The position of deep reminiscence in oscilloscope efficiency
Relating to oscilloscopes, bandwidth, pattern price, and reminiscence depth are persistently cited because the three most essential specs. Reminiscence depth determines the quantity of waveform knowledge that may be captured and saved for evaluation. A bigger reminiscence depth permits for longer time durations to be captured and preserves extra waveform particulars. That is particularly useful when analyzing advanced or intermittent indicators, capturing uncommon occasions, or performing in-depth evaluation and troubleshooting.
Acquisition reminiscence and reminiscence depth
Acquisition reminiscence, additionally known as reminiscence depth, refers back to the variety of samples an oscilloscope can retailer with every acquisition. It’s decided by multiplying the pattern price (MSa/s) by the point captured. For instance, an acquisition reminiscence of 1 Mpts implies that an oscilloscope acquires a million samples in a single acquisition on one channel. Even when a number of channels purchase the identical quantity of reminiscence concurrently, the acquisition reminiscence remains to be 1 Mpts.
Oscilloscopes usually include a predetermined quantity of base acquisition reminiscence as a part of the usual buy. Prior to now, producers produced totally different variations of {hardware} with particular reminiscence capacities. Nevertheless, within the early 2000s, producers adopted a more cost effective method by making a single {hardware} platform with the deepest obtainable reminiscence, which may very well be enabled by way of software program licensing.
This method allowed customers to begin with a lower-priced oscilloscope that had much less reminiscence after which license extra reminiscence as their wants developed. It is very important consult with a datasheet to find out whether or not a specified reminiscence worth is the bottom reminiscence included with the instrument or the utmost worth related to an extra paid choice.
The advantages of elevated reminiscence are evident, however there’s a catch: extra reminiscence means elevated processing necessities, which can lead to slower general operation.
Reminiscence depth will not be a static worth
When buying any instrument, you might uncover that a number of enticing specs are mutually unique. This additionally applies to oscilloscope producers and their acquisition reminiscence depth specs. A promoted reminiscence depth will not be obtainable underneath sure scope settings as a result of tradeoffs inside the oscilloscope structure.
One widespread tradeoff, for example, entails the variety of channels. Oscilloscopes have a set quantity of acquisition reminiscence shared throughout channels, reference waveforms, and different capabilities. The utmost reminiscence specification is mostly relevant when half of the analog channels are lively however decreases by an element of two for every lively channel that shares the identical processing and storage path. For instance, the reminiscence depth could also be 4 Mpts when solely channel 1 is lively however drops to 2 Mpts per channel when channel 2 is turned on.
Deep reminiscence
The definition of deep reminiscence varies amongst oscilloscope distributors and has developed over time. Early digital oscilloscopes had reminiscence measured in lots of of factors, which elevated to Kpts within the Nineteen Nineties. Fashionable oscilloscopes usually provide reminiscence depths within the tens to lots of of Mpts. Distributors could declare their devices have deep reminiscence. That is notably true for older oscilloscope fashions that had been thought-about deep once they had been new however have low reminiscence depth in comparison with present opponents who would possibly present as much as 100 instances extra reminiscence depth.
Deep reminiscence advantages numerous varieties of embedded {hardware} testing, and it’s particularly helpful when capturing very long time intervals. Deep reminiscence means that you can isolate an observable drawback and hint it again to its supply. It additionally helps in fixing advanced points associated to electromagnetic interference (EMI) and crosstalk.
Serial buses like I2C, SPI, RS-232, CAN or LIN, that are generally used for digital designs, will be analyzed extra successfully with deep reminiscence. Whereas protocol triggers help in troubleshooting, visibility throughout a number of bursts or packets of knowledge usually requires capturing a bigger time span. Nevertheless, there are tradeoffs between decreasing the pattern price to seize extra time (which dangers under-sampling the bus) and utilizing segmented reminiscence (which limits evaluation capabilities and inter-segment visibility).
In purposes that require additional evaluation, it’s essential to seize as a lot data as attainable and analyze it afterwards. Oscilloscope instruments and evaluation purposes, in addition to offloading captured knowledge to MATLAB or Python scripts, can facilitate in-depth evaluation and insights.
Reminiscence, pattern price, and bandwidth
Reminiscence depth, pattern price, and bandwidth are carefully associated specs. Having extra reminiscence permits customers to seize a particular period of time or prolong the seize time at a given pattern price:
Reminiscence = (Pattern price) * (Time captured).
With an elevated reminiscence depth, customers can retain the wanted pattern price and even use a sooner pattern price for buying a mix of sluggish and quick indicators. Nevertheless, there’s a restrict to the reminiscence depth when capturing extra time by adjusting the timebase to a slower setting. Past this restrict, the instrument should scale back its pattern price, doubtlessly resulting in undersampled indicators and aliasing. This results in invalid measurement outcomes, and customers could not understand that the pattern price is inadequate for the rated bandwidth of their oscilloscope.
Oscilloscopes don’t present notifications when the pattern price is insufficient for the rated bandwidth, making it difficult to determine undersampling or aliasing points. Oscilloscopes with restricted pattern price exacerbate this problem.
Extra reminiscence permits the person to seize prolonged time intervals whereas preserving an ample pattern price for quick indicators. When a person goals to seize an extended time interval, scopes with restricted reminiscence will inevitably attain their most reminiscence capability. Consequently, the instrument makes a tradeoff, reducing the pattern price to accommodate the desired time interval. Because of this a shallow acquisition reminiscence may end up in a pattern price that’s too low to accurately seize a sign (see Determine 1).
Determine 1 Extra reminiscence means the instrument can seize extra time with out decreasing pattern price. Supply: Rohde & Schwarz
Most oscilloscopes include a default reminiscence restrict to stop efficiency points when deep reminiscence is enabled. As an illustration, one vendor could have a default restrict of 10 Kpts whereas one other could have a restrict of 10 Mpts, although each scopes provide larger most reminiscence capacities.
Customers must manually modify the scope settings to alter the default restrict and make the most of extra reminiscence. This usually entails accessing a handbook acquisition setting dialog the place they’ll management parameters like pattern price.
Some oscilloscopes could not enable impartial management over pattern price, timebase, and reminiscence depth, resulting in a irritating person expertise with restricted workarounds. It’s usually advisable to decide on oscilloscopes that enable the person to independently management all three settings and provide the benefit of capturing off-screen acquisitions.
Segmented reminiscence
Most oscilloscopes provide segmented reminiscence mode, both as a typical characteristic or as an non-obligatory improve. This mode is beneficial for capturing indicators with inactive intervals, similar to serial buses or RF chirps. In segmented reminiscence mode, the oscilloscope saves reminiscence area by capturing solely the lively components of the sign. This allows single-shot captures throughout an extended time period in comparison with steady acquisition.
Segmented mode doesn’t compensate for shallow reminiscence. The quantity of reminiscence obtainable per phase equals most reminiscence divided by the variety of segments. As proven in Determine 2, oscilloscopes with higher reminiscence depth provide extra highly effective segmented reminiscence, permitting for extra segments, longer time per phase or larger general pattern charges.
Determine 2 Segmented reminiscence saves solely a seize window across the set off occasion for extra environment friendly reminiscence utilization. Extra reminiscence means extra segments, extra pattern price, or extra time with every phase. Supply: Rohde & Schwarz
It is very important notice that segmented reminiscence mode has some tradeoffs. It’s a single-shot acquisition and doesn’t work nicely in RUN repetitive mode. As well as, viewing the measurement outcomes entails transferring by way of a number of acquisition screens, and evaluation throughout segments usually has limitations.
Serial bus decode and reminiscence
Oscilloscopes outfitted with serial bus triggering and decode purposes are helpful for debugging and testing. Nevertheless, the correlation between deep reminiscence and the variety of captured packets will be troublesome to find out. Every protocol requires a sure pattern price for proper decoding, and deep reminiscence with protocol decode can pressure the oscilloscope’s processing assets, making it sluggish.
Subsequent era oscilloscopes, such because the R&S MXO 4, clear up this drawback with sensible architectures. For instance, an oscilloscope may have a replica pattern path for protocol decode, enabling dual-path protocol evaluation (see Determine 3). This system permits a excessive variety of packets to be decoded accurately, even with a slower analog pattern price. It additionally shops protocol packets as packets, which leads to a a lot smaller packet data database, that means that the instrument can have a sooner replace price and higher responsiveness.
Determine 3 R&S MXO 4 Collection oscilloscopes have a dual-path protocol evaluation. A separate packet decode reminiscence means a extra responsive instrument with a deterministic variety of most packets that may be captured. Supply: Rohde & Schwarz
The important position of reminiscence depth
Reminiscence depth performs a significant position in oscilloscope efficiency, enabling the seize of longer time intervals and preserving waveform particulars. It enhances {hardware} testing and evaluation by facilitating the decoding of serial bus protocols and offering a complete view of indicators. Nevertheless, there are tradeoffs to contemplate, similar to elevated processing necessities and potential tradeoffs with the variety of channels. Due to this fact, a cautious comparability of producer specs is crucial to make an knowledgeable choice when choosing the precise oscilloscope in your wants. Â Â
Joel Woodward is an oscilloscope product planner at Rohde & Schwarz.
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