A Call to Arms

I’m sure it’s not news anymore to most of the readers here but recently there has been not one, but two devastating blows to the analog electronics industry.  Legendary Linear Tech Applications Engineer Jim Williams passed away on June 10, 2011 and National Semiconductor’s Analog Wizard Bob Pease (the self crowned Czar of Bandgaps) also passed away on June 18, 2011.   EDN author and fellow analog engineer Paul Rako fondly remembers both analog giants in two heartfelt posts on EDN’s website here and here. You can also find a tribute to Williams on Linear Tech’s website (found here) which also links to a collection of his app note guaranteed to provide you with enough reading material for the foreseeable future. [Update] National Semi has also added a tribute to Bob Pease on their website found here.  There’s an excellent video to go along with it that’s well worth the time to watch it (if anyone from Maxim is reading I apologize but your product catalog as Pease’s floor mat was pretty funny).

No one can deny that their unexpected passing is a blow to EE’s everywhere and both men will be greatly missed.  It is unlikely that either Williams or Pease will ever be replaced. In his article on Pease, Rako mentions that there are still many great analog designers in the industry today and while I agree with him, I do claim that we as an industry are currently left with a void to fill in terms of engineers who are as vocal as both Pease and Williams were.  There is now a need for engineers and makers who possess the same passion as these two great men to step up and inspire and teach others with their writing.

My challenge to not only analog fans but all engineers, coders, makers, hackers, etc. is to carry on where Jim Williams and Bob Pease left off.  Be passionate about your work, take pride in it.  Look to teach. Look to inspire. Let your enthusiasm show through in every project. Let people know what we do as engineers may not be easy but the challenge it provides is both exhilarating and at times, fun.  These are the ideals that should be present each and every day you sit down at your bench. You don’t have to be a circuit junky to see  these principles shine through in Williams’ and Pease’s work, they’re pretty self-evident.

So grab your ‘scopes, grab your dev boards, your MakerBots, and your soldering irons (not by the hot end). Take them and make something.  If you’re not a maker, write an article on a bit of theory you’re knowledgeable on or just any topic that interests you.  Throw the results online for others to see be it in your own blog, an Instructable, up on Hack a Day, YouTube, whatever.  Carry on the legacies of passion, knowledge, and dedication left behind by Jim Williams and Bob Pease.  While they can’t be replaced, their memories can be honored through the work of those they inspired.

Sadly, I never had the opportunity to meet nor work with either Pease or Williams. However, the two have inspired me immensely through their countless publications.  Ever since I first stumbled across The Best of Bob Pease on National’s website and Analog Circuit Design: Art, Science and Personalities  a few years ago I’ve been hooked on reading everything these two have put into print.  Both of them have taught me a great deal on not only analog circuits, but also the passion required for really loving the work that you do.  As I prepare to go off into the real world after summer ends and start my own career as an apps engineer, I hope that perhaps one day a few of my own app notes can be as well regarded as those written by Williams and Pease and can inspire budding EE’s the way they have inspired me.

To Jim Williams and Bob Pease, may they rest in peace…


Filter Fiasco: Chapter 1

One of the circuits I have to design for my thesis is a bandpass filter.  Based off the specs I was presented with back in the early fall a filter with f_center = 100 MHz and Q = 250 was required; plus f_center needed to be tunable without changing Q. Not exactly the easiest design in the world but I studied up on a few topologies and settled on the Dual Amplifier Bandpass filter (pages 5.74 and 5.93).  According to Matlab and some hand calculations a 4th order filter was all I needed.

Figure 1: Dual Amplifier Bandpass Filter Schematic

Cut to Rev. 1 of the board and not a single aspect of the filter met spec or even remotely functioned as a bandpass filter.  Simply scoping the output showed my design self-oscillated around 50 MHz, fantastic.  Adjusting the potentiometer I put in place for R2 merely shifted the frequency of oscillation. No amount of debugging or rework could make the filter behave and according to one of my professors, my use pots in the first place was a recipe for disaster because of high parasitics along with poor overall performance at high frequencies. Another important thing to note was my use of a current feedback amplifier as opposed to a conventional voltage feedback amp  because of the higher bandwidth and slew rate they offer at high frequencies (foreshadowing, this will haunt me later on…).

After discussing things with my advisor we decided our first attempt was too ambitious and to spin a second revision of the board only this time with a few changes in the specs.  Mercifully, having a tunable center frequency was no longer required. It was determined that this feature wasn’t necessary in the prototype stage and that designing a new tuning method would take too much effort, thus preventing me from completing more important aspects of the project.  The center frequency was also dropped to 10 MHz which lowered Q down to 25 giving a much more achievable design.  Refining my Matlab simulations and hand calculations showed that I was actually incorrect on my first attempt (whoops) in regards to the number of stages.  With these new specs I would need a sixth order filter.  I decided to keep using a current feedback amplifier though I changed parts from Rev 1 and picked the THS3202 from TI.

With my first design having crashed and burned I turned to PSpice to see if I could get my design working in simulation before spending time in hardware chasing something that may prove to be a dead end.  Using  Intersil’s AN1613 (mentioned in my last post here) I downloaded the Spice model for the THS3202 from TI’s website, incorporated it into my schematic and began simulating.  I eventually got my filter working and meeting spec with the help of some compensation techniques from other app notes I discovered and got the results below in Figures 2 and 3.

BPF Mag Plot

Figure 2: Magnitude Plot of BPF

BPF Current Pulse Response

Figure 3: Vout of BPF to Current Pulse Input

From these figures everything appears to be in order, there’s a nice bandpass shape to the filter that met spec, a decent response to being hit with a 100 uA current pulse for 30 us, yadda yadda. All that should have been left was to slap it on a PCB and make sure it functioned right?  Stay tuned for Chapter 2 as our story continues…


Useful App Notes: Part 1

Over the years I’ve accumulated a rather astounding number of app notes on my hard drive from various companies.  What few ones I’ve read so far are, in my opinion, very useful and should be shared.  The rest I’ve downloaded thinking “This looks like something I should keep around in case it ever comes in handy,” only to never open them and let them collect virtual dust.  Sharing the interesting ones on my blog seemed like a win-win-win situation, I learn things reading app notes, you learn things from reading app notes, and I de-clutter my hard drive/Dropbox account,everybody wins. So without further ado here’s a few of the most useful app notes I’ve come across.

AN1613: From SPICE Netlist to Allegro Design Sub-circuit, Intersil

If you’re like me whenever you come across a part that isn’t in any of the main component libraries when using P-Spice you think to yourself “Is this part really necessary? How well does my design work with this op amp instead of the one I want?”  I did this because up until I found this app note, even if I could find a SPICE model online for a part I wanted, I didn’t know what to do with it.  Enter AN1613.  It goes through an easy step by step process of how to take a SPICE model you’ve found online and actually make it work in a Cadence Allegro Design  simulator.   I’ve used this app note to help simulate parts from a few IC companies with great results.

Op Amps for Everyone, Texas Instruments

While not exactly a single app note, Op Amps for Everyone is still a great design guide to have handy when you need to quickly refresh yourself on a topic or get a basic overview of a concept before researching it in more detail.  It covers everything from basic circuit analysis and feedback theory to filter design, converter interface, and everything in between.  There’s plenty of examples to go along with the theory as well as a whole chapter on layout considerations.

AN95-1: S-Parameter Techniques for Faster, More Accurate Network Design, Hewlett Packard

Here’s a blast from the past, AN95-1 was first released in 1967 by HP. While slightly on the old side, I still found this app note pretty useful when trying to wrap my head around S-Parameters last summer.

Got any favorite app notes you can’t live without? Share them in the comments, I’m always looking to collect more of them.


The Poor Man’s RF Probes

While working on my thesis I often have to measure signals up into 2.45 GHz range be it testing a mixer or determining the transfer function of a filter.  At such high frequencies standard banana jack or alligator clip cables turn into antennas which render any measurements done with them pretty much useless.  I get around this inconvenience by using what the engineers I worked with last summer called “Poor Man’s RF Probes.”  These probes are very easy to use and you can make a pair of your own for $10 assuming you already have shielded BNC to SMA cables and a few SMA billets.  Semi-rigid SMA male to male cables are used to make the probes themselves; here’s a link to the ones that I’m using. There may be cheaper cables out there and if you can find them I’d love to hear about it but in order to make the probes the outer jacket needs to be exposed in order to solder it directly to ground.

To make the probes themselves first cut the semi-rigid cable just below each SMA jack leaving a small amount of shielded cable to strip.  Figure 1 below shows two of the probes I am using at the moment next to an uncut cable and a Digi-Key label for reference.  Each probe is just over an inch long or so giving me enough room to strip the ends and bend the probe as needed while still keeping the overall length short enough to ensure quality measurements.

Figure 1: Semi-Rigid SMA Cable Cut to Length

When stripping the cut cable, I’ve found the easiest way to strip the rigid outer jacket is to use needle nose pliers.  Gripping the end of the jacket with the tip of the pliers and slowly bending it back and forth a few times is usually enough to cause the jacket to break all the way around the cable and you can then just slip it off leaving the center conductor insulator exposed.  The center conductor insulator can be stripped using standard wire strippers (~ 20 Gauge).  The goal is to minimize the amount of exposed center conductor keeping the probe close to the measurement point. I recommend practicing on the unusable  middle portion of the cable that will be left over to get the hang of it before stripping the probes themselves.

Once you get the probes themselves made, figure out where you’re actually placing the probes on your board next. Look for a relatively open area of ground plane close to the pad where you will be measuring from.  Bending the stripped probe slightly is necessary for both a good ground connection and the probe’s mechanical stability.  Note: It’s possible to snap the center conductor from too much bending resulting in a useless probe that will only cause headaches later on so bend with care.

Once you’ve determined where you are going to solder down the probe, use a hobby knife or similar tool and carefully scrape away the solder mask on the ground plane near the measurement site.  Be gentle but firm when scrapping because gouging the board too deeply could short the ground plane to any internal layers that might be in your board.  Figure 2 shows what the site around the input to my filter where I’m placing the probe.  In the picture, I’m placing the conductor of the probe on the unpopulated pad of R2 making use of the 0Ω resistor trick.

My apologies for the so-so photo quality during the remainder of this post; my camera phone only works so well…

Figure 2: Scraped GND Plane Solder Mask Near Probe Site

The third step of the process is to solder down the center conductor of the stripped SMA cable. Usually I’ll tin the pad with a little solder before placing the probe, blob some solder on the iron and while holding the probe in my fingers, tack solder the center pin down (Figure 3).  Note: The probe should be able to stand on its own right now but I wouldn’t move the board or probe too much as you could lift the pad the center conductor is soldered down to.

Figure 3: Tack Soldered Center Conductor

The next to last step is to solder the exposed ground plane to the outer metal jacket of the probe for a solid ground connection.  This will also make the probe mechanically sound so you can now make sure the center conductor is properly soldered down without worry. Note: This step can be done before the previous step if you so choose.  Just don’t be like me and hold the probe with your fingers because if you’re not quick you get burned.  I find this method easier as it make shorting the center conductor to ground less likely.

Figure 4: Soldered Ground Connection

Finally before you can consider yourself done, use a multi-meter and make a quick continuity check. Be sure the center conductor is not shorted to ground and that it is actually connecting to the node you would like it to.  Note: If you have to desolder the probe because of a short or to remove it altogether desolder the center conductor first to avoid lifting the pad.

If you treat these probes with a little care they should last you long enough to justify their cost.  After enough wear and tear though they will become unusable and have to be replaced.  I’ve been using the same two probes for over six months now with no lose in measurement quality and they’ve been removed and resoldered quite a few times.  Before each use however, I do recommend checking the continuity of the center conductor just to be sure it’s still good. Troubleshooting a bad probe isn’t exactly fun and can make you lose your hair when your circuit mysteriously stops working.

Figure 5: Final Probe Placement for Filter Testing


The Value of 0 Ω

I learned the value of 0 Ω resistors while on co-op during the summer and fall of 2008.  My official title was NPI (New Product Introduction) Electrical Intern and I worked in the department in charge of transitioning new products from the R&D stage to production.  My responsibilities included building and documenting the electrical test fixtures that were used out on the production floor to test products at various stages of completion. Being at the end of my second of five years in school, I knew very little about anything at the time, and this being my first real world engineering experience, I was not the one who designed these fixtures.

However, being able to work with the man who did design the circuitry for these fixtures was, I can honestly say, awesome.  Over the six months I worked with him I came to think of his as the Yoda of circuit design. He was an old-time analog guru and I have no idea how long he’s been designing circuits but it seemed like he knew everything. Whenever there was a complex board to test or debug Yoda and I would sit together in his cube,measuring various voltages and what not and he would always patiently explain to me how something worked or why a certain portion of the layout had to be a certain way. I learned a lot from Yoda about design and troubleshooting, but my favorite trick he used was to stick 0 Ω resistors and unpopulated passive component footprints in key areas all over the board.

Figure 1: Example Circuit of Yoda's Techniques

Figure 1 above shows an example circuit containing the techniques I learned from Yoda.  Capacitor C1 and Resistor R3 would be marked NP, for Non-Populated, on the schematic by Yoda indicating to myself that the components were not to be soldered down when building the board.  If, when checking output of the op-amp stage, the output looked a little noisy, placing a capacitor in the feedback path to filter out noise above a certain frequency wouldn’t require a complex rework. The component was simply soldered down on the empty pads and testing could continue.

As testing would go on for the op-amp stage, the empty pads where R3 should be were convenient places to place a multi-meter or oscilloscope probe.  Only after the op-amp was working correctly would I be told to solder a 0 Ω resistor in place for R3 and testing would then continue on down the signal chain.

When it came time to finalize the schematic, if C1 was needed, its value would be inserted into the schematic and the NP marker removed, otherwise it would be deleted.  R3 was typically left in place because if a problem arose with U1 in the future the resistor could simply be tombstoned, thus isolating the op-amp from the rest of the circuit for testing.  This in and of itself was a useful debugging technique for if the problem disappeared after removing R3 then we knew something down the line was affecting the loading on U1 and the op-amp was probably fine.

Another thing I admired Yoda for was that he always came up with multiple circuit blocks to implement a desired function.  The two or three most feasible of these blocks would be placed on the test fixture PCB in parallel with each other and 0 Ω resistors were put in series with their inputs and outputs as shown below in Figure 2.  His most likely choice for that given feature was soldered down first and tested.  Should that block not perform to spec, time was saved because the PCB did not need to be re-spun, just move the 0 Ω’s to an auxiliary block and keep going.

Figure 2: Another Example of Yoda's Techniques

Yoda got away with this because most of the time there wasn’t a tight area requirement for the test fixtures. Placing an extra op-amp or two on the board could be done with only a minimal cost increase.  Typically, electrical components went into a standard sized project box that was machined with the proper holes for connectors and any other mechanical parts and that was about it.  So long as the PCB fit in the box all was good.  This freedom to experiment with new and/or different options for various circuits not only made testing and debugging easier, it also made Yoda a better designer because when it came time to design a new product and its required test fixtures, he already had a sense on what type of circuit might work for a given application.

While these techniques may seem like common sense, I for one am glad I got to learn about them while out in the field. The use of 0 Ω resistors and empty pads in key places are not the type of topics that get covered in core EE courses and I probably would not have learned them otherwise while in school. Currently, I am making great use of these techniques while designing and testing the circuit for my thesis. Having these tricks up my sleeve has been quite handy during the whole process.  Thanks Yoda.


A Bench of My Own…

There’s been a few posts recently about the workbenches and labs people have the privilege to build and/or use and I myself have been suffering from bench envy these last months because I don’t have a bench of my own.  Some good examples include Chris Gammell’s new bench, Miss Outlier’s Place to Tinker, and  even one of my old tweets about a home lab I found on the internet.

While working on my thesis I’m always running between four different labs depending on if I’m soldering my PCB or testing it and also on what specifically I’m soldering or testing.  Due to expansions and add-ons at my school these labs are actually spaced somewhat far apart.  While I’m not hiking for days, it does lead to frustration and a decrease in productivity when even a simple rework, like changing a blown IC can take upwards of twenty minutes with most of that being travel time. (I’m not yet smart enough to design my circuits un-blowup-able on the first try, and yes, un-blowup-able is an industry term, probably.)

At any given time I may need some combination of a network analyzer (filter testing), RF signal generators (mixer testing),  a function generator (ADC testing), oscilloscopes, soldering irons, etc., and sadly there isn’t just one lab which contains everything I need.  Since I’m always walking between labs for long periods of time I can’t leave my test setups in place for when I return.  Other projects going on may need to jump on the equipment and hogging space in multiple labs while I’m not there isn’t exactly proper lab etiquette.

So because of all these hijinks, one of the biggest things I’m looking forward to once I leave school and go off into the real world is having my own bench to work at, or at least having a single lab with most of the equipment I’ll need in it.  I’ve been spoiled by some pretty well equipped labs while out doing co-ops over the years and now that I don’t have my own bench I really miss it.  One summer I even had a whole lab to myself; that was fantastic. I had more cables, scopes, and power supplies than I knew what to do with.

Does anyone else jump from lab to lab like I do, or am I unique in this aspect?


The Inaugural Post

Hey, thanks for checking out the blog.  For those of you who follow me on Twitter @FakeEEQuips, thank you, and I apologize for my lack of ridiculous statements lately but coming up with quality Fake EE Quips is harder than it seems.  I will still be posting ridiculous quips as often as possible but after I started tweeting I realized that 140 characters at  time couldn’t contain me and I wanted to move onto bigger and better things.

A little bit about me, I’m in the home stretch of completing my BS/MS degree in Electrical Engineering and I feel that analog circuit design is the bee’s knees.  In my brief time as an engineer I’ve come to realize that I’d much rather spend hours at a bench with an o’scope and an iron looking for the cause of an oscillation than slogging through a pile of code to find a missing semi-colon or parenthesis.  When I’m not trying to become the next Pease, Williams, or Widlar (ambition FTW), you can usually find me touring breweries, listening to ska music, reading, watching Seinfeld reruns, or perfecting the fine art of just being lazy.

In terms of content besides Fake EE Quips, mostly I will be writing about topics in analog circuit design (IC or board level) and my experiences at school, though other topics will appear from time to time.  At the moment, working to complete my thesis and coursework consumes my time and I don’t have any side electronics projects going on. I’m hoping that after graduation I can start one up and post about it.  Until then I’ll write about projects I’ve worked on in the past along with bits and pieces of my thesis (the unclassified parts, dun dun dun).  I’m also kicking around the idea of starting an App Note of the Week post where I will point out some hopefully interesting/useful App Notes.

That’s about all for now.  I’ll do my best to keep posts coming on a semi-regular basis so things don’t stagnate around here.  Enjoy the blog.