Understanding long/normal memory setting on DSO - Page 1
Forum section: electronics / beginners

author: Dan Moos
author_rank: Frequent Contributor
timestamp: December 31, 2016, 09:22:01 pm
content: Am I correct that the long memory setting on my scope gives me a longer record time at the expense of resolution? In other words, I'm using the same memory account, but long extends the record length by using less samples per second?Second, what are the benefits of choosing one versus the other? I can see why in single shot mode, you might want a longer record time in some instances, but in normal sweep mode, it seems like if my understanding is correct on how it works, normal memory would be best. Is there more to it?

author: bob1033
author_rank: Contributor
timestamp: December 31, 2016, 10:22:57 pm
content: Quote from: Dan Moos on December 31, 2016, 09:22:01 pmThe length of the recording depends on the timebase you currently have selected. If you have ten divisions across your screen and you're at 50msecs/div, you'll have a record length of 0.5 seconds. Likewise, if you're at 500us/div, the record length will be 5ms. The long memory setting on your oscilloscope affects the depth or amount of memory you have for said recording. By having more memory, the scope is able to sustain a higher sampling rate at slower timebases as with long memory as the scope can store 512Kpts to 1Mpts over the record length rather than 16Kpts in normal mode (these arent exact numbers, depends on the scope).Generally speaking, more memory allows you to sustain the scope's maximum sample rate across slower timebases. However, more memory also increases the dead time of the scope, or decreases the update rate of the scope .Using a greater amount of memory puts considerably more strain on the processor and can increase the dead time of the scope. Dead time is how fast an oscilloscope can trigger, process the data it has captured, and then display it to the screen of the oscilloscope. The less dead time you have, the more likely you are to catch an infrequent event. If you're looking at a repeating, regular signal, that's not much of an issue. If you're trying to detect a glitch that happens once every 5 seconds, increased deadtime can decrease the scope's chances of actually detecting it. Some scopes like ones with Megazoom get around this problem of slower updates with more memory using custom asics or processing techniques.

author: Dan Moos
author_rank: Frequent Contributor
timestamp: December 31, 2016, 11:02:53 pm
content: Ok, parts of that sunk in🙋.One thing you said I want clarification on. Do I rightfully understand that what I see on screen is the full record time at that instant? I always thought the record time extend off screen in both directions.Also, I'm not clear what the "dead time" is exactly.Thanks for the help!

author: bob1033
author_rank: Contributor
timestamp: January 01, 2017, 12:47:20 am
content: Scopes including the Rigols have a zoom function separate from the timebase. This article from Rigol makes it seem like the onscreen waveform is the total record time of the scope:https://blog.philippklaus.de/assets/DS1000-Deep-Memory.pdf. I would presume that the time onscreen would usually be the the minimum needed acquisition window: the actual time period the scope samples. See, the acquisition window in seconds can be found by memory depth/samples per second. Through this equation, if I have more memory, I can record a longer period of time with my scope. However, the maximum record time is also bounded by the sample rate.Lets say I have a scope with either 1Mpts or 16Kpts of memory and a sample rate of 500Msamples. I then change the timebase to 200us/divs. If I have 10 divisions on screen, then I need to look at a minimum of 2ms. With 1Mpts and the max sample rate, I can successfully do this (1M/500M=2ms). However, if I only have 16Kpts of memory, I can only record .032ms at the max sample rate. I need to see 2ms at one time, so to compensate my scope lowers its sample rate. In order to achieve a record time of 2ms at 16Kpts, my scope has to sample at 8Msamples. Usually, I wouldn't want to record longer than I'm actually viewing as my scope may have to lower its sample rate for longer times. This leads me to believe that scopes sample to minimum amount of time to fill the screen with data to keep the sample rate the highest.More on acquisition windows, memory, and sample rates:http://www.electronicproducts.com/Test_and_Measurement/Benchtop_Rack_Mountable/Why_oscilloscopes_need_deep_acquisition_memory.aspx

author: tautech
author_rank: Super Contributor
timestamp: January 01, 2017, 01:00:19 am
content: A nice little .gif done by rf-loop showing the interrelationship between memory depth, sampling speed and timebase settings in a SDS1000X in Stop mode.In this post:https://www.eevblog.com/forum/testgear/siglent-sds1000x-series-oscilloscopes/msg933997/#msg933997Bugger the .gif or link won't copy here.

author: bob1033
author_rank: Contributor
timestamp: January 01, 2017, 01:14:49 am
content: As with dead time, after every acquisition window, the scope has to process the data, write to memory, rearm the trigger, update the screen, etc. This requires time for the scope to do, and the signal you're measuring doesn't wait until your scope is ready to take more samples of the signal.First, your scope actively samples the waveform seen in the blue areas. After sampling, the scope processes the data and displays it on screen. The time that takes is seen in grey. During this time, the scope may miss glitches and rare events in the signal. The time of the grey area depends on how fast your scope is at processing the samples taken from the waveform.If you have more memory (say 1Mpts vs 16 Kpts), you have much more data to process and therefore the scope will have to take longer to process it all. This results in greater time spent in the grey area of the diagram that represents dead time.In the gif in tautech's post, at faster timebases you can see the scope only uses the minimum amount of memory needed to show the waveform at the highest sample rate to cut down dead time.

author: w2aew
author_rank: Super Contributor
timestamp: January 01, 2017, 05:21:55 am
content: Quote from: Dan Moos on December 31, 2016, 09:22:01 pmThis video might help:

author: eKretz
author_rank: Frequent Contributor
timestamp: January 01, 2017, 11:45:10 am
content: Quote from: Dan Moos on December 31, 2016, 11:02:53 pmI'm not sure how this works on other scopes, but on my Tek TDS754 at least, IIRC this can be set so that it extends the time recorded on either side of the trigger (so it records off screen for extra divisions) or so that it uses all the extended memory to record higher resolution on one screen's worth of divisions.