Surgeon – My Fallback Career

So I was knuckle deep in a crazy board rework today at work trying to fit a 1206 cap where it didn’t belong because it was left off the schematic and consequently the layout. About halfway through I realized that the rework I was performing required a pretty steady hand, like I imagined a surgeon would need. I chuckled to myself and tweeted that if being an EE didn’t work out I was going to become a surgeon as all the reworks I’ve done over the years count as practice.

Satisfied with my wittiness I pocketed my phone and continued on modifying the board. That would be the end of the story if my brain would ever let anything go. Instead I kept thinking about surgery the rest of the day and realized that actually, EEs are quite a bit like surgeons when you think about it.

  • Operating Room & Table = Lab & Bench
  • Scalpel = X-acto Knife
  •  Heart Rate Monitor, O2 Monitor, etc. = Oscilloscopes, Multi-meters, etc.
  • Cauterization Tool = Soldering Iron
  • Nurses = Techs
  • Tweezers, Hemostats, Cotton Swabs = Tweezers, Hemostats, Cotton Swaps
  • Isopropyl Alcohol, Antiseptics = Isopropyl Alcohol, Board Cleaners
  • Anesthesia = Flux (Just makes the whole process a lot smoother)
  • Microscopic Surgery = Soldering & Scrapping under a microscope
  • Defibrillator = ESD Gun
  • Too much caffeine makes operating harder = ditto
  • Sinus Rhythm = Steady State Response

So I guess all that’s separating me from becoming a surgeon is jargon am I right?


More Vintage Tek Gear

Back in October I posted about some old Tektronix equipment I found at work consisting of a function generator and current probe amps sitting in a power supply chassis. Well after a few more months of rummaging around the various labs and store rooms at work I stumbled across another old piece of Tektronix memorabilia, a Type 576 Curve Tracer. As far as I can tell from perusing the web it dates back to the late 1950’s or early 1960’s. One site, Barry Tech, claims the curve tracer is from 1969 while Vintage Tek simply states the 576 was developed sometime between 1960 and the early 70’s when the series was discontinued. The 576 is also the second curve tracer offered by Tekronix designed with transistors instead of tubes, the first being the 575.

[Update: The guys over at Vintage Tek got in touch with me and as it turns out the 576 first appeared in the March 1969 Tektronix catalog.]

Spec Overview from the datasheet found on Valuetronics:

  • Can deliver up to 220W of peak power to the DUT.
  • Voltage steps can be as large as 1500 V and current steps can hit 20A peak. If you purchase the 176 high current plug-in the 576 can hit up to 200A!
  • Absolute accuracy of 2% of total output including settling, or 1% of amplitude setting.
  • Pulse generator capable of 1 to 10 repeated steps.
  • Built in beta and gm calculator to save the designer from pesky arithmetic

The 576 Curve Tracer in all its magnificent glory. Couldn't find a picture of the insides online sadly.

I think I definitely prefer knobs over touchscreens on my test equipment. Fiddling with this guy just felt great. Not that I want to give up my modern gear completely mind you, I'm just sayin'.

Note the force and sense lines for the collector and emitter terminals. Also this thing could dish out some serious power to the DUT, up to 220W!

Looking on Barry Tech I saw they also provided a link to the 576’s manual you can check out here. Tek was kind enough to provide a complete section dedicated to the circuit description that I definitely plan to check out in my free time. The circuit description section includes pieces on the control loop compensation, logic diagrams, timing charts, and at first glance, what appears to be pretty detailed explanations of it all. Check out Figure 3-6 to see a diagram of the discrete A/D used in the tracer designed with only a handful of diodes and resistors. Supporting the DIY movement before it was fashionable, the manual also contains a complete section on basic maintenance and troubleshooting including where to locate various circuits, key performance specs, and a soldering guide.

Unfortunately I don’t think this 576 sees much action anymore at work. To my knowledge nothing in our product catalog needs to be tested using at the individual device level and if it did it would probably be done using a wafer probe station instead. I’ll ask some of the designers and test engineers if it’s ever used anymore and report back.


Jim Williams, AN13, & Op Amp Wizardry

Not too long ago I was reading through one of Jim Williams’ famous App Notes, AN13, High Speed Comparator Techniques. It’s an older App Note that was published back in 1985 and I didn’t really have a specific reason for reading it other than thinking it looked interesting and I wanted to learn more about comparators. For a comprehensive overview of AN13 I recommend reading Dr. Lundberg’s (aka Dr. Analog) three part summary over on his blog Reading Jim Williams.

The first section of AN13 is extremely informative and makes the app note well worth the read in my opinion. Entitled “The Rouges Gallery of High Speed Comparator Problems” this portion of the app note highlights common pitfalls of comparator circuits including bypassing, ground planes, probe compensation, and much more. As useful as The Rouges Gallery is what intrigues me the most in AN13 doesn’t actually have anything to do with comparators and is found in the first circuit of the Applications Section. The overall circuit is a Voltage to Frequency Converter shown in Figure 16 of AN13 and reproduced below with the part that fascinates me most boxed in red.

What Jim Williams has done is replace the input stage of A1, the LT318A, with a pair of 2N4393 JFETs. These JFETs drive the output stage of A1 via the two Comp pins of the amplifier. A1’s true inputs are shutdown by shorting them directly to the -15V supply leaving the rest of the amplifier free to serve Williams’ needs for this particular application. According to page 8 of the app note this trick was done “for low bias, high-speed operation.”  Now I don’t have a whole lot of experience using op amps with Comp pins to begin with let alone understand their internal architecture enough to hack them like Jim did. As far as I knew something like this wasn’t even possible and it definitely wasn’t brought up in any of the classes I took in school! Needless to say, upon seeing it done here in AN13 I was a little stunned.

After my initial shock I decided to look at the LT318A a little more closely to try to see how Jim Williams had pulled off this neat little trick. Linear Tech is usually pretty good about providing schematics  of their op amps with a description in their datasheets and I was hoping this was the case with the 318. As it turns out, the LT318A datasheet is a bit sparse compared to other datasheets from Linear but fortunately for me it does contain a schematic of the part. Unfortunately for me, it doesn’t appear to be a simplified version and there isn’t a functional description. Looking at the schematic in detail I could tell this wasn’t the basic op-amp architecture I was used to dealing with but I decided to dive in anyways. I’ve copied the LT318A schematic here and marked it up into functional blocks as best as I’m able to but if anyone out there has more info on this op amp or sees somewhere that I went wrong please let me know.

From studying the datasheet schematic of the LT318A I was able to understand more of how Jim’s op amp hack works. Connecting the amps inputs on pins 2 and 3 to the negative rail turns off the differential amplifier, the heart of which are the input transistors Q1-Q4 with Q13 and Q14 being the active loads. As a side note there may be some common-mode feedback on the diff pair but I’m not 100% sure. Thinking about what was said in the app note I would have to assume using two JFETs in place of the input circuit on the op amp would lower bias requirements and cutting out all those transistors speeds up operation of the overall application circuit.

The output from the input differential stage is then fed into what I assume to be a second gain stage  (not sure what Q21 does, this may be a weird folded cascode configuration too). From Figure 16 in AN13, you can see that Jim Williams has the JFET input transistors driving pins 1 and 5 on the LT318A. Sure enough these two pins correspond to the output of the diff pair on the amplifier and feed directly into the second gain stage. Following the gain stage comes what appears to be a Class AB output stage.

I enjoyed this little exercise of trying to understand some of Jim Williams’ techniques. While I may not have exactly figured out how the LT318A works I feel I did understand the high-level thought process behind this neat little trick. I also saw that my BJT design skills are a little rusty and that perhaps I should dust off Gray & Meyer or Sedra-Smith and brush up on the topic. Who knows, there could be a revisit to this post in the future…

Any neat op amp hacks of your own? See a mistake in my analysis of the LT318A? Let me know in the comments. Oh, and Happy New Year!


State of the Fake EE Union

Well hello there. It certainly has been a while since I’ve posted. Sorry about that, between the holidays and submitting a draft of my thesis to my adviser before Christmas I was running around with my head cut off and couldn’t seem to find the time to write anything. Just to keep everyone updated and what not here’s a couple of noteworthy news items:

1) Back on December, 20 I had an article published over on Engineer Blogs about the differences between school and work. Chris and the other EB writers were kind enough to offer me a spot guest writing for their site after I submitted the piece for their consideration. For the time being I will be submitting stories to EB for your enjoyment on a part-time basis and will work up to one day joining the team as a full-time author.

2) Though I’m writing for Engineer Blogs on occasion I will still be posting content here to this blog when I get a chance too. Fake EE Quips will stay more technical in nature (circuits, app notes, etc.) while any stories that I feel relate to engineering as a whole I will publish on EB. I’m currently trying to get a post out here before the new year so be on the lookout that in the next day or so (it’s about Jim Williams and a crazy op-amp trick). I’ve also got some more ideas bouncing around in my head but my lack of a home lab makes it difficult to prototype and test circuits. Hoping to acquire a scope and power supply in the new year but we’ll see…

3) The Amp Hour Bingo will be getting some updates in order to make it easier to win and more fun to play. For those of you who play along like I do I’m sure you’ve noticed it’s basically impossible to BINGO while listening to the podcast. As far as I know I’m the only one who has gotten a BINGO and it’s only happened once. Roel and I brainstormed a bit and came up with a way to place the tiles more efficiently so the most common aspects of The Amp Hour are always used when a board is generated and the rest serve as filler. Other features to add were also discussed including saving a board and posting results Twitter. I don’t know when exactly these changes will be put into place since I’m dead weight when it comes to coding but when Roel finishes them I’ll be sure to make it known.

So yeah, that’s what I’ve got for you at the moment. Thanks for checking out the site and reading my posts over these last 8 months or so. It’s been a lot of fun so far and I plan to keep writing in 2012 and longer assuming the world doesn’t end next December. Once my thesis is finally wrapped up I can even post on a more frequent basis too!

Happy New Year,

Carmen


Expanding the Analog Geek’s Toolkit

So I spent the other weekend in Washington, D.C. visiting a friend for his birthday. While I was waiting for my plane to take off in RDU Alan over at Tektronix was kind enough to provide me with some pre-flight entertainment. Back in 1987, Alan assembled The Ultimate Analog Engineer’s Toolkit designed to provide solace to engineers dealing with such problems as noise, the Miller Effect, and management, among other things. My personal favorites were the box of dBs (positive and negative flavors of course) and the Parasitic Pesticide. I tried to order some samples directly from Alan himself but he said his stock has run dry. I hope he can get another shipment delivered in time for the holiday season.

Anyways, as I was sitting in the terminal after reading it and I started to come up with a few additions to the toolkit and I present them to you here.

The Low Flow Current Adapter – We’re all familiar with the low flow shower heads installed in bathrooms nationwide to reduce water consumption. The Low Flow Current Adapter works in much the same way only on current instead of H2O. Plug this bad boy in between the power source and your circuit and watch as your product’s power consumption plummets before your very eyes.

Frequency Shaper Tool - Sometimes designs run over budget and when this happens you can’t always afford the box of high quality dBs needed to make a circuit meet the required specs placing a lot of “hertz” on you the designer. This is where the patented Frequency Shaper Tool comes in handy. While not as precise as a Box of dBs its low cost and reliable performance makes it ideal for the engineer on a budget. The Shaper allows a designer to grab a hold of an amplifier or filter’s frequency response and bend it to the desired shape. Need to nudge a zero in the stop band? Squeeze just a little more rolloff out of a filter? Reach for the Frequency Shaper and bend a transfer function into submission.

DocuGel – Come across an old PCB, product, or IC and have no idea what it does or how to use it? Did someone who’s clearly not as bright as you are forget to document their work and now the burden to upgrade the device is on you? Spread some DocuGel on the offending product and leave it sit overnight on your bench. Return the next morning and find datasheets, BOMs, gerber files, and schematics laying on your bench!

Grounding Stakes- Nasty PCB layout causing you problems? Is your circuit’s performance suffering at the hands of ground refusing to sit at 0V? Pound a ground stake into an open portion of your board and watch your problems disappear. Let your circuit know you only accept 0V, no more, no less.

I accept checks and all major credit cards :-)


Vintage Tek Gear

I was poking around in the Design Lab at work and happened to come across a few pieces of old Tek Gear. There’s one FG504 Function Generator and two AM503 Current Probe Amplifier modules together in a TM504 Power Supply chassis. The FG504 can generate a sine, square, or triangle waveform up to 30Vpp over an impressive 1mHz to 40MHz frequency range. It also has a host of other features too including burst and sweep modes. The AM503 can be attached to any oscilloscope and allows for current measurements of up to 100 A depending on the current probe used with it. I’m not sure if this stuff gets used much anymore, if at all, but it’s still cool none the less so I snapped a few pictures to share my findings. I also did a Google search for each module and found a few interesting sites which I linked to at the end. Enjoy!

Results of my Google searching:

  • What looks like a product page from a catalog for the function generator
  • Catalog page for the current probe amp. The AM501 Op Amp Module also on this page looks pretty cool too. I’d definitely like to play around with one of those.
  • An ebay page selling the power supply (includes pics of just the supply itself).
  • A manual for newer versions of the AM503 (AM503B and AM5030)
  • Website which shows a picture of the inside of the FG504, all through-hole and some crazy looking traces.

Want to see some more Vintage Tek Gear? Check out my new post on the 576 Curve Tracer.


Useful App Notes: Part 2

AN104 – Load Transient Response Testing for Voltage Regulators, Linear Tech

An app note written by the late great Jim Williams back in 2006 discussing how to test linear regulators by applying a load transient to the output. These techniques discussed are also directly applicable to switching regulators as well making the app note doubly useful. My favorite parts of this app note are Figures 6 and 8 which show two interesting transient generator circuits, one BJT based and the other FET based. At the moment I’m trying to gather up the necessary parts to prototype one of the circuits and possibly throw it in a project box to use at work. A great resource for anyone doing work in power electronics.

ANA – The Monolithic Operational Amplifier: A Tutorial Study, National Semi

Want a fantastic reference about the intricacies of op amp design? Look no further than this app note written by Bob Widlar. Covering everything from input/output stages, to techniques for increasing slew rate and proper layout, any and all relevant topics related to op amp design are discussed.  There’s even a section on thermal feedback which is interesting as it’s not a topic typically covered all to often. While I don’t think I would recommend this app note as a stand alone reference for a beginner, it’s perfect for someone looking to expand their knowledge on op amp design. If you are a beginner I would suggest pairing ANA with either Johns & Martin or Gray & Meyer. Fun Fact: Gray and Meyer are cited several times in the bibliography.

The Data Conversion Handbook – Analog Devices

Not exactly an app note but still another good reference to keep at hand. If you’re looking for a general overview of ADCs and DACs  along with a bunch of other useful information I highly recommend this book. Chapter 1 is an interesting history of converter technologies throughout the ages. Chapter 2 discusses sampling theory and defines converter specs. Chapter 3 covers the various topologies of DACs and ADCs from a primarily applications standpoint rather than a design one but is still incredibly detailed.  Chapters 6 and 7 cover interfacing and all the various support circuitry that goes into properly using converters in your design. Chapter 9 has a some good information regarding board layout, passive components, prototyping, and more. I read through most of this handbook two summers ago during an internship while waiting for automated tests to run and I can say I learned quite a bit.


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