DSO versus DPO – What’s the difference?

Blog Post created by JohnnieHancock Employee on Sep 1, 2016

DSO stands for Digital Storage Oscilloscope. DPO stands for Digital Phosphor Oscilloscope. A DPO is also a DSO. And a DSO can also be a DPO. So what exactly is a DSO and what is a DPO?

A DSO is typically a real-time sampling oscilloscope. Real-time sampling simply means that the scope is able to capture signals in a single acquisition utilizing a high sample rate analog-to-digital (ADC). In other words, a DSO doesn’t utilize repetitive acquisitions to “build-up” sufficient samples to represent the signal under test (equivalent-time sampling), although this is a not a hard-and-fast rule.

As mentioned before, a DPO is also DSO. But a DPO adds one additional element that allows it to better represent the signal’s third dimension. The first two obvious dimensions are voltage and time. The third and less obvious dimension is frequency-of-occurrence, which is represented by trace intensity on a scope’s display. If you can beckon back to the old analog scope days you may recall that these oscilloscopes were able to display a range of trace intensities. This can provide valuable insight into the true analog characteristics of a signal under test. This is especially true for complex-modulated analog signals as shown below, as well as for digital signals that contain varying degrees of noise and/or jitter.

With older analog scope technology, trace intensity variation was a natural phenomenon based on how much time the electron beam remained within a XY region on the inside face of the cathode ray tube (CRT). The inside face of CRTs of analog oscilloscopes are coated with a material called phosphor. When electrons strike the phosphor, the phosphor begins to glow. The more electrons that strike the phosphor in a given region of the CRT for a given amount of time, the brighter the phosphor glows.

When DSOs were birthed in the early 1980’s, this third dimension of trace intensity was initially lost as shown in the screen image below.

As technology progressed, oscilloscope vendors developed a technique that could closely emulate the display quality of analog oscilloscopes utilizing digital signal processing to bring the third dimension back from the grave as shown in the screen image below.

Basically, by counting the number of hits (digital samples) in particular XY regions of a bit map — sometimes called buckets — pixel intensity could be digitally modified to represent trace intensity modulation of phosphor. This is where the term Digital Phosphor Oscilloscope (DPO) came from.

So why doesn’t Keysight have DPOs? Actually, we do. But we don’t call them DPOs. Nearly all of Keysight’s DSOs employ trace intensity modulation. In fact, Keysight’s oscilloscope display technology provides the highest quality trace intensity modulation due to the fact that Keysight scopes have the industry’s fastest waveform update rates with deep memory acquisitions. This provides more hits in XY regions (buckets) in a shorter amount of time to provide a higher level of statistical information for which to base pixel intensity upon.

So why doesn’t Keysight call them DPOs? Keysight believes there is enough confusion concerning different names for the same basic instrument. My large screen flat-panel television that I watch Rockies baseball games on is still just a TV even though it utilizes an entirely different technology than older CRT-based televisions. And besides, why use an old analog technology term when many of today’s younger engineers have never used an analog oscilloscope and don’t have a clue what phosphor has to do with an oscilloscope? Same goes for the term “sweep”. Refer to one my previous blog posts titled, “Oscilloscope Triggering: When is Normal not so Normal?”.

Maybe we should call them DSOWDPTMSCDSAAMDMCs (Digital Storage Oscilloscope with Digital Phosphor Technology, Mixed Signal Channels, Digital Signal Analysis, and Mixed Domain Measurement Capabilities). But in my eyes, it’s still just a scope! And if you are an Aussie, it will always be a CRO (pronounced “crow”).

For more information, see this application note on Oscilloscope Display Quality.