Author Topic: How does the trace appear to go backward  (Read 1849 times)

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Offline itdontgo

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How does the trace appear to go backward
« on: May 17, 2012, 08:11:26 am »
I'm no big scope expert so there may be a good reason for this but I dont understand how the trace can appear to go back on itself.  It's a simple one axis input (not x-y).

Any ideas?  It'll be around 50Hz as it's noise from a linear PSU.  You could make good use of the MC34262 in this application!





Online rolycat

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Re: How does the trace appear to go backward
« Reply #1 on: May 17, 2012, 08:38:05 am »
It appears to go backwards because it is going backwards.

The electron beam in the CRT normally scans from left to right due to a ramp voltage applied to the deflection plates, but it takes a finite time to return to the start point, and this is what you can see.

Most scopes have a circuit which cuts the intensity during this process, but on some scopes retrace blanking can be turned off, and on others the circuit can fail.

« Last Edit: May 17, 2012, 08:40:42 am by rolycat »
 

Offline itdontgo

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Re: How does the trace appear to go backward
« Reply #2 on: May 17, 2012, 08:53:38 am »
OK you can see it going back in a straight line but that's not quite what I was refering to.  What is not clear from the picture is the small teardrop shapes it forms in the red circles.  I dont see how that can be explained by the trace sawtooth drive signal unless there is some noise on that signal within the scope i.e. the scope is faulty.

I could be completely wrong if course!


Offline steve_w

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Re: How does the trace appear to go backward
« Reply #3 on: May 17, 2012, 09:49:49 am »
looks like a triggering problem.  reduce the intensity of trace and remove the flyback.  Then see if you adjust the triggering for a more stable display.

Use and external trigger for best results.

regards

Steve W
So long and thanks for all the fish
 

Offline Gall

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Re: How does the trace appear to go backward
« Reply #4 on: May 17, 2012, 10:33:50 am »
I saw exactly that on an old Russian scope due to "dry" electrolythic capacitor.

The trace IS actually going backwards due to scan or trigger circuit malfunction.
The difficult we do today; the impossible takes a little longer.
 

Online rolycat

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Re: How does the trace appear to go backward
« Reply #5 on: May 17, 2012, 11:22:01 am »
It's hard to tell if it's a scope malfunction or dodgy triggering given the messy signal, but the fact that you can see the retrace does suggest that your old soldier may be overdue for retirement.

What happens if you feed it a clean sine or triangle wave and get a good trigger? If that waveform goes backwards as well then you need to repair your scope (or buy a less knackered one).

 


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