标签: oscilloscopes

My aim is to boost your value in only twenty minutes a day. That is the amount of time that commercials take in a one-hour television program. So instead of advancing forward with the DVR or going to the fridge because that food commercial triggered your hunger to go get a snack, here’s the second best thing you can do besides an abdominal workout or throwing dumb bells around. Bettering your engineering skills involves both the physical circuit evaluation and solving equations. As it turns out, you can probably do both for an investment of under $100. In this world of surface mount, highly integrated silicon, expensive software, and sophisticated test equipment, how can that be? Well, relax folks. I’m a power engineer. I’m expected to achieve 100 percent efficiency at no cost. Of course you’ll settle for high efficiency at some cost. However, I’m a cheap skate when it comes to investing in my business. I look for deals that allow you to build a lab at minimal expense. Solving equations is a great way to improve your value. The time spent designing in most engineering jobs is 5 to 10 percent. Or at least that was my experience at larger corporations. That number increased at startups but not much considering I developed web pages, marketing plans, business plans, etc. With such a little amount of time invested, you can quickly lose your skills. This happened to me when I decided to go back to graduate school after three and a half years in industry. In order to refresh my knowledge, I went down to the college bookstore and bought a book titled Schaum's Outline of Electric Circuits. Nowadays you just look it up on Amazon (Reference 2, 3). They are up to the sixth edition versus the 2nd edition I used 25 years ago when dinosaurs ruled the earth and cars consumed fuel at a rate faster than Seattle-ites consume coffee. As for the physical part of improving your value, life is so unfair. Components have shrunk to the point where us fifty-somethings can't even see the darn things. Further frustrating you is the inability to get a scope probe on the lead let alone hook it with the probe. As a final blow, who wants to spend tens of thousands of dollars outfitting a lab? Fear not my friends; El Cheapo to the rescue. The best way to solve the dilemma of the physical circuit is to invest in the old style plugin breadboard with leaded components. I know what you’re thinking: "Good luck finding one and then purchasing the components individually let alone finding leaded ones." I have a solution. It even has jumper wires so you don’t end up stripping that old Ethernet cable that’s long since been replaced by wireless in your home. That will keep you from scrounging around in the attic like an uninvited chipmunk for the second time. Those of you who suffered the fate in the 1990s are probably grimacing in agony at that memory. My solution for a physical lab platform is in the form of the Radio Shack Electronics Learning Lab (Reference 1). Prior to financial strangulation by divorce and cheaper labor, I intended to get one of these for each of my sons. I would wait for Christmas so that the price would drop from $59 to $49. I was pleasantly surprised to find out that these are currently (as of this writing August 2015) going for only $31 smackers.   These make for excellent hobby kits that will have your kids focusing less on thumb pressing their gizmo and more on the physical world and the holidays are just around the corner. No, Radio Shack doesn’t pay me although I could use a break on last minute components for my prototype. I will say that Mouser, Coilcraft, and Fairchild have been more than responsive for providing fast turnarounds. Perhaps you are thinking, "Big deal getting me a thirty dollar circuit that I have to analyze with a kilobucks scope." As it turns out, there are several smartphone applications such as Oscilloscope Pro (See Reference 6) that turn your phone into an oscilloscope. Perform a Google search and you will find both Android and iPhone applications to suit your needs. Just remember, there are voltage limitations to adhere to unless you wish to fry your phone. Don’t be plugging these into the wall until you understand the allowed input ranges. Like oscilloscopes, digital multimeters (DMMs) have really come down in price. Walmart has DMMs (See Reference 8) for under $10. My suggestion is to get one that has the ability to read current as well as voltage. However you might still have to insert a low value resistor in order to read current with an oscilloscope as current probes are not easily adapted to phone based scope applications. I typically parallel ten 1-ohm resistors to ensure accuracy for measuring current. Here is one final hint for you. Although the Radio Shack Learning Lab is battery powered, you just might need to make yourself a DC power source. Instead of hiring me, buying a demo board, or stretching a long lead from the cigarette lighter in your car; grab yourself a bunch of these jacks (490-PJ-002AH , See Reference 7) that fit the plug on most computer power supplies. Although they are surface mount products, you can solder wires to them or order through-hole versions. In addition to smartphone-based oscilloscope applications, you can find some neat little signal generators too. Some versions have PWM capability. Just remember to turn the sound down as the whole office will glare at you like they did me. The one I used would activate the phone’s speaker if I didn’t have a load plugged into the audio jack. Don’t tell your offspring that these signal generator apps are available as they need to experience the frustration of wiring a 555 timer for themselves. It probably won’t work anyway as they have most likely already downloaded the sig gen app between times when they were getting game cheat codes and overusing the world “like”. By the way, I developed an app that counts that overuse for you!   Scott Deuty

Every day I am astounded at the versatility and uses of oscilloscopes ( aka cathode-ray oscilloscope or digital storage oscilloscope ) as well logic analysers and a range of ancillary tools such as logic analysers and function generators. Normally associated with hardware debug such tools are also in a variety of ways in software debugging as well. But the best way to understand the capabilities of any system – hardware or software – is to do a tear-down. Normally this is something developers do when they are tasked with building some device or system for some market or design need: find a similar device and take it apart and see that others did to make it work. I think a similar strategy is useful in evaluating and using the tools and better yet. Better yet, once you understand the principles at work from study and use: build one yourself. That is what the authors of " Create a DIY scope/logic analyser " have done: using a programmable SoC, they have built much of the functionality of a standard off the shelf oscilloscope into just a few components. Although they have squeezed a lot of capability out of just a few components, it would be interesting to see how it compares to traditional factory-made alternatives.  

In the Jurassic period, all oscilloscopes and logic analysers were large, hulking bench instruments that were incredibly expensive. However, a decade or so ago a variety of low-cost scopes and analysers appeared. They are not analogous to the little shrew-like mammal that replaced the dinosaurs, since the high-end devices are essential for lots of engineering work and will never disappear. Instead, these USB-connected instruments fill a niche for those with limited budgets who are not working on bleeding-edge projects. Over the years I've reviewed a number of these. Saleae's new Logic 16 is a sixteen channel USB-based logic analyser (LA). It's one of the very few cross platform LAs available, with application software that runs under Windows, Linux, and on the Mac. Its speed is a function of the number of channels being used. With three the unit clocks data in at up to 100MHz. Nine channels (somewhat puzzling; one would think eight would be more likely) drops it to 32MHz, and half that speed is attainable when all 16 channels are in use. With all of the mad voltage levels now used in digital circuits, it's nice that Logic supports two voltage ranges: 1.8 to 3.6V and 3.6 to 5V. Capture data is streamed through short on-device buffers to the host computer, which can handle up to 1 TB of data given minimal transitions (the data is compressed). As a practical matter the software suggests that most host computers can handle up to about 1.8 GB. Like with all of these USB devices one must not overrun the USB connection, but I had no problems at all using a four-year old MacBook Pro with a 16MHz acquisition rate. A built-in protocol analyser decodes CAN, DMX-512, I2C, I2S/PCM, Manchester, 1-Wire, async serial, simple clocked parallel, SPI, and UNI/O. The Logic 16 comes in a stunning metal case with non-slip rubberized bottom. It's spare, like an iPhone. Beautiful. There are no controls, just connectors for the probes and USB, and one LED. It comes with a zip-up carrying case that is so elegant it could be a high-couture fashion accessory. And it will all fit nicely in a handbag for those ad hoc debugging sessions that always seem to come up at wedding receptions and cocktail parties. Unlike some USB LAs, the Logic 16 does come with micro-grabbers for each of the channels and ground. Installation is straight-forward, but it does seem to want to install its own USB driver (on the PC). I turned to a Mac installation at that point, since my USB ports were busy handling other experiments. It's easy to connect the probe wires, and it's just as easy to connect them incorrectly. Doesn't the black wire go to ground? Nope. The instructions are very clear about this. The ground wire is carefully labelled. Black is input 0, because, as is common on these small logic analysers, the wire colours use the resistor colour code to denote their bit position. The unit comes with neither software nor manual. Cookies, though, are included with the suggestion that, after downloading the application from the company's web site, one should snack on them. A 23-page manual is online. The residents of Ganssleville enjoyed the Chips Ahoy. The display is very simple, stark almost, with nothing extraneous on the screen. The black background mirrors the inky black of the machined aluminium case. The ease of use is unmatched by any other USB logic analyser I've tried. Navigation is breathtakingly fast and smooth.   Screen shot of the Logic 16's interface.   Some of these small LAs falter when uploading data to the host computer. It took about seven seconds to acquire and upload 100 million samples (of all 16 channels) gathered at a 16MHz rate... which is exactly the time required to gather the data. The upload is seemingly instantaneous. Trigger modes are limited. Any single channel can start the logic analyser on an up- or down-going edge, and that can be combined with a logic one, zero, or don't-care on any combination of the other channels. There are no complex triggering modes, and it's not possible to trigger on just a simple binary pattern sans edge. The reasoning is that the Logic 16 can acquire such a vast amount of data that the event of interest will likely be captured. The company tells me they are working on enhancements to the trigger, as well as adding a search feature to the application. Width, period, duty cycle and frequency measurements are displayed for the data at the mouse position. Two cursors show absolute time from the trigger event, plus the time between cursors. Interestingly, the delta time measurement also shows its accuracy in per cent due to quantisation from the acquisition rate. The binary value of the data at the mouse cursor position is displayed; it can also show hex, decimal, etc. equivalents. But those are shown after the binary, and eat up a lot of screen space. I wish these additional radix displays were below the binary rather than adjacent to it. Saleae has a community site where they make an SDK available. This means you could develop your own protocol analyser. The source for all of their analysers is there, providing a framework on which to base a custom version. I looked at the I 2 C module and found that there's not a lot of code required. The SDK found me longing for single analogue channel as it would be so easy to make this a data logger, and analogue is often more interesting to log than digital signals. But that would be a different instrument. You can give the application itself a whirl, as it will go into a simulation mode if a Logic 16 isn't detected. I generally prefer a dedicated bench instrument rather than a USB device, as the former's knobs are so much easier to manipulate when probing a board. But the Logic 16's UI is so intuitive, and so easy to manipulate, that it outshines the bench analysers I've used. Like all of these low-cost LAs the Logic 16 is not feature-rich. Its weakest point is the lack of triggering flexibility. But that aside, it does a good job of providing much of what a much more expensive bench analyser will do. Too my knowledge it's the only such product that supports Windows, Linux and Mac. The price ($299) is right, it'll slip into a shirt pocket, and the engineering is something Steve Jobs would envy.  

How do you build and verify the instrumentation which allows you to do leading-edge work, and advance the state of the art? This has always been an engineering and science dilemma. How do you confirm vital metrology, which must itself be better than what you are now trying to measure? How do you actually know that what you think you are measuring is really what is there, anyway? Or, to cite the Roman poet Juvenal, "who will guard the guards themselves?" Obviously, there is no single or simple answer to this, but we do know one thing: it isn't easy. Yet vendors such as Agilent Technologies continuously work the problem, both by developing radically new instrumentation and by looking to squeeze that last bit of performance perfection out of what we already have. I saw a clear demonstration of this when Agilent showed two new test/calibration products, for different situations but with a common underlying theme. One is the N2809A PrecisionProbe software for their Infiniium 90000 X-Series and 90000A Series of oscilloscopes; the other is a "system impulse response correction" (SIRC) calibration product for multimode optical receivers with bandwidths up to and beyond 25GHz, and single-mode receivers nearing 100GHz bandwidth. The PrecisionProbe product lets users fully characterize the signal path's passive components (cables or fixtures), and then compensates automatically during signal acquisition and analysis. For example, it allows you to take your GHz cables—which were not absolutely perfect to begin with, and have likely endured a hard life of bending, twisting, and maybe even some kinks and crushing—and develop a profile of that specific item, which is then associated with it for all future measurements, as a downloadable file. As a result, the scope assesses only the device under test (DUT) in its displays and readouts, while cable or fixture is taken "out of the picture", so to speak. It's always been possible, of course, to work around these imperfections to some extent, and engineers have done so for many years. But it's a complex process, often involving a vector network analyzer (VNA) and maintenance of a paper trail of the numerous calibration factors and points for that setup, as well as physically disconnecting and reconnecting each item to be checked. That's both labor-intensive and can introduce new errors, since things never go back quite the way they were. While the idea of measurement and compenzation is certainly not new, making it practical at the bandwidths of these scopes required using the scope's internal 15ps rise-time, indium phosphide (InP) pulser IC as an signal source to the outside. Having this source is only part of the story. The Agilent scopes embed a DSP hardware accelerator to perform the compenzation calculations (frequency response, loss, linearity, skew, artifacts, and phase shift) in real-time, so the scope incurs a throughput loss of only a few percent—practically invisible to the user. Advancing the state of the art in instrumentation has always been among the toughest challenges engineers face. You have to determine the sources of error, then figure out ways to minimize them, cancel them, or compensate for them—and also how to verify that what you are seeing is reality, and not a figment of the test and/or your own mental "projection". The recently deceased analog-expert Jim Williams often explored this, and one of his first published articles, "This 30-ppm scale proves that analog designs aren't dead yet" is still a remarkable exposition on this subject. Do you address these sorts of challenges? Have you ever had to? Has your equipment, fixturing, overall setup, or even your own assumptions ever led you astray—and for how long?