30 September 2012

Vintage scopes are better part 5

Vintage scopes are better. Back in February, I did a four-part series on why vintage oscilloscopes are better that their modern counterparts (see part one, part two, part three, and part four).  Here's another reason that recently bit me.

Reason number 5: Aliasing.

Another reason that digital scopes are inferior to analog scopes is the inherent aliasing that occurs in any sampled-data system.  If the time base is set incorrectly (very incorrectly), aliasing can cause significant confusion and can mislead a novice user.

I have heard people say "Modern digital oscilloscope do not have a problem with aliasing."  This statement is demonstrably false. Here is an example of a Tektronix TDS3012B scope looking at a 8 MHz crystal oscillator.  (Embarrassingly, this example occurred in front of a room full of students. Although I realized what was happening, and quickly fixed it, it still angered and disappointed me.)

Here is the waveform shown at 40 milliseconds per division.  The period appears to be about 32 ms, but the scope is in roll ("strip chart") mode, so I can't get a stationary trace on the screen.  The frequency measurement reports about 31 Hz, which matches the period observed.

The waveform at 4 milliseconds per division. The period still appears to be about 32 ms (eight divisions for the brightest trace), but I can't seem to get a stable trigger. Despite the fact that the oscilloscope says that it's triggering, the waveform is rolling around the screen with several ghost images. The frequency measurement reports about 31 Hz, which still matches the period observed.

The waveform at 400 microseconds per division. Now the waveform looks like a smudge, and the frequency measurement reports 500 kHz, which doesn't make sense. The "Low resolution" label is the first indication that the scope thinks that there might be something wrong.

The waveform at 40 microseconds per division. The waveform still looks like a smudge, but the frequency measurement now reports 8.25 MHz, which is in the right ballpark. At least the smudge is uniform across the screen, which implies the time base is much too slow. The "Low resolution" label is still present.

The waveform at 4 microseconds per division. Now we can see many, many rising and falling transitions, and we know that the time base is set too slow.  The frequency measurement is 8.015 MHz, which is much better.

The waveform at 400 nanoseconds per division. Finally, we get a clear view of the actual waveform, and a frequency measurement accurate to three significant digits.

The waveform at 40 nanoseconds per division. At last, the correct time-base setting. Now we can see the whole waveform, at the right speed, and we get an accurate frequency measurement.

Go back and look at the first two pictures again. Imagine yourself as an undergraduate student, racing to complete a laboratory project by the end of the class period. You don't have a lot of experience with oscilloscopes, and you're just trying to get a frequency measurement. You can see the period on the screen, the automatic frequency measurement matches, and there is no other indication that the scope is lying to you. Would you take the time to get a stable fixed trigger?  Or would you just press the STOP button, make the measurement, and go on the the next step?

I admit that this problem is mostly due to user error (when looking at a new waveform, start at the fastest time scale and gradually slow down the horizontal sweep until you see the signal). However, the conclusion is sound: modern digital scopes DO have a problem with aliasing, whereas vintage analog (non-sampling, of course) scopes do not.

26 September 2012

EE Prototyping 2

Today was the Course Fair, where the professors hold a "trade show", of sorts, and advertise their classes for the Spring term.  I brought along some show-and-tell items to promote the EE Prototyping class.  First off, the following description for the course catalog (written with help from the seminar) was submitted:
Through a series of projects, we will learn to design, build, and debug electronic prototype systems. We will cover multiple aspects of the prototyping process, including circuit and system design, soldering, deadbugging, troubleshooting, component selection, schematic capture, printed-circuit board (PCB) layout, PCB fabrication, PCB assembly, and thermal analysis. We will discuss the tradeoffs among "faster, better, cheaper", and explore examples in the realms of analog, digital, RF, and power. In addition to hands-on reverse engineering and fabrication experience, students will learn technical communication through design documentation.
Secondly, I brought a number of boards to demonstrate the right ways and wrong ways to build circuits (stealing some ideas from Appendix F in App Note 47). First the wrong ways: solderless breadboards and wire wrap (just say no!):

And now the right ways: dead bug on copper clad, as preferred by Jim Williams (described in detail in App Note 47; the board shown is actually a simple oscillator that Jim built for me); and, of course, the good life, a custom PCB (like this Analog Devices eval board):

I also brought along some other examples of interesting PCB boards. Exhibit A: The low-cost circuit board from a floppy disk drive (a single-sided PCB, that includes a "square-wave" trace around the motor for the position encoder).

Exhibit B: The controller board from an inkjet printer, which includes a wide variety of IC packages such as the socketed DIP, several SOICs, and that EPSON ASIC in the middle with a million pins on tiny spacing.

Exhibit C: A ruggedized power supply (you can tell it's rugged from all the epoxy holding the parts in place).

And finally, Exhibit D: The PCB inside a Spectral Synthesis ADDA2218, an 18-bit analog-to-digital audio converter, which uses colored FR4 (the color isn't painted on, it's impregnated in the fiberglass). I thought this was a neat touch, for a circuit board that only one in a thousand customers would ever see (only those willing to void their warranty!).

The reading assignment this week for the seminar was Sections 9.1 and 9.2 (Hardware Design Techniques: Passive Components and PC Board Design Issues) of the Analog Devices Data Conversion Handbook.

Next week, I'll talk about some of the proposed class projects.

24 September 2012

Scope Sunday 38

I forgot to mention: I attended the MIT Flea Market last weekend, but I didn't buy anything oscilloscope related. (I almost bought a Tektronix 7904 with a bad focus problem, but somebody else beat me to it. That's OK; I didn't really need another 7904 project.) I did, however, happen to wander through the Stata Center, which stands on the former site of MIT Building 20.  On one wall in the lobby, there is a display of photographs of famous people from Building 20, including this one of Norbert Wiener with a Tektronix 541 scope in the background.

I also took my kids to a small community museum last weekend, where they had a large model train layout (my son lost his mind). In one corner of the museum they had the following sign.

Of course, I left a post-it note that said "vintage Tektronix oscilloscopes". Unfortunately, the display case for these "member collections" was only the size of a small bookcase (about 30 inches square and 8 inches deep), and it was currently filled with porcelain cat figurines... but I did ponder for a minute, "Wouldn't it be cool to put on a temporary museum display of my scope collection?"  I wonder if there are any local museums that would be interested in that?

22 September 2012

EE Prototyping 1

I'm working on a new project-based class this fall, and I wish I could get Jim's advice. I think he would love it...

Here's the background: Olin College has a popular course called "Introduction to Mechanical Prototyping":
Introduction to Mechanical Prototyping is an elective course with no prerequisites that can be taken by any interested Olin student. This course is all about learning to build things. Through a series of accelerated design projects, we will learn to design, build, and debug mechanical systems. We will cover multiple different fabrication approaches including sheet metal, 3D printing, molding rigid and compliant polymers, and long fiber-reinforced plastics. In addition to hands-on fabrication experience, students will learn and master the technical communication of mechanical design through design reports and professional engineering drawing practices.
Students also get hands-on experience with reverse engineering and modelling, as well as practical CAD and fabrication techniques. The resulting projects are really cool (see the web site for more).

This course has caused some long-simmering jealousy among the EE students. "We want something like that, except for us", with requisite pointing, was the demand. Several students asked me (and other faculty members), "Can you teach a class on EE prototyping next Spring?" Well, sure. But what does "EE prototyping" mean? Circuit design? Breadboarding? Soldering? Troubleshooting? Component selection? PCB design? PCB fabrication? PCB assembly? Value/cost engineering? Design for manufacturing?

Of course, a class on practical electrical engineering could be a really useful thing, but what topics should be included? What should be the mix of lecture time versus project time? Should the class cover analog, digital, RF, power, or all four? (Are there enough weeks in the semester for all four?) What are the best references? (Jim's application notes? Bob Pease's book? Howard Johnson's books? The free Analog Devices handbooks?)

To help solve the problem (and get some help), I've convened a seminar this Fall called "How to teach EE prototyping?" Eight students are going to help me brainstorm ideas, develop assignments, and generally design the course. I've promised them that the seminar is going to be equal parts round-table discussion, show-and-tell time, individual research, group activities, application-note club, rumors, innuendos, and lies. We have five goals:
  1. document the needs and wants for this class,
  2. manage expectations and preconceptions,
  3. achieve sufficient buy-in for the course,
  4. plan some of the course content and deliverables, and
  5. spend some time chatting about circuits.
My hope is to create a useful, practical, hands-on course that Jim would approve of. To set the mood, I started the students off with a hefty dose of App Note 47. The first reading assignment was
  • Mr. Murphy’s gallery of high-speed amplifier problems (pp. 7-15),
  • The tutorial section (pp. 15-32), and
  • Appendix F, Additional comments on breadboarding (pp. 98-112).
I'll let you know how it goes.

02 September 2012

Scope Sunday 37

Remember how I mentioned last week that I hadn't bought any scopes this year, and I then I bought two 422 portables at the Boxboro flea market?  Well, funny thing: I found another deal on Craigslist this week...

The listing advertised two 547 scopes for $50 in western Massachusetts.  Good deal (I've paid a little more for a 547, but I've also paid less).  I called and said I'd take them.  Turns out the seller was a high-school science teacher who had gotten the scopes from a neighbor of his parents to give to his school.  Unfortunately, the school didn't want them because they didn't have the space, and now he needed to get rid of them. (Unfortunately, his parents live in Ohio, so my thoughts of meeting this excellent neighbor were quickly dashed.)  So, we made plans to meet up.  The seller was driving into Boston this weekend to help his son move, so we met at a gas station along Route 2.

What made this transaction a unexpectedly GREAT deal were the accessories that came with the scopes.  The scopes are nice, but the accessories (some of which are shown above) are fantastic:
  1. A Model 500 scope cart, painted Tektronix grey instead of Tektronix blue.  This cart should match my Tek 511A.  It has a drawer, but no storage for plug-ins (of course, the early Tek scopes didn't have plug-ins, but that big blank panel still seems like a lot of wasted space).
  2. A complete C-12 camera system.  This is my first scope camera, and I'm kind of excited about it. Anybody have a good source for Polaroid film?  (Gotta love that smell.)
  3. A mint-condition vinyl cover, with the Tektronix logo on the side (very nice).
  4. A pair of 1A4 plug-ins, in addition to the two 1A1 plug-ins that came installed in the scopes.
  5. Original bound manuals for 547, 1A1, 1A2, the camera, and a 549 (for some reason).
  6. A photocopied manual for the 1A4 plug-ins.
  7. A 1971 Tektronix catalog.  So cool.
  8. Three extra 154-0568-00 CRTs.  Unfortunately, they're all the same and two of them are labeled "gassy".  When I saw the box of extra CRTs, I had hoped for different phosphors, but, alas, no.
So I actually got all of this for $50 and a scope.  When the seller told me that he was a science teacher, who wanted a scope for his school, I offered him the smallest scope that I had (it was a "modern" 20-MHz, two-channel, no-name scope, but it works, and it's smaller and lighter than a 422 or 453).  I hope it's small enough that he can find room for it.  I wish my high-school science club had had an oscilloscope!