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

[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]

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?

[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.

[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

[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.

[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?

[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]

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

I will try it this afternoon,and by they way about the sample.I just wanted to let you guys know,In a previous post I said I would check the "secondary sample also" well there is no secondary sample.This machine feeds one sample from positive pulse and another from the consecutive negative pulse to the differential amp.Thats a change of pace right. And the sample can not start as soon as the other PI machines I have,you cant move the sample up to the egde of the saturation plateau.It can just include a few uS of the linear range of the amp and then the rest of the sample is at zero.I works similarly when you are inching it closer,when you go past a certain point it will not auto tune any longer and no output audio.I set the machine up yesterday for the best performance and its running at 4khz,with a 100uS TX width,and it likes a 40uS sample width the best. And another interesting thing about it since I have it running stable"albeit the preamp output" is when you place the coil on the ground and press the auto tune it oscillates at about 1hz,when you lift the coil it loses some ground influence and it goes quiet,sort of a silent search.Pretty neat I think.

[Brian Deese]

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.

Now I have a question for you,in your bipolar pulsing detector do you use two coils wound in opposite directions or the same coil pulsed with opposite polarity? The thought that I am having is that if you have two coils encapsulated in the search head,in that close proximity,and they share a common ground does that add difficulty in critical damping? In this design it has two damping resistors of course, and the coil who is not active is held to zero with 390R. Just a thought,maybe not a good one.

[Davor]

First off, a log amp is my brain child, not a real implementation yet. However, your detector is capable of employing such idea - as is. It depends upon an inverting opamp frontend.

Second, this double coil business is a nice example of using exactly the same circuitry for opposite pulses. You must have both damping resistors and they must be the same value. In effect it is a 1:1 transformer, and with good coupling factor as well. I'd expect coupling better than 0.95.

From your plots I don't see any ringing, and you should try finding a root cause of your detector's bad behaviour elsewhere.

There are several trimmers to check. Did you confirm DC offset to be zero at 5534 output? Did you check timings?

[bklein]

I wonder about the output not being symmetrical. Is the 5534 supposed to output to the negative rail? I can't determine this from my limited googling. What is the negative voltage IC as I can't make that out. I would think ideal would be symmetrical +\- supplies and rail to rail output here. I also wonder about your battery - a chance it needs to be more beefy than a 9v or AA cells. Try a bench supply. How old is this? Maybe weak ESR electrolytics.

[Davor]

... I also wonder about your battery - a chance it needs to be more beefy than a 9v or AA cells.

Actually not. Aside from 5534 all other opamps are military grade 324s, 393s, cmos and such, and I don't expect it to use more than 50mA at average, mostly for supplying the coil.

As I said - I like this design.

[Brian Deese]

I wonder about the output not being symmetrical. Is the 5534 supposed to output to the negative rail? I can't determine this from my limited googling. What is the negative voltage IC as I can't make that out. I would think ideal would be symmetrical +\- supplies and rail to rail output here. I also wonder about your battery - a chance it needs to be more beefy than a 9v or AA cells. Try a bench supply. How old is this? Maybe weak ESR electrolytics.

Read post 38 there is no voltage reg ic. It has linear regs that are built with discreet components,I have already hashed out the voltage supply levels. Also notice this machine pulses the coil with VC and not B+. The positive reg transistor is the bigest part on the whole board.

[bklein]

Post 13 shows the complete schematic. U2 SI7657? is a negative voltage converter by the looks of it. But can't find anything on that part so I must have the part number wrong. What do you have for V- here? I'm assuming it is the source for the comparator circuits.

I didn't really get an answer: Should the output if the 5534 be rail to rail? Post #15 shows alternating + and - preamp outputs that I imagine must be symmetrically sampled about the zero V reference point. Problem is that the amplitudes differ significantly. You've got a -1.5 division max. signal vs a +3 division one. I would think they should be the same peak amplitude if they are being sampled equivalently about the zero reference point.

Looks like I messed up about dynamic damping - instead these are the TX circuits.
So I go back to whether or not the positive and negative amplitudes out of the preamp should be identical - if yes, are these parts ok?

Lastly, as for the power draw - it will be the TX that draws the most. Is it enough to skew the negative rail? Whether or not the IC's are "military" or not is irrelevant.
In fact, these days what the heck does "military" mean? Made in China or not made in China?

[Mechanic]

Hi Barry,

Here's the datasheet : http://pdf1.alldatasheet.com/datashe...62-SI7661.html

It is a charge pump and could well be(or surrounding components) a source of trouble as to the low -voltage. The datasheet says voltage ranges from 4.5 to 20v. It would be worth checking the electrolytic caps around this area.

As to the 5534, yes it should go to the rails - a little bit, if the input signal and gain is high enough.

Cheers Mick