TP4 waveform wrong?

[exciter]

Hi Reg!
im sorry for my many questions
so i followed your instructions.
here a picture. i hope i have understand it:
i dont tuned the main/sec width yet.

[Reg]

Hi Exciter,

It is easier to see the adjustments of the delay using a dual scope, but it can be done with a single trace scope also.

If you increase the sensitivity a little on the scope and then look at TP 4, the output of the preamp and look carefully at the trace, you should see a small bump or ripple move left or right when you adjust the delay. This bump or ripple will slide along the decay curve signal trace.

What you will see is somewhat like you see on Fig's 27, 28. and 30 on the HH article. If you look closely at those pictures (pics) and look at the tp 6 signal which is the upper trace, you will see a small wiggle or bump directly on the decay curve, directly above the pulse on the bottom trace. That small bump or wiggle is the delay signal. If you adjust the delay pot, you should see that little wiggle move left or right, depending upon which way you turn it.

Also, if you adjust the sample width pot, you should see that wiggle get wider and narrower.

Now, this sample or wiggle indication is not a large signal on the decay curve, so you will have to look carefully to see it, but it can be seen. Some times it is easier to see by increasing the sensitivity of the scope so the decay curve signal is bigger.

Let me know if this makes sense.

Reg

[exciter]

oh i see, i need a two channel scope ;(
only have one channel...

by the way- why does the edit button disappear several minutes after posting??

[exciter]

shouldnt i be able to adjust main delay at Tp6??
when measuring with my scope on tp6, moving the main delay pot changes nothing on screen but audio tone is changing, beeping...
when changing secondary delay, i can change delay on tp8.

[exciter]

where do you measure the sample time????
directly on the pots??
R42 - is main sample time?
R45 - the second ""

tunes my pulse width to 50us like you said.

Best regards.

I´ve shielded my coil with antistatic-fail now.

[exciter]

Okay thank you.
so pulse-width are these little lines.
will check this.

[Reg]

Hi Exciter,

I gave you some bad advice when I mentioned you might start with a 100 usec pulse width. I have modified my HH so I have a wider range of pulse widths than what is originally designed so I can adjust for a wider pulse width than originally designed.

My simulation package indicates what 1843 indicated and that is the pulse width on the HH can be varied between about 10 usec and 75 usec or so on a standard HH with no mods.

So, to begin with, simply adjust the pulse width for somewhere between 50 usec and 75 usec, rather than the 100 usec I originally mentioned. This still gives you a good starting place to check the rest of the detector.

Sorry about the confusion.

The minimal width of about 10 usec or so is too short and using it will result in a detector lacking sensitivity. The 50 usec to 75 usec range will work just fine for most applications.

Cheers,

Reg

[1843]

exciter,
Check R1...R4 and C8 and make sure their values are right.

[Carl-NC]

What you have marked on the oscope pic is closer to the period, not the pulse width. It shows a period T=300us, which is a frequency f = 1/T = 3.33 kHz. This sounds way too high for a mid-setting of the pots. Check the component values around the 555.

The pulsewidth is the width of the skinny part of the pulse... looks to be 10us or less, which is too low for a mid-setting, and too low in general.

- Carl

[exciter]

hi, thanks.
but my understanding of pulswidth is right, isnt it?
is 150us okay???
my pulswidth changes much with freqency for my understanding.

[Carl-NC]

Sorry, been out-of-town with very limited access... Reg, thanks for helping out.

I agree, my TP4 waveforms look wrong, but I'm sure they are what I measured. Not sure why there is no positive pulse, I'll have to look again.

Yes, PW will change slightly with frequency change... nature of the 555.

- Carl

[exciter]

i can get 150us, with tuning freq and width. its the smallest value i can get.
what i dont understand is, that when i change pulsewidth to 150us and after this play around with freq. pulsewidth will also change or not?
i asked because you said i should tune width to 100us and then tune the freq...
but this will change pulsewidth, so it isnt any longer 100us...

[exciter]

so i checked my voltages. they are fine, also battery (12,22V).
u said pulse width 100us. Am i right to measure it on TP1? pulse width is that what i have marked in this picture or not?
this was with both pot (freq, width) in middle position. timebase 0,1ms, so one div should be 100us.
But i cant tune pulse-width to get 100us???

[exciter]

Hi Reg!
I mean i cant [IMG]file:///C:/DOKUME%7E1/ADMINI%7E1.EXC/LOKALE%7E1/Temp/moz-screenshot.jpg[/IMG][IMG]file:///C:/DOKUME%7E1/ADMINI%7E1.EXC/LOKALE%7E1/Temp/moz-screenshot-1.jpg[/IMG] stabilize the scope signal.
It just jumps...there ist only one timebase setting to get it standing still on the scope.
maybe i should try it without any shielding.
Or i should go for an shielded case...
willl try it in a few hours

[Reg]

Hi Exciter,

When you change the pulse width, you also change the pulse frequency. You probably should set the pulse width at a fixed value, maybe 100 usec and then adjust the pulse frequency to see if things get better or worse. If things are jumping around, you need to look at the +5 and -5V as well as the battery voltage to see if they are jumping around also. I would measure them with a scope and a meter. Any one of them changing much at all can cause a problem.

What isn't known right now is whether the jumping around is because of external noise or internal problems or internal instability. External noise can make a detector problem look like the problem is internal. If you can remove the coil and the noise or jumping problem goes away or reduces significantly, then external noise is probably the main problem. However, external noise can point out internal weaknesses also.

If you built your PI so you can use a DD type coil, then I suggest you built a noise canceling coil that is ground canceling also. If you didn't modify your PI, then you can simply take the coil you have and twist it into a figure 8. This should reduce the external noise. To make a figure 8, simply pinch the coil in the middle and twist the top half one half turn. This makes half of the coil up and the other half down.

This type of coil reduces the effects of external noise.

Now, the audio can cause problems also if you have the adjustments too high. By this I mean, if R35 and/R41 are set for minimum resistance or near minimum resistance, they can cause the detector to be unstable at times because the high current draw of the audio can react with the pulse. I would recommend you set R 35 and R 41 for about mid position to start to see if that helps. I suspect your audio will be weak, but that is ok to begin with.

I am not sure what you mean when you say you can't measure any pulse. Are you saying you can't stabilize the scope signal so you can measure things?

Reg