Scope Sunday 40

Two weeks ago (second weekend in October), I was in California, and I scheduled some time to do some junk shopping. I stopped by some of the surplus stores that Jim introduced me to, including Excess Solutions in Milpitas, Weirdstuff in Sunnyvale, and HSC in Santa Clara.

In addition to some surplus junk, Excess Solutions has rows and rows of components. I bought some of the capacitors that I used in my Capacitor Quiz last week.

I also stopped at HSC Electronic Supply, where they have a new banner sign with an unnecessary apostrophe.

They did have some interesting Tektronix hardware, but most of it was too rich for my blood, like this Type G plug-in unit, which was missing all its knobs and tubes, priced at $25.

However, this 3S1 dual-trace sampler was just too good to resist. It seems to be complete, but there were at least two broken components on the B channel (the vertical-position pot and a trimmer capacitor near the sampling bridge, C433). I don't actually have any 560-series mainframes (well, I have one), but I couldn't pass it up. Due to the damage, I talked the manager down to $25, and packed it into my carry-on bag for the flight home.

Saturday morning (being the second Saturday of the month) should have been the Electronics Flea Market at De Anza, but it was canceled due to a conflict with Pacificon. However, there was a swap meet scheduled for Saturday morning at Pacificon, so I went to that. It was hopping well before dawn (unfortunately I didn't have a flashlight). Here's a picture I took while it was still pitch black out.

There was some nice Tektronix gear there, including a rack-mount 545B, a nice 465B, some 7000 gear, and this 491 spectrum analyzer.

Of course, The Fates taunted me. Having just bought the 3S1 last night, I was tempted by a whole lot of 560-series equipment, including three 561 mainframes, 17 assorted plug-in units, and a cart. Of course, there was no way to get it home, or arrange to get it home (my plane departed a few hours later, early that afternoon), and I am confident that I can find these scopes closer to home. Still, funny.

Eagle-eyed observers will notice a pristine copy of Stan Griffiths' book, Oscilloscopes: Selecting and Restoring a Classic, on top of the 561B. Unfortunately, the seller would not sell it alone; it was part of the whole giant 560-series package (which was wise). You can buy a scan of it from the VintageTEK museum store. I hope somebody bought the whole lot of scopes (I probably would have, if it wasn't 3000 miles from home).

The trip home was uneventful. I carried the 3S1 sampler in my carry-on briefcase, but I had no problems at the airport. The TSA agents did test it for explosive residue, but they didn't even ask me what it was. Having breezed through security, I really regret not buying the 465B scope that I saw.

EE Prototyping 4

Design of the "EE Prototyping" course moves steadily forward.  Unfortunately, there hasn't been a lot to blog about because the topics we've been discussing have been mostly formalities:

1. How often should the course meet? Twice a week? Three times a week? What is the balance between lecture time and laboratory time for this course? Should we use afternoon lectures and evening labs (like some other courses at Olin)? How long should the class/lab sessions be?
2. Should the course satisfy "Graduation Requirement A" or "Graduation Requirement B" or both? What are the necessary attributes of a course that satisfies "Graduation Requirement B"?
3. What topics should be covered in lecture?
4. How should the reading assignments be ordered and structured?

Some of these issues are settled, and some are still in flux.  We did come to a good, workable decision on the class schedule, inspired by other lab classes and the sophomore design course.  One of the standard time slots at Olin is twice-a-week at 3:20pm to 5:00pm, but some courses use an extended slot of 3:20pm to 6:00pm. This course will be scheduled for the extended slot.  On lecture days, we'll wrap up by 5pm, on laboratory days, the students can stay until 6pm if they need to. Several times during the term, we'll have in-class team design reviews, and we'll use the full period those weeks.

We've also been brainstorming about lecture topics for a few weeks.  Some of the suggested topics include:

1. Schematic dos and don'ts
2. Grounding: analog versus digital, ground planes, stars, and the chassis
3. What's so special about 50 ohms?
4. Noise and non-noise
5. Proper use of oscilloscopes and probes
6. Op-amp applications and nonidealities
7. Care and feeding of A/D converters
8. The wonderful (horrible) world of capacitors
9. Heat sinks and thermal problems
10. Power supply design

There are many other topics that have been suggested. This list just includes the "bite-sized" ones.  Some of the other suggestions could be entire courses in their own right (like analog filter design, phase-lock loops, power converters, and motor drivers). We plan to start narrowing this list down to a reasonable syllabus in the next few weeks.

The seminar did work on a little class project that I'll talk about next week, but here's a hint:

The reading assignment last week was two chapters in Jim Williams' 1995 book: Chapter 1, "The importance of fixing" (that's another hint!) and Chapter 17, "There's no place like home".  This week, the reading assignment is to start reading Bob Pease's book, Troubleshooting Analog Circuits.

Capacitor Quiz

Pop quiz!

Examine the markings on the following capacitors and determine their values. To assist you, I've captioned each capacitor with a copy of its text. Click on the picture to get a larger version.

Hint: Here's a link to an explanation of capacitor markings on Wikipedia. Hint hint: You'd probably have better luck just guessing.

The answers are below, contained in the first comment.  No peeking!

Scope Sunday 39

Currently I am teaching an industry seminar on analog-to-digital converter technology. I was doing a little "pre-assignment" reading (reading text before assigning it to my students) of Chapter 5 in the Analog Devices Data Conversion Handbook. Normally, I wouldn't draw attention to an error in another author's work (I have plenty of my own errors to worry about!), but this one was particularly egregious. On page 5.15 I came across the following discussion of measuring settling time and oscilloscope overdrive:

Modern digital storage scopes (DSOs) and digital phosphor scopes (DPOs) are popular and offer an excellent solution for performing settling time measurements as well as many other waveform analysis functions... These scopes offer real-time sampling rates of several GHz and are much less sensitive to overdrive than older analog scopes or traditional sampling scopes. [pg. 5.15]

Unfortunately, this statement is backwards. Traditional sampling scopes are virtually immune to overdrive (see part three of my series "Vintage scopes are better"), but modern DSOs and DPOs can be just as sensitive to overdrive as older analog scopes.

The text continues:

From a historical perspective, older analog oscilloscopes were sensitive to overdrive and could not be used to make accurate step function settling time without adding additional circuitry. Quite a bit of work was done during the 1980s on circuits to cancel out portions of the step function using Schottky diodes, current sources, etc. [pp. 5.15-16]

This statement seems to be an indictment of Jim's careful settling-time-measurement work in application notes such as App Note 74, but the objection is erroneous. The authors qualify their statement in the first sentence of the following paragraph:

Even with modern DSOs and DPOs, overdrive should still be checked by changing the scope sensitivity by a known factor and making sure that all portions of the waveform change proportionally. Measuring the mid-scale settling time can also subject the scope to considerable overdrive if there is a large glitch. The sensitivity of the scope should be sufficient to measure the desired error band. A sensitivity of 1-mV/division allows the measurement of a 0.25-mV error band if care is taken (one major vertical division is usually divided into five smaller ones, corresponding to 0.2 mV/small division). [pg. 5.16]

The first sentence is good advice (ALWAYS verify your measurement chain!), but the last sentence is terrible advice. You cannot measure the fine settling time of a DAC by simply cranking up the vertical sensitivity. It doesn't matter if your oscilloscope is vintage analog or a modern DSO, if ANY part of the waveform is off the screen, you can't trust the results.

Here's a quick example, using the same TDS3012B (and the same four-volt square-wave oscillator) from my post on aliasing last month. The falling edge of the square wave exhibits a little undershoot. At 500 mV/div, the undershoot appears to be about 800 mV (at the bottom of the screen). Note that some of the four-volt waveform is off the top of the screen.

However, if we move the trace up to the top of the screen, now the undershoot appears to be about 1400 mV (note that the vertical scale is still 500 mV/div).

The only change between these two screenshots is a small rotation of the vertical position knob. Clearly, oscilloscope overdrive is a concern, even in a modern DSO.

EE Prototyping 3

There are several major projects that we're considering for the course.  Two of my favorite ideas involve taking stuff apart.  The first idea is just to collect a bunch of random electronic products and disassemble them to explore a variety of construction methods.

• Printers and scanners
• WiFi routers and ethernet switches
• Cable TV boxes
• AM/FM radio receivers
• Laptops and cell phones (modern and ancient)
• Heavy-duty adjustable power supplies
• Function generators, oscilloscopes, and other lab equipments

The second idea is a focused effort in reverse engineering. The class will be split up into an even number of teams.  Each team gets a product that they have to reverse engineer and create a complete documentation package for, including schematic, bill of materials, and maybe even a working simulation (but no board layout and no pictures). Then, each team trades their documentation package with another team, and they have to build a working copy of the object just from the documentation (without ever seeing the other team's original).

My thought is to use cheapo guitar pedals for this project. The circuits are relatively simple, most use commodity parts, and they're fun.  Plus, when you're troubleshooting an audio project, in addition to looking at the waveforms on an oscilloscope, you can listen to it (and tell that something is wrong from the sound it makes or doesn't make). To get an idea of the complexity of this project, I went to my local Music Go Round shop, and bought all the pedals that I could find for less than $10.  Here they are, the lambs for the slaughter (with a couple others... the Boss DS-1 and Joyo JF-06 were a little more than $10).

I had the students in the seminar take them all apart and look at the circuit boards and the chips used. The Danelectro BLT Slap Echo has two boards, a switch board and a sound board.  The sound board has a PT2399 echo chip on it, which is no surprise.

The Behringer CS400 Compressor/Sustainer uses a lot of tiny surface-mount parts.

The Ibanez PL5 Powerlead uses all through-hole components on a single-sided board.

The Danelectro D-6 Fab Flange also has two boards inside.  The switch and amplifier board has some CMOS 4053 switches and some TL072 dual op amps on it.

The sound board uses a pair of BL3207 bucket-brigade chip (with a BL3102 clock driver chip)

The sound board in the Danelectro D-2 Fab Overdrive just uses a TL072 dual op amp and some passive components.

I think these will make great projects.

The reading assignment this week is Sections 9.5, 9.6, and 9.8 (Hardware Design Techniques: Thermal Management, EMI/RFI Considerations, and Breadboard and Prototyping) of the Analog Devices Data Conversion Handbook.