Need Guru to look at 5534 scope shot,something is wrong.

[Mechanic]

Hi Brian,

What ever diodes you use, they need to be fairly low capacitance. The other thing you need to consider is the forward voltage of the diode. Effectively, if we use 2 diodes that have a forward rating of .6v, then our signal gets clipped at 1.2v, which in reality is still pretty high. I would just try and experiment with different diodes and using just one or 2 diodes in series whilst comparing the settling time.

As far as the opposite signal you see on tp7, in the coil head, consider it as one big coil with a center tap. The center tap is ground, and the tx pulses are fed in from each side. Therefore the signal seen on tp7 will be opposite for 1 of the pulses.

Cheers Mick

[Brian Deese]

If the question is, "Why does the opamp output have an extra transition?" then I don't see what the problem is. From what I see (post #15, bottom pic) the output of the preamp is doing what it should be doing, because the input definitely bounces negative when the diode clamp recovers.

But if the question is, "Why does the flyback have a negative bounce?" then the answer is probably, "The damping R is too high." That is, if you are looking for a nice, well-behave critically damped waveform. But that's not always the best waveform to have, and may be slightly slower than being a little underdamped.

I see the negative bounce,I just dont know enough about op amps to know what I am looking at.So correct me if I am wrong,this is what I think I have learned from this thread. High gain in the amp causes slow recovery from saturation.And the slow recovery is why the output of the amp last longer in time than the negative bounce which it is amplifying. I will investigate further and see if the bounce can be made to go away.A question about the diodes,are the by206 good units? I see the 4148 in this position more often.And the dumbest question of all which has perplexed me since I started working on this thing.How do I see the opposite polarity pulse on TP7? Inductive coupling between the two coils inside the search head?

[Davor]

This is EXACTLY my log amp solution! Antiparallel diodes in feedback for a bipolar signal.

Brian, you are probing the inverting input of an opamp. If everything works in normal active regime it should read zero (0) volts against the non-inverting input, usually signal ground. In this case you don't have it. Opamp is not able to cope with input swing of a PI spike, so anti-parallel diodes kick in, and opamp saturates. Once current through diodes reaches equilibrium with current passing through feedback resistor, the opamp turn backs to normal working regime, but it takes some time, and at the very end of this transition it goes below zero - thanks to opamp's compensation circuitry and delays. Then it reaches normal active regime and voltage thereafter is zero. It is perfectly normal.

[Mechanic]

Hi Brian,

Why not try the diodes in the feedback loop. Just some general purpose diodes will do. Bav99 will do too. This should speed up the opamp recovery and enable an earlier sample, thus more sensitivity.

Cheers Mick

[Carl-NC]

If the question is, "Why does the opamp output have an extra transition?" then I don't see what the problem is. From what I see (post #15, bottom pic) the output of the preamp is doing what it should be doing, because the input definitely bounces negative when the diode clamp recovers.

But if the question is, "Why does the flyback have a negative bounce?" then the answer is probably, "The damping R is too high." That is, if you are looking for a nice, well-behave critically damped waveform. But that's not always the best waveform to have, and may be slightly slower than being a little underdamped.

[Brian Deese]

But you don't have any ringing at the input of a preamp - it is a perfectly normal behaviour of an inverting opamp recovering from saturation - nothing else.

You better try finding your culprit elsewhere.

Can you re word that,I am a little confused about the perfectly normal behaviour part.Are you saying the output of the 5534 should look the way it does? Or should not look the way it does?

[Davor]

But you don't have any ringing at the input of a preamp - it is a perfectly normal behaviour of an inverting opamp recovering from saturation - nothing else.

You better try finding your culprit elsewhere.

[Brian Deese]

Hi Brian Deese. You had a detector in working order, but you were not satisfied with the detection of small objects (aluminum cylinder 10mm tall,8mm diameter at 20cm... its too slow to signal...) http://www.geotech1.com/forums/showthread.php?t=17571
What did you do after?

The preamp output has always looked like it does now,(it crosses zero one time too many)it does not look like typical PI.It looks like it is ringing in the output of preamp but not in the input of preamp or measured directly at the coil,that is the purpose of this thread to determine why that is.Now the machine will work just as it always has if I set the sample pulse on the decay curve,I did that this morning actually,I wanted to check the detection distances while adjusting the preamp offset,and I got no gains by offsetting it,so I set it at zero.I changed some timing resistors so that I have control over the sample time,and width,and tx width,and freq. I have adusted it every way possible and the "ringing" is consistent.I didnt know much when I posted that thread,and now I have built a couple PI's and have a little more knowlege.It does respond slow,I had to study and find the vco stage in the schematic and if you lower the value of C31 it will respond faster but I am not sure it is as stable.I have the original value in it now and when the sample pulse is in the right place and you hold the auto tune it clicks at one click per second,solid,stable no variation.An iron ring 50mm diameter will increase the freq to 2 clicks per second at about 30cm which is not terrible,so there you have it.It works but I dont think the preamp output is supposed to look like that.

[maikl]

Hi Brian Deese. You had a detector in working order, but you were not satisfied with the detection of small objects (aluminum cylinder 10mm tall,8mm diameter at 20cm... its too slow to signal...) http://www.geotech1.com/forums/showthread.php?t=17571
What did you do after?

[Brian Deese]

HI Brian,

Have you checked the 1k series resistor on the input path to the preamp to make sure it is 1k. Although this would not be causing the under damping problem anyway as the parallel diodes have stopped conducting by the time the problem shows up. Hmmmph!

Have you checked that the coil ground(0v) is all connected properly and that the damping resistor is connected to it properly?
Have you checked that the coil is wired correctly to the plug?

As far as I can tell the tx transistors seem to be working properly, the power supply seems ok too.

Can you check when the first sample is being taken by the 4016 (pins 5 & 13) compared to the preamp output?

Do you have access to another coil?

Cheers Mick

Well I will check all those things again,the coil wires are coax and they plug directly into the board,the coax socket is soldered directly to the ground plane on the pcb,and they are gold plated.I have had the damping resistors out a couple times and the board is fragile,and double sided so I have a couple spots that dont look perfect but I am pretty sure there are all solid solder connections,I will double check.I dont have another coil,would be nice.However you know there are two coils in this search head and they both measure almost identical in inductance and resistance it is hard to imagine there is a problem.I have shortened the coax and reused the coil side connectors, so I will put my work under the microscope today.I do recall it was very tedious,the coax plugs are tiny about 1/4" diameter outside.I can move the sample with the trimmer,I dont know where it is presently,I will check that and the second sample also. What if I bread boarded an 5534 and fed it all the corresponding pins of the in circuit chip,except for pin 6 of course,and check the output? Would that be viable? And Carl if you are reading this I dont mean to ignore your initial suggestion but I think in the past I have had a pot soldered in to the damping resistor sockets and the mystery pulse didnt go away even with very low R. I have to buy some sockets before I destroy this board,it will not tolerate much more desoldering of the damping resistors.

[ivconic]

HI Brian,

Have you checked the 1k series resistor on the input path to the preamp to make sure it is 1k. Although this would not be causing the under damping problem anyway as the parallel diodes have stopped conducting by the time the problem shows up. Hmmmph!

Have you checked that the coil ground(0v) is all connected properly and that the damping resistor is connected to it properly?
Have you checked that the coil is wired correctly to the plug?

As far as I can tell the tx transistors seem to be working properly, the power supply seems ok too.

Can you check when the first sample is being taken by the 4016 (pins 5 & 13) compared to the preamp output?

Do you have access to another coil?

Cheers Mick

Yes, those are good questions. Great chances that there is some trivial problem there.

[Mechanic]

HI Brian,

Have you checked the 1k series resistor on the input path to the preamp to make sure it is 1k. Although this would not be causing the under damping problem anyway as the parallel diodes have stopped conducting by the time the problem shows up. Hmmmph!

Have you checked that the coil ground(0v) is all connected properly and that the damping resistor is connected to it properly?
Have you checked that the coil is wired correctly to the plug?

As far as I can tell the tx transistors seem to be working properly, the power supply seems ok too.

Can you check when the first sample is being taken by the 4016 (pins 5 & 13) compared to the preamp output?

Do you have access to another coil?

Cheers Mick

[maikl]

Here you can find how old detector http://www.geotech1.com/forums/showthread.php?t=12357
Here scheme with better resolution http://www.geotech1.com/forums/showthread.php?t=18157

[ivconic]

I am clueless after reading your fresh infos.
Loooks like some component turns to higher tolerance than it should be.
How old is detector? Used much?
I assumed coil is original.

[Brian Deese]

Ok I got to looking at the divider( R7 and R8 ). at the input of U1A.I found an resister divider calculator to do the math.So U1A is going to be output high until the non inverting input matches the inverting input 0V,so we consider that pin to be zero volts if the regulator is working,then we say VC is 3.48V then do the math of the resistor divider and we get VE = 2.9V which is roughly what I have at 2.82V. So before I put my foot in my mouth I applied the same math to the positive regulator,D4 a 1.2v reference diode,connected to the non inverting input of U1B,and another divider to the inverting input.assume once again the center of this divider is 1.2V given that the regulator is working,insert the resistor values in the calculator and you get VC = 3.34V and this calculation is made with no respect to the battery voltage.Only the 1.2v ref diode and the resistor divider values are used. So there was no room for error like the above calculation for the neg,where I had to enter the VC value. So isnt 3.3V a pretty standard control voltage? I think I am solid now on the voltage supplies.