Avoiding jitter

[Ben]

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@John P wrote:

...To give a better picture of what is actually going on, I'd suggest triggering the scope from the square wave (which has a very fast rise time) so you will have a relatively constant time reference.  The sine wave will have some noise on its output (which is probably normal) but I wouldn't expect any time jitter based on how you are configuring your outputs.

Best Regards,

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Agreed and adding to that, if the scope has a low-pass filter for the trigger use it to reject the higer freq components of the sq wave... or switch to decaf.

Ben

[Markitus]

Thanks for your complete answer John,

With the trigger on the square this vibration is much lower, and now it is something around 20ns. I know this is something obvious to do, but I tried it before and it didn't solve the problem, so i thought it was not a trigger issue, It works now though.

Concerning the voltage "vibration",here there is a video showing it. You can see one of the points "oscillating", with an amplitude around 1mV i think. Is it as low as it can get?Can it be reduced?

Thanks for all your help

[johnsold]

Markitus,

That signal is rather noisy to be talking about "oscillations." It appears that you have broadband noise of about 1 mV rms along with some spikes which are porbably generated by a different process. In addition there is some kind of periodic (probably) signal in there as well. It also has an amplitude around 1 mV. It appears that the period may be a few microseconds but it might be much different due to the sampling processes in the scope.

You have been talking about sine waves and square waves yet your video shows a square wave which appears to be the trigger signal, a ramp or sawtooth signal, and signal with a step which occurs after the square wave rising edge and the noise I described above at the top. 

Where is this signal coming from? Which signal has the disturbances your supervisor does not like. What about the disturbances is a problem for the suprevisor?

Lynn

[Markitus]

I am sorry the video is confusing, my fault.

The orange signal is the square one, the pink is a slow triangle, wich is supposed to move the stage on the y axis.

The blue point is one of the sinewaves points, and it is on a different scale-grid. It is in a 5mV-10ns

I'd rather show the whole wave, but the oscillation is too small to be seen on a vid.

The disturbance my supervisor don't like is on the sinusoidal and the square. This perturbance should be less than the accuracy of the stage ( around 1nm). So I should reduce on 1 order of magnitude that noise/vibration

[johnsold]

Markitus,

I am color blind so I am having some trouble with your descriptions. The camera was also overexposed so the traces tend to be washed out.

I am wondering if you have some powerline frequency interference due possibly to a ground loop or something like that.  It can be difficult to suppress such signals to the microvolt level.  Since it is apparent that you are not familiar with electronic measurements (based on the terminology that you tend to misuse), you may have some connection problems.  Can you post an electrical schematic diagram showing the connections to your DAQ system, stage, oscilloscope, and anything else elelctrial in the system?

Lynn

[Markitus]

Thanks for your answer john,

I apologize for the misuse of those terms, it is clear i don't know too much about electronics, and the fact that english is not my language is not helping me either.

Before I start trying to do what you ask me to, I have a question:

Using only one signal, in the simplest program, I see that my points on the oscilloscope are around 2 mV height. Is that caused by what you think could be the power interference, or this power interefence are the source of the movement of the point?  Should my points, in normal circumstances, be smaller? Because there is no point trying to solve the 1mV "movement" of the wave, if the points are as big.

My card is 16-bit, I assume I should have higher "resolution" on those points, is that correct?

Thanks for helping me,

marc

PS: I still haven't connected the stage to the daqcard, my supervisor, wisely, has not allowed me to do that until he is satisfied with what he sees on the oscilloscope

[johnsold]

marc,

You do well in a language that is not your primary one. Better than I would do in your language, certainly. I doubt your English teachers were electronics experts so you are, in effect, learning two languages, the technical terminology being the second of them.  I am trying to be sure that we are understanding each other.

The 2 mV you see on the oscilloscope may be a combination of several things. All electronics devices including the oscilloscope have some internal noise. You can see how much noise the oscilloscope has by disconnecting all probes and wires from its inputs and grounding the inputs.  What you see under these conditions is the minimum noise contributed by the instrument.  Ideally this should be just a slight amount of randomness on the trace - no more than about twice the thickness of the trace. Next reconnect your probes or test leads to the circuit or DAQ device but leave the power turned off to everything except the oscilloscope. The signal you see now will be any noise picked up by the leads and circuitry outside the oscilloscope plus the internal noise observed earlier. In the best case this will be almost identical to the internal noise of the oscilloscope. You may also see power line frequency noise or other signals picked up, such as from switching power supplies, nearby radio transmitters, or other sources. Finally turn everything on. Now you have the total signal you have been observing. Anything which changed from the previous test is due to the perfromance of the other equipment.

One way to detect power line frequency interference is to set the triggering on the oscilloscope to "Line," which triggers it from the power line. Then the power line frequency part of the signal becomes stationary on the screen and everything else "rolls" by. It can be tricky to observe but with a bit of practice you can tell whether there is power line frequency interference present. If the oscilloscope has an Averaging mode, that can help because it tends to reduce the magnitude of the signals which are not synchronized with the power line.

With a 16-bit A/D converter and a +/-10 V scale you will have a resolution of about 0.3 mV although the noise and accuracy of the DAQ device overall may prevent it from being exactly that good.

Lynn