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Old 14th Dec 2017, 8:52 pm   #1
Al (astral highway)
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Default ‘Scope trace instability ???

Hey folks,

I’m experimenting with ways to control the on-off ratio and pulse width of a circuit. The driver produces a waveform like this ,40uS to max 250uS pulse width and 40-200Hz.

The trace appears to wobble slightly in either plane. There is nothing wrong with the scope input amplifiers as its native square wave is perfect.

I had wondered if this could be because I have lifted the earth of the scope? I had also wondered if there is a small DC component in the signal, possibly.

Any ideas welcome , thank you
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Old 14th Dec 2017, 9:48 pm   #2
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Default Re: ‘Scope trace instability ???

Try DC coupling the scope and checking again. Also try 10x probe if you aren’t already.

This may be an artifact of the coupling cap plus the source impedance.
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Old 14th Dec 2017, 10:54 pm   #3
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Default Re: ‘Scope trace instability ???

It looks to me like the scope is set up so that it triggers on the rising edge at the end of the waveform. Is that what you intended? If so, then any noise or jitter on the output from your circuit, especially in that negative-going pulse, will show up as a movement of the trace.

Try moving the trigger level to be above zero so that the scope triggers on the leading edge.

Chris
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Old 15th Dec 2017, 1:04 pm   #4
Skywave
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Question Re: ‘Scope trace instability ???

Quote:
Originally Posted by astral highway View Post
I had wondered if this could be because I have lifted the earth off the scope?
Presumably, that's the mains earth connection, yes? If so, why did you do that?

Al.
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Old 15th Dec 2017, 1:16 pm   #5
ms660
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Default Re: ‘Scope trace instability ???

Interesting....The OP (Al...Astral) says:

"I had wondered if this could be because I have lifted the earth of the scope?"

Quote:
Originally Posted by astral highway View Post
I had wondered if this could be because I have lifted the earth of the scope? I had also wondered if there is a small DC component in the signal, possibly.
The quote says:

"I had wondered if this could be because I have lifted the earth off the scope?"

Quote:
Originally Posted by Skywave View Post
Quote:
Originally Posted by astral highway View Post
I had wondered if this could be because I have lifted the earth off the scope?
Presumably, that's the mains earth connection, yes? If so, why did you do that?
Could mean different?

Lawrence.

Last edited by ms660; 15th Dec 2017 at 1:32 pm.
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Old 16th Dec 2017, 12:55 pm   #6
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Default Re: ‘Scope trace instability ???

Al,

Assuming you have tried most things, such as the HF reject button on the sync options (if there is one on that scope) likely the signal has some amplitude or phase jitter. Amplitude jitter causes phase jitter in a circuit like a scope sync circuit where some slice level is involved because of the waveform rising and falling edges so it can upset the scope's lock.

The circuit generating the the signal might have an issue causing erratic timing.

Make sure the scope's earth clip is connected directly to the earth system of the timing circuit you are monitoring and that there is no supply ripple on the timing circuit generating the pulse.

Hugo.
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Old 16th Dec 2017, 6:45 pm   #7
Al (astral highway)
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Default Re: ‘Scope trace instability ???

Chris, moving the trigger level did the trick l. Thank you .

Al, to be clear , I lifted the distal end of the scope’s mains safety earth. You can see why this could be useful at times. The unit is double insulated so not prone to add fuel to your most obvious concern. If you want to start a debate on why this isor isn’t a good idea, please start a new thread.

Hugo, yes, I think some amplitude variation and hence phase jitter as you describe. The signal source is battery powered and I will put more coupling on the supply rails and report back.
Good call on connecting both ground clips together, I’ll try that, too.

If this doesn’t quite produce the cleanest signal possible I will put the signal into a Hex Schmitt tigger
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Old 16th Dec 2017, 9:53 pm   #8
Argus25
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Default Re: ‘Scope trace instability ???

Al,
Schmitt triggers are good for helping a switching circircuit respond to and ignore slow rising input signals with noise on them. It prevents multiple transitions on the output at the switching point.

But if the incoming signal has a phase or timing jitter the Schmitt won't get rid of that, because it still switches initially at least on the leading edge at some specific threshold voltage.

It will help stop amplitude jitter on a sloping waveform from causing multiple transitions at the output, but if there is a lot of amplitude jitter the output phase will bounce around after its past through a Schmitt trigger.

It's best to make sure the signal source is as clean and jitter free as possible rather than trying to clean it up later.
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Old 17th Dec 2017, 5:49 pm   #9
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Arrow Re: ‘Scope trace instability ???

Quote:
Originally Posted by astral highway View Post
Al, to be clear, I lifted the distal end of the scope’s mains safety earth. You can see why this could be useful at times. The unit is double insulated so not prone to add fuel to your most obvious concern. If you want to start a debate on why this is or isn’t a good idea, please start a new thread.
O.K.: two points arising.

One, there have been extensive threads and discussions on this forum about disconnecting the earth lead from mains-powered equipment and the corresponding use of isolating transformers: there is nothing I can add of any worth to what has been previously written.
Two, I ask again: why was it necessary to remove that earth? (I will respect your privacy if you choose not to answer that and shan't ask again).

Finally, please don't misunderstand me: I'm only trying to help.

Al.
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Old 18th Dec 2017, 12:59 pm   #10
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Default Re: ‘Scope trace instability ???

Hey Al, you obviously have some anxiety around this practice. It’s widely acknowledged to be controversial, but acceptable to some in some circumstances.

I won’t dilute this thread by heading off in this direction but I’m sure that if you have deep feelings about it, and started a thread, an enlivened conversation could result.

I hope you do feel so inclined as your contributions are very detailed and often thought-provoking. So please, go ahead!!
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Old 18th Dec 2017, 10:41 pm   #11
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Arrow Re: ‘Scope trace instability ???

Quote:
Originally Posted by astral highway View Post
Al, you obviously have some anxiety around this practice. It’s widely acknowledged to be controversial, but acceptable to some in some circumstances.
I use isol. transformers a great deal. Some here form part of a semi-permanent mains-distribution system in my workshop. No: for myself, I don't have "anxiety around this practice": any 'anxiety' I may have is what I read by, or hear about, people who don't use them when they should, or use them when they are not necessary. (The latter is obviously 'forgivable'; not so the former).

I'll look in the archives of this forum to see if I can dig out past relevant threads.

----- Later -----

I've found what I was looking for & fortunately this post is still in its 'edit' period.
Yes, there have been a great many discussions about this topic - some controversial, some a bit heated. However, to me, the most enlightening and relevant thread to this post is here:

https://www.vintage-radio.net/forum/...=134177&page=3

The contributions by Mark (mhennessey) make for particular interesting reading.

Al.

Last edited by Skywave; 18th Dec 2017 at 10:58 pm.
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Old 18th Dec 2017, 11:26 pm   #12
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Default Re: ‘Scope trace instability ???

Progress: I’ve beefed up the decoupling in my frequency /pulse width variable impulse generator, reducing jitter to an acceptable (almost non-existent) level. A pair of clamping diodes on the output is also an improvement. I may have to do more as the unit will be coupled by twisted pair a metre or so from the input logic.
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