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What acquisition mode should be used on the oscilloscope ? I have a GW Instek GDS-1052-U. I can choose between Normal, Average and Peak Detect. Which is the best ? Also, when I am using Average I can select a number from 2 to 256. If I select 256, the trace is at first very noisy and it getting less noisy in a few seconds: Average 256, when I connect the probe to the LM317 power supply: https://imgur.com/a/LE8ZF Average 256, after about 10 seconds: https://imgur.com/a/MWcZr Average 256, after about another 10 seconds: https://imgur.com/a/QfPwn

It is normal for the trace to be more noisy at the moment when I connect it to the signal source and after a few seconds to get less noisy ?

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    \$\begingroup\$ Which is the best ? How can anyone answer that when you do not define what you think is "best". If there was one "best" mode, why would the other modes be there? Have a look at a video by Dave from the EEVBlog about acquisition modes: youtube.com/watch?v=036Ems1z-o0 What do you expect to happen when the Averaging setting is changed? You really should do more research before asking: I changed this, now my trace changes, is that normal? \$\endgroup\$ – Bimpelrekkie Feb 3 '18 at 16:16
  • \$\begingroup\$ Thanks for the reply @Bimpelrekkie I do not know what can happen if I change the Averaging setting. \$\endgroup\$ – mike_mike Feb 3 '18 at 16:21
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    \$\begingroup\$ What do you think? I can tell you yes/no but what does that teach you? Think about what averaging is and what is does and its influence on a trace. You might even want to look at this video that I found after searching for "oscilloscope averaging" on Youtube: youtube.com/watch?v=TWCFrQPddVM \$\endgroup\$ – Bimpelrekkie Feb 3 '18 at 16:22
  • \$\begingroup\$ I think that if I set the scope to the averaging mode, then the scope will make some more readings (if I set the number to 256, then it will make 256 readings) of the input signal and after that it will make a average of those signals. And if the scope will make more readings, it will take some time to show the average on the scree. It is true ? \$\endgroup\$ – mike_mike Feb 3 '18 at 16:24
  • \$\begingroup\$ See, you do understand/know. So what would do that to the trace, I mean using more averaging makes the trace... because we average out the noise more. \$\endgroup\$ – Bimpelrekkie Feb 3 '18 at 16:25
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Start with Normal
If you are only interested in DC voltage, you can go to Average to help reduce noise. But why use a 'scope? Use a multimeter instead - it is almost always more accurate.

Perhaps you are interested in noise pulses on that LM317 power supply, but the noise hides them, and you wish to use averaging to see those pulses more clearly.
In this case, you must establish a reliable trigger. You must establish and verify that you have a reliable trigger in Normal mode. If those noise pulses are related to the line frequency (50 Hz, or 60 Hz), then a line trigger is a good choice.

Otherwise, you might trigger from the signal source (Channel 1 or Channel 2 for example). And the trigger must be reliable before you go to averaging display. An untriggered source signal or improperly-triggered source signal gives an undulating and incorrect display in the Average mode.

It should be obvious that a single event (rather than a repeating waveform) cannot be averaged in Average mode.
The default whenever you probe something new, should be Normal.

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If you think about it, now averging 256 samples reduces the p-p noise 1/16 or 6.25% of original.

SNR with random noise reduces with time according to the square root of the number of samples average, your observations are "log normal" (pun intended)

However just as in photography, it is always best to capture a better image by technique than try to edit it after with filtering. Averaging is just like a Low Pass filter with 2nd order random noise rejection. Quite often the noise can be random, or recurring from inductive ground leads and switched loads.

Since noise has no DC content ( we call that drift or DC error or Load regulation error) it is best to measure it AC coupled and due to stray inductive effects of the probe near 20MHz, and noise due to no load effects, it is often standard to measure supply noise with 50 ohm cable and 50 ohm scope termination ( Ac coupled). After following this method, use probe leads <=1cm near the output of the regulator or after distribution at the load, consider your scope options.

  • pk-pk mode is best for visualizing all noise effects, averaging mode for finding dominant synchnonouse events and remove random noise, but non-avearging mode is usually best with vector dots linked for a continuous trace just to look like the best analog scopes. Memory effects like P-P are just for looking at phantom noise when your DSO cannot display computed RMS, Avg, Vpp to understand where it is coming from.

Triggering to find the synchronous noise events requires skillful use of HF, LF DC, AC and threshold setting as well as choosing external events on other channels. Such as using PWM to trigger a load pulse and then examine the effect on load regulation to determine the ESR and output impedance of the regulator.

But if you understand then the raw non-averaged ( Normal ) signal is best with good probing methods.

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  • \$\begingroup\$ Thanks for the reply @Tony Stewart. EE since '75 . So when you say that it is "log normal" you mean that what I observed on the scopes screen (the reduction of noise over time) is normal ? I am asking because I don't know the meaning of the expression "log normal". \$\endgroup\$ – mike_mike Feb 3 '18 at 16:49
  • \$\begingroup\$ Yes normal but "Normal Display" mode is best. Since jitter reduces 1/ squared it has a logarithmic effect so 4 samples is 1/2 the noise 100 samples with 1/10th 1million samples is 0.1% of the noise, so it follows a log curve but that's normal \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Feb 3 '18 at 16:55
  • \$\begingroup\$ @mike_mike What is a Lognormal Distribution? has an explanation, although you will need further reading to find the meanings of the terms used in that. \$\endgroup\$ – Andrew Morton Feb 3 '18 at 16:58

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