μblog: engineering from the trenches

Dear illustrious reader,

You have come here to find (hopefully) some great (terrible) information, or perhaps something relating to engineering. Not this time, this time I will use this place to put forth a public complaint, as well as the counter point. My gripe is with Comcast. Having just written a small summary regarding comping custom Fonera images, I figured that I would try to write something up regarding cross-compiling from a favorite operating system called FreeBSD. I followed the same footsteps, create a VMware configuration, download the latest FreeBSD ISO (i386) but then it stuck me, lets use BitTorrent! System activates and then nothing else works, meaning, while I am downloading the ISO, I can’t even check my email. Others have seen some of the same problems. So if Comcast is actively downgrading my connection using some silly tool, what can I do? There is no answer. I live in the middle of nowhere there is only one viable broadband provider with whom I must live.

The counter point is that Comcast owns the infrastructure, so they should be able to shape traffic regardless of any group of individuals that stand in their way.

Summary: The United States is an area that is sparsely populated so that most geographical areas can be controlled by one or two monopolies without competition.

Please feel free to comment to let us know how excellent/terrible your service has been and where you are located.

Dear illustrious reader, I commend you for making this far. Thank you for reading my blog.

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It is true that I hold a high regard for current sources due to their limitless applications in the biosciences. Current sources are at the heart of electroplating systems for electrode manufacture, stimulation of tissue and imaging. This is only a small part of the reason that I find this 1973 application note so appealing. The most important message, to me, is the complete walk-through from the basic governing equations based on ideal op-amps, to non-ideal characteristics, to error propagation.

In this case, the current source becomes less important than the design process. One of the subtle issues that can be seen from the equations is that resistor matching can degrade circuit accuracy just as much as the op-amp quality. For this reason, it may be beneficial to spend an $0.05 on more accurate resistors and save $1.00 on an op-amp. The converse is also true: you can spend an extra $1.00 on a more precise op-amp to save $0.05 on passive components. The standard error from your complete circuit may end up being comparable in the two cases.

( an587-d.pdf )

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This week, we have the 39VF800A 8Mbit parallel flash from SST. As expected, there is not too much to see as the main part of the chip is made up of a repeating pattern of tiny transistors.

SST39VF800A-10x-stitch.jpg

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I have been looking at semiconductor reliability handbooks (provided by most semiconductor manufacturers) and have found that Sony has one that is put together very well. Their overview of the reliability problem is on par with textbooks and features nice illustrations. From an integrated circuit design standpoint, the second chapter was the most interesting as it outlined many of the failure mechanisms that occur in modern ICs. This provides insight into design constraints and criteria which can provide higher yield and over-all reliability. The other three chapters are also interesting, however, they focus more on a the process engineering approach.

Why was I looking at these handbooks? I recently read an anonymous insider interview, meaning it might be false, regarding failure rates of Microsoft XBOX360 game consoles. Some individuals put average failure rates at around 20-30% which cannot be acceptable for consumer devices. The insider puts most of the blame on poor thermal management leading to mainboard warping, which then leads to solder joint failures (covered in the Sony handbook). Poor design and haste lead to these problems, and in the end, the incurred losses may have justified pushing back the initial deadline.

( qrhb.pdf )

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For what it is worth, the La Fonera is still one of the better deals on basic embedded systems on the internet. I have looked at it before and shelved it for quite some time until I needed it again for a prank (open wifi, http redirection, etc). The available documentation, at the time of writing, is a bit spotty but one can gather enough information to build and test firmware based on the OpenWRT project. This guide will hopefully illustrate the complete process from the very start to actually running the custom firmware.

(more…)

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I must admit that I goofed this one up. I misplaced the piece of paper with the chip identification before uncapping this chip, as a consequence, this is a mystery chip labeled P3727 (on the die). I guess this one will have to be just another pretty face.

tip3727d-10x-stitch.jpg

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Although Wolfson Microelectronics produces some fine integrated circuits, their application note section is somewhat out of the way and doesn’t like to be linked to directly. This didn’t stop me from looking around and finding some potentially useful app notes:

A.C. Coupling Capacitor Selection

Recommended Output Filters for Wolfson Audio DACs 

Class D Headphone Filter Component Selection

Issues When Grounding D.C. Coupled Headphone Outputs

The main reason that I was looking there in the first place was that I was getting excessive noise when coupling a portable audio player to an audio system I am working on and couldn’t figure out why. When I took everything apart, I found that output stage of the audio device was being pulled up to a higher voltage than expected by the coupling on the input stage and thereby biasing the input stage of the audio amplifier incorrectly. After some careful circuit modifications, the signal integrity was returned with fairly good low frequency response. At this point, my audio circuit experience is still minimal, I hope to post some designs once I get something worthwhile going.

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Today we have the ADS830 (8bit, 80MSPS) analog to digital converter from Texas Instruments. This is a pipelined ADC so it is likely that each of the vertical sections in the center (there are six) probably contains a two-bit flash ADC, DAC and appropriate amplifiers for taking the difference and gain after each stage. It is likely that there is a single voltage reference location that supplies all of the comparators. Given that this is an 8bit ADC that incorporates some sort of digital error correction, and there are six 2bit stages, there may be some stage redundancy to allow for higher certainty of the lower bits.

ads831-10x-stitch.jpg

ads831-40x-stitch.jpg

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Most scholarly articles that have some substance have a reference section at the end. The purpose of this section is to support scientific statements and to aid readers who would like to learn more about some of the article’s sub-topics. In rare occasions, authors use the reference section to show their scholastic diligence by citing some very old  or obscure sources. An example would be to include Laplace’s or Poisson’s equations in your paper, where relevant to electromagnetics, and then cite Maxwell’s Treatise on Electricity and Magnetism from the 1870s.  These are basic electromagnetics equations that are part of standard physics and engineering textbooks, which should be cited instead of Maxwell’s works. The rationale is that modern books are much easier to locate and are much easier to read. On the other hand, a good reason to cite Maxwell in a paper would be to discuss some particular part that is either unavailable or is contrasted to modern literature.

The purpose of this post is to present the best selling mathematical textbook in known history: Euclid’s Elements of Geometry. This contains a reproduction of the original Greek text along side a translation to modern English. Much of the contents of this tome are covered in typical high-school curricula, however, if you want to bolster your appearance of scholarship, this is probably one of the oldest works that anyone in the sciences will ever cite.

( elements.pdf )

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In analog circuits, it is not uncommon to see noise at 60Hz and harmonics of 60Hz. While some portion of this noise, especially in sensitive equipment, can be attributed to pickup of local electromagnetic fields, there is also an issue of noise coupling in through DC power supplies. Even the best regulators still have a finite power supply rejection ratio and let in a little bit of noise from the power lines. The problem is compounded when power circuits are specifically designed filter 60Hz noise but are unable to cope with higher order harmonics.

Reactive/non-linear loads are the core of the problem when it comes to many 60Hz harmonic noise problems. The supply cables have finite impedance, therefore, any harmonics on the current load will also be reflected on the delivered voltage, and vice versa, resulting in some level of harmonic contamination for all of the devices sharing the particular power circuit. A simple example is figure above. When the capacitor bank is charged, the bridge rectifier diodes will only conduct for the brief period of time when their output voltage is slightly higher than the capacitor voltage resulting in current spikes as indicated. Since the current spikes are much narrower than the rectified voltage signal, they will contain higher frequency harmonics.

This is where power factor correction and better design can come in to save the day. The primary idea behind power factor correction is to remove any apparent reactance or non-linearities from the load and make it look like a plain resistance. Another key feature of power factor correction is maximizing the power delivered to a device as the real power, related to the in-phase components of the voltage and current supplied, is what typically does the work in the system. Although this second feature is very important over all, it is rarely important to neurology related electronics as they are typically based on sensitive amplifiers and not heavy electrical loads. (The obvious counter example would be an fMRI machine.)

In any case, here is a short (11 page) primer on power factor correction from Fairchild Semiconductor followed by the more substantial (208 page) power factor correction handbook from On Semiconductor.

( an-42047.pdf ) ( HBD853-D.PDF )

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