TDS 2000B series Measurements and Averaging - Page 1
Forum section: electronics / testgear

author: KaloyanP
author_rank: Newbie
timestamp: July 19, 2013, 07:50:25 pm
content: Hello, I work at a PCB assembly&test facility with some pretty ancient automated test machines. Today we had the idea to improve the workflow by remote controlling the Tek 2000B scopes through USB. Hopefully we will be able to make our test software set the scopes up automatically and take measurements. So my question is the following:Are the automatic measurements affected by the averaging function of the scope? There are a lot of tests where we have to measure things like Vpp, rise time, etc, and it is always easier to just use the built in measurements. However, when the specs are too strict, the measured Pk value might be larger than the actual peak voltage of the low frequency signal, so we have to use cursors, which, of course, requires someone to sit in front of the scope.The manual says that the scope examines the whole 2500 point buffer to perform the automatic measurement. However, I don't know if this means the 2,500 sample points taken in the last trigger, or the 2,500 sample points "displayed" on the screen.So: does the averaging function affect the automatic measurements of the scope?Thanks a lot,Kal

author: jpb
author_rank: Super Contributor
timestamp: July 19, 2013, 08:15:05 pm
content: Your question isn't very clear. I don't know the Tek commands but for most scopes the remote commands just match those on the scope itself. So if you set average acquisition remotely that is what you get, if you set normal mode that is what you get.

author: KaloyanP
author_rank: Newbie
timestamp: July 19, 2013, 09:15:46 pm
content: P.S. I reread what I had written and now I realise where the confusion stems from: when I say "automatic measurement" I mean the values you get when you press the "MEAS" button on the scope. I am wondering whether these values are affected by the averaging function of the scope or whether they are only done on the raw data captured during the last trigger event.Sorry, I will reiterateWhen you are measuring a signal, say, a 50us square wave pulse of 1Vpp, you will get a certain amount of random noise due to the preamps, sampling, etc. This noise is mostly random and of very high frequency. Thus, the scope has the option to average several consecutive waveforms (this preserves the repetitive high- frequency components of the periodic signal). Without averaging, your 1Vpp signal will register as 1.1 or even worse when displaying the automatic measurements due to this noise. However, +/-10% is a pretty wide spec, sometimes we have to deal with much tighter specs like +/-5 or smaller. Thus, the automatic measurement is out of range while the actual low frequency waveform is happily within spec.Thus, we are not sure if, with averaging turned on, the average waveform on the screen is used to calculate the measurement values or the actual (noisy) sample data for the last trigger.

author: jpb
author_rank: Super Contributor
timestamp: July 19, 2013, 10:20:17 pm
content: I would have thought it was the average waveform (the one displayed) that was used, but as I have a LeCroy WaveJet and not a Tek this is only supposition on my part - I don't know.But it is easy to find out I would have thought just by trying it. Use the cursors to measure values from the screen with average mode on and then compare them to the automatic measurements.