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An MSO for the Masses, by Jack Ganssle (Reviewed January, 2010)
I love my job. In addition to meeting lots of really interesting engineers who are creating the next wave of innovation, vendors send me their latest wares for a look-see. Most, of course, are hoping to get some free ink to help promote the product, though I rarely do product pieces. But a few of the goodies are cool enough that I think there's value to the readers of this site for an unbiased review.
Tektronix sent me their MSO 2024 MSO recently. I've been a fan of Tek scopes since first using them 40 years ago while in high school. My dad worked for a start-up space company, and we kids were required to provide free janitorial duty on weekends. But that did give me access to the lab, which had a number of hulking vacuum tube-based Tektronix scopes firmly seated on caster-equipped carts.
The company had a number of 545s. Those units were like all of the Tek scopes of the era: massive, reliable, sporting a panel covered with a sea of knobs, switches and levers, all to draw a couple of traces on a five-inch screen. Pull the access covers and you were presented with a wonderland of truly beautiful electronic and mechanical engineering. The time base selector switch, for instance, had a fabulously machined mechanism to propagate a hard twist of the knob into the scope's bowels.
Tektronix was long-heralded for their engineering, and a succession of improved scopes kept coming to market. Perhaps my favorite was the 7000 series that had sci-fi looking illuminated clear plastic pushbuttons to control many functions. We bought a lot of these at the company where I worked in the 1970s. Their quality polished the Tek name.
The MSO 2024 is a another quality Tek product whose major flaw is that the company wants the demo unit back. Maybe it'll get lost in shipment. Or the 75 year-old technophobe next door might sneak in at night and steal the thing. Surely there's some story I can concoct that Tek's marketing folks will believe.
The MSO 2024 is very different from a 545 and 7000 series. As has become standard in this product sector the CRT has given way to a flat-screen display, which greatly reduces the depth of the instrument to about 5 inches. Scopes no longer ask for a serious commitment of bench space. At 8 pounds it's as easy to move as a laptop.
As the name implies, this is a Mixed Signal Oscilloscope, which is a hybrid of logic analyzer and traditional o-scope. In the case of the 2024 there are 4 analog vertical inputs plus a 16 channel logic analyzer. An MSO can display any mix of digital and analog data at the same time. Key to the magic of an MSO is that any input can trigger the sweep. Want to see the output of a programmable gain amplifier when the software commands a gain of 8? Hang the logic analyzer probes on the amplifier's digital gain input, an analog channel on the output, and trigger on 1000b. Since embedded is the intersection of hardware and software, I've long thought that an MSO is an indispensable piece of equipment.
First, a summary of the specs. The 2024 is a 200 MHz scope that samples at 1 GS/sec. It stores a million data points. A few caveats: 8 of the digital inputs can sample at full speed; the other 8 run at 500 MHz. And when an analog channel is set to the highest gain (2 mV/division) the bandwidth drops to 20 MHz. The time base can run from 2 ns/division all the way to 100 sec/div, just about enough time for a decent nap.
An MSO without cross-triggering would be nothing more than an odd mix of logic analyzer and scope in one inconvenient box, so vendors focus on an extensive array of features to start the sweep. The 2024's triggering modes are too extensive to list. But in addition to cross-trigger, it can start on detection of runts (pulses that cross one user-set threshold but don't make it to another), pulse width (those that are >, <, =, or != to user-set times), all sorts of logic conditions, and much more. Optional little memory sticks add bus triggers, which I'll describe later.
Then there's the screen. It's 7" TFT full color display with 480 x 234 pixels. The display is very bright and vibrant. I dragged it outside on a sunny day. Even with the light reflected off that deep layer of bright snow that pummeled Maryland I found the display easy to see.
With a nod to their long history in this industry, Tek dubs the display “Digital Phosphor." I found that sort of amusing, thinking that by the same logic one could call a calculator a “digital slide rule." Turns out there's a 1972 patent for an “Electronic Digital Slide Rule" (US patent 3,676,656) with the usual wonderful pre-microprocessor complexity of the era. To be fair, Tek uses the term to refer to a parallel processing approach that tosses acquired waveforms to the screen at high rates to mimic an analog scope.
When confronted with a new piece of test equipment I always power up and punch buttons to get a general feel for the device. Then I reach for the manual. Whoops - there is none. Tektronix supplies a PDF version on disk. Actually, two disks, which contain a confusing mix of files, some of which seem to be the same.
In the olden days scopes inevitably had an attached pouch that was a perfect place to store the manual. Ironically, the 2024 comes with a pouch - a soft briefcase really - which one puts the instrument into. But I think a device of this sophistication really requires a bound instruction manual. The PDF version is a very good quick-start, but does not delve into some of the more complex features in enough depth.
I have one other, minor, beef with the 2024. It's generally pretty responsive, but some functions are sluggish. The vertical and horizontal position controls respond just slowly enough to be mildly objectionable, though the other knobs show no real delays. Panning along a waveform is not smooth; it's a little jerky, and the screen flashes a little when the scope rewrites text on the screen. The unit has a powerful search capability - a necessary feature with a million sample points- but it can take 12 seconds to grind through the buffer. Bootup is also dilatory, taking 63 seconds from hitting the power switch to displaying a signal. That's slower than the ancient vacuum-tube 545. These are minor concerns, but point to the use of an underpowered CPU. The scope's most important feature is showing waveforms, and at this the 2024 is very fast indeed.
I reviewed several USB scopes in May, 2005. These pocket-sized put the UI on your computer. But when holding three probes inside a chassis it's awfully hard to mouse around to reset a virtual knob. I prefer the traditional physical knobs on a full-sized instrument like the 2024. Their “feel" is very important to me. Tek has done a good job on this instrument. The horizontal and vertical controls have satisfying detents while not being as hard to twist as on old Tek equipment. There's just enough room to get a good grasp on each without activating an adjacent one.
This powerful machine has far more functionality than there is space for knobs. Like most similar instruments, in addition to dedicated switches it uses “soft" switches whose functions' varies. There are seven buttons along the bottom of the display (called “bottom bezel" controls) and five grouped vertically (“side bezel") next to it. Press, say, “Trigger Menu" and the screen displays labels for the bottom bezel switches. One is “Coupling." Press the control associated with that label and the side bezel switches get their own labels to allow one to pick AC, DC or other trigger coupling parameters.
Two “multipurpose" dedicated rotary knobs are used for a variety of purposes, like moving cursors and selecting more sub-menu items from the display.
I found the user interface confusing at first. You'll need to read the manual, uh, PDF, to get the hang of the 2024's operation. Menus seem to lurk on screen when not needed. But there is a method to the madness and it doesn't take long to get the hang of the instrument. With practice its operation became second nature and easy to use. But you'll want to keep the manual, uh, PDF, around as some of the seldom-used functions get lost in the menu structure.
Digital scopes store more data records than one would generally display on the screen. Each vendor has their own way, or ways, to scroll through the sweep. But perhaps a little background is in order, first.
The horizontal sweep control on an analog scope sets the rate at which the CRT's electron beam sweeps from left to right across the screen. Select, say, 1 usec/div and it will takes a microsecond to move the beam one division.
Digital scopes are substantially different. The horizontal sweep control - called “scale" on the 2024 - sets both the acquisition rate and the display rate. If the unit is set to acquire 1 million samples at a time, crank the 2024's scale to 10 nsec/div and the scope will go to its highest acquisition rate of 1 Gs/sec. It will suck in 1 msec worth of data, each point 1 nsec apart, but will display only 100 nsec (10 nsec/div times the ten displayed divisions) worth of data.
The other 0.9999 msec of data that's not on-screen could be terribly important. The pan and zoom controls give you a mechanism to examine it all. The 2024 uses two concentric rotary knobs for this: the big outer one pans left and right while the inner control sets the zoom level. A million points is an awful lot of data; moving through it with a conventional knob could induce carpal tunnel syndrome. But Tek came up with a very nice feature: the more one twists the knob, the faster the thing pans.
Early digital scopes could automatically figure a signal's frequency. Tek takes this much further, as the 2024 can take and display 29 different measurements, including rise and fall time, RMS, pulse counts, phase and far more. There's a real yin and yang here; I love having that data on-screen, but begrudge the (small) amount of LCD space required.
The camelcase FilterVu feature is a variable low-pass filter that can be set from 1 Hz to the scope's full 200 MHz, depending on the horizontal scale setting. But it gets better: the unit shows the filtered signal, of course, as well as the unfiltered one at a lower brightness as a ghostly image hovering in the background.
An FFT option turns the unit into a poor person's spectrum analyzer (showing amplitude vs frequency rather than amplitude vs time).
I mentioned bus monitoring earlier. Optional memory sticks turn the 2024 into a protocol analyzer for I2C, SPI, RS-232, and RS-422/485. Alas, there's no Ethernet support. Various display modes can show the bit stream, and even label the packets with their hex values. A cool feature is the automatic tagging of errors, like missing ACKs. This display is so pretty and full of information I planned to publish a screen shot, but the unit they sent me had a bad USB port so I couldn't save the image.
There's plenty more. Lots of modes, gobs of features. This little puppy does practically everything. And that can be a problem. After an involved setup for a particular problem so many parameters in a maze of menus might have changed that you can no longer measure a simple signal. But press Default Setup and the instrument strips away the complexity and figures out appropriate settings. It's not perfect - a 5 volt square wave sets the device to 1 V/div instead of a much more appropriate 2 V/div - but I found myself using the feature often.
At $5400 this is a great scope, well suited to a lot of embedded needs. It's three instruments in one: oscilloscope, logic analyzer and protocol analyzer (though the latter is an extra-cost option), which seamlessly lets you work in all of the domains at once. After a little time getting acquainted with it you'll find the a delight to use.
Amy at Tektronix wants the demo unit back. She's in Oregon 3000 miles away. What will she do, send Vito and the mob to pry it out of my hands? Hmmm.