Unusual TP4 Waveform

[Molzar]

Hi,

Still having some problems with the HH. Took a scope to look at some of the waveforms. All seem to be OK except for TP4. There is a large spike after the pulse goes negative. This is with the coil attached and is similar with either coil. Damping resistor is 680 ohms.

Any suggestions on what is causing this?

Molzar

[Carl-NC]

Looks like ringing. What does TP2 look like? Try a 560 or 470 ohm damping R and see if it goes away.

- Carl

[1843]

Hi Carl,

Sometimes when I look at the op-amp input (TP2 in HH), it is decent, but output of the op-amp has ringing (especially when PW be set to high). Only by lowering the damper resistor more, the op-amp output ringing fades.
I am confused why different op-amps has a different ringing and waveform....?
For say, in one of the my PIs when the opamp is LF356 and a 1k damper resistor, its output has'nt ringing, and when I replace it by an LF357, the damper resistor must be 680R to getting a good curve.
What is the reason?

Kind regards,
1843

[Carl-NC]

It is probably due to speed limitations of the opamp. You need one that can handle high slew rates, which means a wide bandwidth. Some opamps have methods of compensation that help stability, so you don't have to slow down the input signal.

- Carl

[1843]

If it is due to slew rate, so LF357 would be better, but in actual LF356 shows a better performance.
LF356 with a high damper resistor has a decent curve while LF357 has ringing by the same resistor; although slew rate of LF356 is 12V/us and LF357 50V/us.
About the compensation, both of them have a such ability.

Regards

[Molzar]

Replaced damping resistor with 470 ohm. This made the waveform look correct except the negative portion of the waveform's (after Q3 turns off) duration is huge, approximately 200us. This does not seem correct in looking at other's waveforms (~10us?). Varing the pulse width did not affect the negative duration. There is now no respone at all to metal objects as I believe I am sampling in this area that is at a constant negative voltage. With the 680 ohm in, the durations are much closer to what I've seen posted.

Prior to changing to the 470 ohm resistor I changed the opamp from a TI 5534P to a ON Semi 5534A this didn't change the waveform look much but there was a marked improvement in metal response. Will do some more experimenting tomorrow.

TP2 looked OK at 680 ohms, have not checked at 470.

Any other suggestions?

[gef12]

op amp

You have to be careful as to what opamp you use.
Check with Reg. I found the Phillips ones were the best even thou they do not make them anymore. I think the ON 5534 are the next best or close to the Phillips brand. TE not so good.
gef

[Molzar]

Components

Thanks Gef12. I'm beginning to realize that the components (and manufactures) you use have a large impact on the performance of the HH.

After rechecking most of the board (and finding nothing) I started trying some different components. Plugged in a Fairchild FQP11N40C in Q3 (in place of the NJS? IRF740), this improved the spike dramatically (about ½). Then tried a Fairchild IRF 710B and the spike disappeared!

Also obtained some of the Philips NE5534A, these worked a as good or better than the ON Semi components. Had to order a bunch of these though, if anyone is interested I’d like to sell/trade the surplus.

Also I tried LT082 for IC8, this did not work well, LT072 works OK, and a Nat Semi LMC6482AIN (CMOS) worked very well.

The unit works but I still have one problem. The audio coming from the unit is still erratic. On other project detectors I’ve built the audio is smooth and steady. This unit has the same general sound, but it is uneven. Say like morse code instead a steady constant beat. I don’t think this is interference as it is basically the same in many different environments.

How is the VCO on HH supposed to sound?

Molzar

[BIPOLAR PI]

Question to Car , BBsailor & Regl

What advantage give PI whit bipolar transmit pulse?
Little noise, many dept, stable in bad ground???

[Carl-NC]

What advantage give PI whit bipolar transmit pulse?
Little noise, many dept, stable in bad ground???

In mine detectors, you want a net zero magnetic field, so they often use bipolar pulsing. Otherwise, I'm not sure... I've long suspected that a properly designed bipolar pulse might be Useful in distinguishing ferrous/non-ferrous.

- Carl

[Ray-NM]

.

The unit works but I still have one problem. The audio coming from the unit is still erratic. On other project detectors I’ve built the audio is smooth and steady. This unit has the same general sound, but it is uneven. Say like morse code instead a steady constant beat. I don’t think this is interference as it is basically the same in many different environments.

How is the VCO on HH supposed to sound?

Molzar

If you have a scope hook it up to tp9 and slow the scope way down and you should see a varying sine wave. watch as it changes and correlate the different peaks to your vco output. You might see what is happening that way.
adjust your sensisitivty to as low as it will go Max resistance on the 5k pot and probably set the threshold to zero volts if possible. I find that the vco somehow has a definate effect on the output at tp9. Not positive on how to correct this but have thought of using an optoisolater at the output of the final amp so that the feedback from the vco wont affect the final op amp. Those are just my thoughts let some of the others with more knowledge explain what is happening and what a possible fix might be.
RayNM

[Molzar]

Audio Sound

Ray,

Thanks for your reply. So I take it from your reply that this irratic audio is "normal" for this design. That's all I wanted to know, as other designs I've built have cleaner audio. I didn't want to start the final assembly until I was sure I hadn't screwed something up.

Changed to the Non VCO last night, I think I like this better, but still has some oscillation in the signal.

Read some of your other posts and found the answer to my original question about TP4! http://thunting.com/geotech/forums/s...ad.php?t=13136

What MOSFETs have you found to work well?

Thanks for your help.

Molzar

[Wahyu]

Hi,

Still having some problems with the HH. Took a scope to look at some of the waveforms. All seem to be OK except for TP4. There is a large spike after the pulse goes negative. This is with the coil attached and is similar with either coil. Damping resistor is 680 ohms.

Any suggestions on what is causing this?

Molzar

Hello Molzar,
I also encountered this problem.
My previous use AWG 22
for this problem I replace the wire email with a small size AWG 28 - AWG 30

I found some wire emails have poor results, I do not know why like this


Wahyu.

[Qiaozhi]

Hello Molzar,
I also encountered this problem.
My previous use AWG 22
for this problem I replace the wire email with a small size AWG 28 - AWG 30

I found some wire emails have poor results, I do not know why like this
[ATTACH]39481[/ATTACH]

Wahyu.

That's not due to "bad wire". As you can see in the scope image, the coil is underdamped. You must change the damping resistor to a lower value for that particular coil.

[Wahyu]

Hello Qiaozhi,

I have been looking for a critical damping for this problem, but the curve as red circles remain, after I change the wire emails, the curve becomes normal.

although coil is underdamped but if you use a good wire,result not like the red circle

underdamped for good wire result like this ;


Wahyu.