tx(on schematic surrounds Rx). The coil is balanced for zero signal at amplifier out when Tx on and off. Recorded the signal during Tx on and off. Normally don't record during Tx on, noisier and a PI uses decay Tx off.

[green]

Looking at schematic reply#168. Doesn't show MUR460 diode in series with Tx coil. Tried it both ways, ended with it included. Tx in a constant current loop or constant rate loop. Wasn't thinking about effect Tx(Rd) has on constant rate coil current. Need to determine if a problem.

[Skippy]

Puzzled me. I'm contemplating whether the target is coupling more than wanted with the TX and RX coil. These both have loads on them (resistors), so will absorb energy from nearby inductors, like our target. This would shorten the decay time of any current flowing in the target. I'm trying to work out, if the above is true, why there's more resistive load during Transmit than during Receive. The RX coil is unchanged, so I'm looking at the TX circuitry. If that's a perfect current sink, then it behaves like " a very large voltage source, through a very large resistor" , so it shouldn't be adding any damping to the TX coil. If it were flawed, and behaved like a current sink with a parallel resistance, then the opportunity to remove more energy from the target exists.
I'm a bit short of ideas. One obvious thing is the 12 V power rail. If that drops/rises, could it effectively increase the coil damping? I'm unsure about it from a theoretical viewpoint, I would just take the simple way out and try more PSU decoupling, of the right type, in the correct place.
The place being from the bottom end of the 0.5R current-feedback res, to the coil +ve end.
Another possibility is the op-amp current regulator is not working correctly due to bad wiring, eg. no proper star point for the -ve supply. The star point should be the bottom end of the 0.5R sense resistor in the fet source circuit. The main PSU decoupling goes there. The bottom end of the 750R/0.1u/1u current-setting components go there. The wire going to your bench PSU/battery(?) goes there.
Other things I thought of had the opposite effect unfortunately. Eg. if the fet didn't turn off fully, it would add damping during the off-time, shortening the target TC's during this period.
I suppose it would be rude to ask for a photo of the board.

[green]

Puzzled me. I'm contemplating whether the target is coupling more than wanted with the TX and RX coil. These both have loads on them (resistors), so will absorb energy from nearby inductors, like our target. This would shorten the decay time of any current flowing in the target. I'm trying to work out, if the above is true, why there's more resistive load during Transmit than during Receive. The RX coil is unchanged, so I'm looking at the TX circuitry. If that's a perfect current sink, then it behaves like " a very large voltage source, through a very large resistor" , so it shouldn't be adding any damping to the TX coil. If it were flawed, and behaved like a current sink with a parallel resistance, then the opportunity to remove more energy from the target exists.
I'm a bit short of ideas. One obvious thing is the 12 V power rail. If that drops/rises, could it effectively increase the coil damping? I'm unsure about it from a theoretical viewpoint, I would just take the simple way out and try more PSU decoupling, of the right type, in the correct place.
The place being from the bottom end of the 0.5R current-feedback res, to the coil +ve end.
Another possibility is the op-amp current regulator is not working correctly due to bad wiring, eg. no proper star point for the -ve supply. The star point should be the bottom end of the 0.5R sense resistor in the fet source circuit. The main PSU decoupling goes there. The bottom end of the 750R/0.1u/1u current-setting components go there. The wire going to your bench PSU/battery(?) goes there.
Other things I thought of had the opposite effect unfortunately. Eg. if the fet didn't turn off fully, it would add damping during the off-time, shortening the target TC's during this period.
I suppose it would be rude to ask for a photo of the board.

Including photo of board.

Another test, Tx on with AWG10 40mm ring. Tx time 100us, adjusted space(target to coil)for near full scale reading. Recorded data 100 and 200us Tx, target not moved.

Then adjusted space(target to coil)for near full scale reading at 200us Tx. Recorded data 200 and 400us Tx, target not moved. Charted data. Inserted a line parallel to the shorter Tx, added a second line same slope and positioned it near the longer Tx. Appears the shorter the Tx the steeper the decay slope(shorter TC). Could skin effect be the cause?

Forgot to add, Y axis, log scale .4V/decade, 25mV in=0V out to chart.
12V battery
board, copper both sides, top three sections, left (12V), middle digital (5V), right (analog common), bottom(-battery)

[Skippy]

Oh! That's ... a work in progress. It raises a number of questions ... is that why you have noise concerns ? Is that why you get odd, inconsistent data?.
More specifically: long leads flapping around when there's a high-current pulse generator nearby isn't good. And the general spread-out nature of it raises issues about the quality of the ground(s) and the power supplies.
And all the advice about power supply connection, decoupling connection, having the star point at the 0.5R ( I assume it's two 1R0's on top of each other?), needs implementing.

And re: my idea of there being too much coupling between target and Rx/Tx coils, I've just taken a fresh look at some charts you posted a few days ago:
http://www.geotech1.com/forums/attac...4&d=1548613325
..and noticed that targets on top of the coil gave slightly lower TC's than those 16mm away, which supports my thinking. Though it is only a slight change.
Seeing as the 40mm test coils are very similar in size to half of your 1.5" 8-coil, I wonder if they show this effect more strongly? Just an idea, I guess too close and you'll just get overloaded RX amp problems?
And re: the latest 100/200/400us slope, I found it hard to work out which was which at first. The shortest transmit time results in the biggest response from the target? This is related to the business of 'optimum' TX time being 4 times longer than the target TC ? But this is a 200 us target, so why doesn't 400 us give a bigger response.? Non-expert here, don't forget.

[green]

Oh! That's ... a work in progress. It raises a number of questions ... is that why you have noise concerns ? Is that why you get odd, inconsistent data?.
More specifically: long leads flapping around when there's a high-current pulse generator nearby isn't good. And the general spread-out nature of it raises issues about the quality of the ground(s) and the power supplies.
And all the advice about power supply connection, decoupling connection, having the star point at the 0.5R ( I assume it's two 1R0's on top of each other?), needs implementing.

And re: my idea of there being too much coupling between target and Rx/Tx coils, I've just taken a fresh look at some charts you posted a few days ago:
http://www.geotech1.com/forums/attac...4&d=1548613325
..and noticed that targets on top of the coil gave slightly lower TC's than those 16mm away, which supports my thinking. Though it is only a slight change.
Seeing as the 40mm test coils are very similar in size to half of your 1.5" 8-coil, I wonder if they show this effect more strongly? Just an idea, I guess too close and you'll just get overloaded RX amp problems?
And re: the latest 100/200/400us slope, I found it hard to work out which was which at first. The shortest transmit time results in the biggest response from the target? This is related to the business of 'optimum' TX time being 4 times longer than the target TC ? But this is a 200 us target, so why doesn't 400 us give a bigger response.? Non-expert here, don't forget.

Amplitude is proportional to rate, Tx on. One half the Tx time, twice the rate(1A peak). Should be twice the signal. Close to 3divisions near start of decay(3divisions=twice the signal). Same with your VLF, double Tx amplitude doubles Rx signal?

Noise is lower with Rx input shorted. Coil pickup causing a lot of the noise, some coming from the scope.

( I assume it's two 1R0's on top of each other?)1RO's? I used one board, copper both sides?

[Skippy]

Two 1R0's means two one Ohm resistors. It's common engineering practice to replace the easily-lost decimal point with the multiplier, example 2K2, 1M5, 4R7, R22 for: 2.2K, 1.5M, 4.7R, 0.22R.
These fellows, one hiding under the other:

[Tinkerer]

Amplitude is proportional to rate, Tx on. One half the Tx time, twice the rate(1A peak). Should be twice the signal. Close to 3divisions near start of decay(3divisions=twice the signal). Same with your VLF, double Tx amplitude doubles Rx signal?

Noise is lower with Rx input shorted. Coil pickup causing a lot of the noise, some coming from the scope.

( I assume it's two 1R0's on top of each other?)1RO's? I used one board, copper both sides?

Some 17 years or so ago, Eric Foster and David Johnson had a discussion about PI TX on http://www.findmall.com/list.php?34 Basically the question was: What kicks the target. If I remember right, the two opinions were different, but both very interesting. I know I have saved some of the discussion, but am unable to find it right now. It could be interesting to compare the output relative to the 2 different opinions.

[green]

Some 17 years or so ago, Eric Foster and David Johnson had a discussion about PI TX on http://www.findmall.com/list.php?34 Basically the question was: What kicks the target. If I remember right, the two opinions were different, but both very interesting. I know I have saved some of the discussion, but am unable to find it right now. It could be interesting to compare the output relative to the 2 different opinions.

My reply was about Rx signal vs constant rate Tx when Tx is on. Before the kick.

Discussion after the kick could be interesting also.

[Tinkerer]

My reply was about Rx signal vs constant rate Tx when Tx is on. Before the kick.

Discussion after the kick could be interesting also.

Sorry, I thought that you were looking at the RX signal during the TX time OFF.

I am still looking for the discussion mentioned above. I am sure I have it on some backup disk.

The current wave shape as well as the di/dt, during TX ON, can have a considerable influence on certain targets.

[green]

Sorry, I thought that you were looking at the RX signal during the TX time OFF.

I am still looking for the discussion mentioned above. I am sure I have it on some backup disk.

The current wave shape as well as the di/dt, during TX ON, can have a considerable influence on certain targets.

Nothing to be sorry about. Good to see you are reading the thread. Any comments welcome.

[green]

Oh! That's ... a work in progress. It raises a number of questions ... is that why you have noise concerns ? Is that why you get odd, inconsistent data?.
More specifically: long leads flapping around when there's a high-current pulse generator nearby isn't good. And the general spread-out nature of it raises issues about the quality of the ground(s) and the power supplies.
And all the advice about power supply connection, decoupling connection, having the star point at the 0.5R ( I assume it's two 1R0's on top of each other?), needs implementing.

And re: my idea of there being too much coupling between target and Rx/Tx coils, I've just taken a fresh look at some charts you posted a few days ago:
http://www.geotech1.com/forums/attac...4&d=1548613325
..and noticed that targets on top of the coil gave slightly lower TC's than those 16mm away, which supports my thinking. Though it is only a slight change.
Seeing as the 40mm test coils are very similar in size to half of your 1.5" 8-coil, I wonder if they show this effect more strongly? Just an idea, I guess too close and you'll just get overloaded RX amp problems?
And re: the latest 100/200/400us slope, I found it hard to work out which was which at first. The shortest transmit time results in the biggest response from the target? This is related to the business of 'optimum' TX time being 4 times longer than the target TC ? But this is a 200 us target, so why doesn't 400 us give a bigger response.? Non-expert here, don't forget.

Some thoughts, maybe correct or not. When I design a circuit it usually needs modification or additions. Building on copper clad boards makes it easier for me. Not a good way to build a circuit if making very many. I've thought the copper clad acted as a star point and where thought it was necessary connected components at same place on buss(750R, .5R and the caps for current fdbk and command as an example). Building this way might not be best for noise. The noise thread was about determining if noise was what was expected or worse. Example: I use a differential amplifier with 1k to common at each input, gain about 450 and BW about(DC to 1MHz). Input open, p-p output noise=15mV minimum. Input shorted, p-p output noise=2.7mV with 1nV/rt Hz input noise. Something to compare my amplifier with to see if improvements could be made. You are probably right about leads flapping in the air, not sure if worse than Rx next to Tx with 500V spikes on Tx. There are no spikes after the amplifier has settled. Any thoughts or corrections appreciated.

Inconsistent data: not listing ambient temperature, target position and orientation, coil size, Tx time, constant rate or constant current might have some effect on decay rate along with circuit calibration. You have done some measurements with a VLF. Have you checked if target position or orientation effects your VLF measurement? Eric is the only one that I can remember charting decay with a PI. Keep hoping for some more input to compare with.

[Skippy]

"I've thought the copper clad acted as a star point"
The clue is in the name - Point. If a high current flows through a conductor, even a sheet of copper, it creates a voltage drop, bouncing the ground level to other parts of the circuit. Worse still when considering step changes in current, such as may occur with digital logic, or PI detectors. Then the inductance of the conductor also comes into play, as you're aware; put a dI/dt through an inductor, and a V is generated.
"Have you checked if target position or orientation affects your VLF measurement?"
I'm very aware that it can do, so I try and be consistent. Not too far away from the centre sweet-spot for static measurements. Avoiding changing meter ranges, as the calibration is slightly different. EMI and temperature variations are always a pain.
One of the problems with using a commercial machine is that you don't really know the 'specs' .. what does/doesn't drift with temperature etc.

You're correct about having nothing/no-one to compare to. I get the feeling that I'm the only ever to have made some of the measurement I've done. Clearly I'm not, I'm sure George Payne had to do a lot of this kind of stuff back in the 1980's, but the work of professional/commercial designers is very closely guarded.