标签: scope

Anyone here who still remembers the Heathkit "Stackables"? These were presented with a blue plastic box and a white front panel in build-it-yourself kit form.   As I recall, the Ix-528x series included a multimeter, RF signal generator, audio signal generator, signal tracer, RLC bridge, and -- possibly (I can't remember for sure) -- a power supply. These were a family of low-cost basic test instruments for the electronics experimenter/hobbyist/repair person. Their notable claim to fame was that they were all built into the same plastic cabinet that featured molded feet that interlocked with a molded top ridge so they could be stacked vertically and stably on a benchtop, thus saving a lot of scarce and valuable real-estate.   Think for just a moment about this simple concept, and how it can similarly apply in your food pantry. Lately I've noticed two types of tin cans -- the older style that can be vertically stacked because their bottoms mate with the tops of the cans beneath them (e.g., Campbell's soup tins), and a newer style that was designed by some dimwit who did not think to include the stackability feature (e.g., Kroger-brand veggies). Try to stack these types and they come crashing down like a house of cards. No question as to the preferred type to make the best use of the limited horizontal space in a pantry.   Other than the intentional Heathkit stackable feature, most older styles of test equipment were not designed to be stackable. However, since they typically came in flat-topped cabinet enclosures, this was an inherent attribute. Even units from different manufactures could easily be stacked on a bench into a reasonably stable "tower of power" -- just look at any of illustrator Daniel Guidera's monthly EETimes caption contest cartoons for examples.   Then, along came the aesthetic enclosure designers hired by marketeering managers with no brains. Fancy curves and non-flat tops -- test equipment styling started looking more like sports cars than truly functional items. The result is that many of them can no longer be stacked.   Keeping this in mind, let's look at how some of these non-mating test equipment boxes can be made to fit together on a limited-space benchtop. Consider the DMM sitting on top of a power supply as shown below. The unfolded front tilt-flap of the DMM keeps it from sliding backwards off the tilted-upwards power supply, but it tends to slip forward and hide the power supply's display.     However, when the tilt-flap folded against the DMM bottom as shown below, it prevents the DMM feet from engaging the power supply and it slides off.     One solution is to forcibly remove the annoying tilt-flap from the DMM, thereby allowing its feet to hold it somewhat in place. But the serial number that is on the tilt-flap is no longer part of the DMM, which could raise some issues with the ISO-9000 calibration auditor.   Next, consider the frequency counter sitting on top of a function generator as shown below. The front lip of the frequency counter holds it from slipping off the function generator, but it does make the function generator's button labels hard to see.     Swapping these two devices round and placing the function generator on top of the frequency counter doesn't work; the function generator's tilt-riser holds it from slipping off, but hides the counter's digits.   Even identical equipment made by the same dimwit manufacturer who never considered stackability can be stacked with the aid of series-connected cable tie-wraps as shown below (don't use duct tape because it can cover up the ventilation holes). By the way, if you have to tie-wrap-stack two scopes together to make a 4-channel scope, you (or your corporate fiscal expenditure manager) might be a Redneck (with apologies to Jeff Foxworthy).     Note that these Atten brand "Scopes from Hell" have a couple of functional bugs (among many) that are clearly visible in the above photo. One of these bugs is fairly obvious -- the other is a little more hidden. Can you spot these bugs? If so, please post a comment below. Also, please offer any suggestions you have for stacking these types of devices on your benchtop.   Glen Chenier Engineer

Do you still remember the Heathkit "Stackables"? These were presented with a blue plastic box and a white front panel in build-it-yourself kit form.   As I recall, the Ix-528x series included a multimeter, RF signal generator, audio signal generator, signal tracer, RLC bridge, and -- possibly (I can't remember for sure) -- a power supply. These were a family of low-cost basic test instruments for the electronics experimenter/hobbyist/repair person. Their notable claim to fame was that they were all built into the same plastic cabinet that featured molded feet that interlocked with a molded top ridge so they could be stacked vertically and stably on a benchtop, thus saving a lot of scarce and valuable real-estate.   Think for just a moment about this simple concept, and how it can similarly apply in your food pantry. Lately I've noticed two types of tin cans -- the older style that can be vertically stacked because their bottoms mate with the tops of the cans beneath them (e.g., Campbell's soup tins), and a newer style that was designed by some dimwit who did not think to include the stackability feature (e.g., Kroger-brand veggies). Try to stack these types and they come crashing down like a house of cards. No question as to the preferred type to make the best use of the limited horizontal space in a pantry.   Other than the intentional Heathkit stackable feature, most older styles of test equipment were not designed to be stackable. However, since they typically came in flat-topped cabinet enclosures, this was an inherent attribute. Even units from different manufactures could easily be stacked on a bench into a reasonably stable "tower of power" -- just look at any of illustrator Daniel Guidera's monthly EETimes caption contest cartoons for examples.   Then, along came the aesthetic enclosure designers hired by marketeering managers with no brains. Fancy curves and non-flat tops -- test equipment styling started looking more like sports cars than truly functional items. The result is that many of them can no longer be stacked.   Keeping this in mind, let's look at how some of these non-mating test equipment boxes can be made to fit together on a limited-space benchtop. Consider the DMM sitting on top of a power supply as shown below. The unfolded front tilt-flap of the DMM keeps it from sliding backwards off the tilted-upwards power supply, but it tends to slip forward and hide the power supply's display.     However, when the tilt-flap folded against the DMM bottom as shown below, it prevents the DMM feet from engaging the power supply and it slides off.     One solution is to forcibly remove the annoying tilt-flap from the DMM, thereby allowing its feet to hold it somewhat in place. But the serial number that is on the tilt-flap is no longer part of the DMM, which could raise some issues with the ISO-9000 calibration auditor.   Next, consider the frequency counter sitting on top of a function generator as shown below. The front lip of the frequency counter holds it from slipping off the function generator, but it does make the function generator's button labels hard to see.     Swapping these two devices round and placing the function generator on top of the frequency counter doesn't work; the function generator's tilt-riser holds it from slipping off, but hides the counter's digits.   Even identical equipment made by the same dimwit manufacturer who never considered stackability can be stacked with the aid of series-connected cable tie-wraps as shown below (don't use duct tape because it can cover up the ventilation holes). By the way, if you have to tie-wrap-stack two scopes together to make a 4-channel scope, you (or your corporate fiscal expenditure manager) might be a Redneck (with apologies to Jeff Foxworthy).     Note that these Atten brand "Scopes from Hell" have a couple of functional bugs (among many) that are clearly visible in the above photo. One of these bugs is fairly obvious -- the other is a little more hidden. Can you spot these bugs? If so, please post a comment below. Also, please offer any suggestions you have for stacking these types of devices on your benchtop.   Glen Chenier Engineer

It was the early 1970s when I was sent overseas with three impressively sized racks of equipment controlled by a Honeywell H316 computer. Along with them came the orders to install the system and get it running. My adventure began when my boss sent me to a local one-week Honeywell computer course on machine language. That single course was the only training I received on this new system. Apparently, I couldn't be spared for the longer hardware courses, though my boss did allow a technician (who was going to the same location as the equipment) to attend. I only had to install the computer system, train everyone how to use it, and leave. That was the plan. The peripheral devices included a paper-tape reader, teletype terminal, large reel-to-reel digital magnetic tape drive, high-speed printer, data interface, and a huge hard disc. The hard disc was about a foot high and completely sealed inside a cylindrical enclosure that was pressurized with an included nitrogen bottle and pressure regulator. As I later discovered, it was fortunate that all the hard disc electronics boards were mounted outside the pressurized enclosure. "This is not me (or the system I installed), but it is a similar picture that I found on the Internet."—Steven Karty This was back in the olden days before BIOS ROMs told the computer what to do after being turned on. So I had to "fat-finger" in around 30 16bit words of instruction, which told the computer how to read the punched paper-tape reader output. Then I had to load an ASCII punched paper-tape into the paper-tape reader, which told the computer how to read the magnetic tape drive's output. Then I had to make sure that the large magnetic tape reels, which contained the computer program, were mounted and rewound to their beginning. Then, when I had everything ready, I would simply hit the start button, the computer would read the punched paper-tape, the magnetic tape reels would spin, and the whole system would start. This initialisation procedure had to be repeated each time the system was powered on. Before the system could be shipped, it had to be packed. Before it could be packed, everything heavy had to be removed from the racks and packaged separately. Although other people did the packing and crating, I first had to disconnect and remove the equipment from the racks and make sure that I would remember how to reinstall it. Everything went smoothly—at first. The equipment, the technician, and I arrived intact at our destination. I reinstalled and reconnected all the equipment, cued everything up, and hit start. Then things got rough. The paper-tape reader ran, but the magnetic tape reels refused to budge. Most of the hardware peripheral interfaces were not only unique and custom-designed, but also poorly documented. I called the technician over and asked for his help. We single-stepped through the instructions where the computer was stuck and figured out that the computer was waiting for the hard disc interface. The computer could not go onto the next step and tell the magnetic tape reels to spin until this disc interface was ready. After using a Tektronix scope to trace through the disc interface, we concluded that the interface was waiting for a signal from the hard disc. The technician then abandoned me, saying he had been trained only on the interface and not on the hard disc. As he slipped out, he mumbled that the "origin" signal from the hard disc seemed to be missing. I realised that I would be blamed if I couldn't fix the system. That I hadn't been allowed to attend any hardware courses was irrelevant. Unfortunately, there were no replacement boards for the hard disc. Fortunately, the system documentation included schematic diagrams of the hard disc electronics boards. Deserted by the technician and feeling very lonely, I picked up the scope probes and began tracing through every circuit where I thought the origin signal was supposed to go. I finally found the origin signal at the input to a potted delay line. But I didn't see anything at the delay line's output. In desperation, I decided to solder a jumper wire around the delay line. When I then repeated the initialisation procedure, everything worked perfectly! The manufacturer of the hard disc later said the design had enough margin so it did not need a delay line, but it sent a replacement anyway. In the end, all it took was just a piece of wire to fix this computer system. But I never would have found the problem, and thus its solution, without an oscilloscope. And I would have lost interest long before finding the problem if using a Tektronix scope were not so much fun. I met another technician (who had spent two years where I installed the equipment) before I left on this trip, and asked him for any hints about the site. He said it was nice and safe, so I wouldn't have any problems. That was only partly true, because I had stomach problems the entire time. I saw him after I returned and asked if he ever had any gastrointestinal issues. He said that while he always felt fine, his wife suffered from the same problem as I had described to him in such graphical detail. When I asked him what might account for the difference, he said his wife drank the local water but he drank nothing but beer. I didn't bother asking him why he forgot to tell me that before I left. But I still wonder if he brushed his teeth with beer. I still get a kick out of using Tektronix scopes because their triggered sweep circuits always work perfectly—which is why I finally bought my own. But it's an old analogue scope with a CRT. The new Tektronix scopes with their digital displays are way more fun. Steven Karty built an oscilloscope from an EICO kit in the mid-1950s when he was 10 years old. He started working in radio and TV repair shops at 13, became an amateur radio operator at 14. He has used Tektronix scopes almost exclusively for the last 48 years, and he has BSEE. He submitted this article as part of Frankenstein's Fix, a design contest hosted by EE Times (US).  

Over the years, I've evaluated many small USB scopes and logic analysers. These devices are generally pocket sized, very inexpensive, offer a lot of functionality for the money, and use your PC for the user interface. Most have beautiful screens. But I'm not a fan of screen-based controls. Obviously, one saves a ton of money by leveraging the PC's resources, but it is frustrating to have to grab a mouse and then carefully move a virtual control to change a channel's vertical resolution or fiddle with the triggering or time base. You just can't beat a stand-alone instrument for ease of use when holding two probes in your hands, a third in your mouth, and another between two toes. Somehow one manages to just barely nudge that vertical control with the knuckles or nose. The PC interface is far less amenable in those circumstances. Owning an iPhone and iPad, I find myself occasionally trying the two-finger squeeze on my four-year old MacBook when using one of these USB instruments. One quickly gets used to new interfaces and it's a bit jarring when the Mac doesn't respond to touch commands. While at Best Buy the other day, trying to avoid the 18-year-old salespeople, I looked at a variety of PCs with touch screens. The screens worked surprisingly well. Clearly touch and gesture will dominate over the next few years. When will the USB instrument folks provide a touch-screen interface? I envision one with large controls that can be activated by the crudest of motions for those times all four limbs are holding probes. A little nose action could move a vertical gain slider up or down; a brush of the elbow changes the time/division. What about voice activation? That might be even better than the UI provided by a bench scope. "Scope: trigger at 2.4V." "Scope: 20 msec/division." It might be hard to choke out an intelligible command with a X10 in your mouth, but perhaps a grunt filter could translate. Just a few days ago my assistant had to stand over my bench and punch the "single sweep" button when I asked because my hands were balancing three probes; it sure would have been nice to just command "Scope: single sweep." The instruments would need addresses of course, so in a lab full of engineers Joe's commands won't drive Bob's scope. Shortly after saving this article to disc, an email arrived from the NSA. They seem to know my Agilent's IP address, and wrote that if I say "NSA: single sweep" near the PC's mike they'll take care of it for me.

In a series of columns recently, Guide to probing and Another guide to probing , I discussed some of the issues that arise in using scope probes as frequencies increase. Alas, too few engineers do a real analysis. Turns out, a decent (think over $300) 10 pF 10X probe at 100MHz looks like a 160Ω load! That's 14 mA at 2.3V, which is more than many gates (not to mention CPUs) can drive. In other words, putting a probe on a perfectly good circuit may cause the system to stop functioning. But it gets worse as the frequency goes up. In the following graph probe impedance in ohms is shown on the vertical axis and frequency along the horizontal. Impedances are shown for three different probes. Note that 10 pF is pretty common for decent probes; better ones, like Tektronix's very nice 5 pF TPP1000 approach a thousand dollars.     Agilent recently introduced the Infinium 90000, a 33GHz scope. ( I want one of those! ) But how do you probe a signal that fast? A 5 pF probe will look like a one ohm load at that speed. That's over 2 amps at 2.3V. The answer, of course, is to buy special probes, which run about $30k a pop. A set of four will buy a house in the Midwest. For those of us working at more modest frequencies, say 50 to a few hundred MHz, one can steal an idea from High-Speed Digital Design by Howard Johnson and Martin Graham. I briefly mentioned this in the second of the two referenced articles, but quite a few people asked for more details.   The probe is simplicity itself. Note in the figure above that a typical quarter watt resistor has about 0.5 pF of capacitance. Get a meter of RG-58/U coax. On one end install a decent BNC connector ( or, just buy cable with a preinstalled BNC ) and solder the resistor (with short leads) to the inner conductor at the far end. Then as show in the figure below solder the other end of the resistor to the node being probed, and solder the braid ( very short ) to a ground near the node. The result looks like this:   ( Note the SOT-23 package I'm probing is so small you can't really see it in this picture, or in real life if you have the slightest myopia ).ÿ It's very important to set your scope's input impedance to 50Ω, since most scopes default to 1 M? or so. If your scope doesn't have a selectable impedance buy a 50 attenuator ( Agilent's N5442A or Test Products International's 120082, which is $56 from Digi-key ). Now you have a 0.5 pF probe which divides the input by a factor of 21 ( i.e., this is a 21X probe ). And its performance, shown in the red line in the figure below is pretty stunning:   The downside is that this probe isn't as easy to use as one from a vendor. After all, you will have to solder it in place every time it's moved. An alternative is a commercial low-capacitance probe, which will set you back about five grand each. Probably the most important take-away here is to understand that everything is part of your circuit. Even humidity can affect sensitive analogue designs. And your test equipment is part of the circuit. No scope, logic analyser or any other device is perfect; they will all interact in lesser or larger ways with your board. Always do an engineering analysis to understand how things will behave.