Archive for category tech notes
Prototype, progress, and the first digital camera
Posted by Michael Beach in prototype, tech notes on February 13th, 2010
We make prototypes. Sometimes it’s hard to imagine how a product will evolve from the first (clumsy?) prototype into a sleek product, once it gets ‘real’ and becomes a manufactured (perfected?) item.
Since we all know what digital cameras look like now, here’s a photo (ironically a digital photo) of an early Kodak prototype digital camera, to show how things change and improve: (note that we don’t need the cassette tape to store images any more).
The full article can be found here: link to story about first digital camera at Retrothing
Low noise, low drift techniques
Posted by Michael Beach in low noise design, tech notes, technical articles on February 12th, 2010
This presentation has a set of clear, well thought out images describing how chopper techniques can reduce 1/f noise, reduce drift, and even how to cancel the nasty charge injection of FET switches. It shows how modulation can reduce noise in a sensor amplifier system.
link to pdf of Kofi Makinwa\’s presentation, Dynamic Offset Cancellation Techniques
I first learned of Kofi Makinwa’s excellent work through the recent IEEE Solid State Circuits magazine, Winter 2010, Vol. 2, No. 1. He demonstrates a clever accelerometer that uses a small air volume as the ‘proof mass’. The Wheatstone bridge has been around a long time, but it’s clear it can be taught some new tricks. This is the first I’ve heard of a ‘nested chopper’ architecture. Great stuff. Check out Makinwa’s other publications at the IEEE.
I’ve spent some time trying to squeeze good data from MEMS sensors, and I know how difficult it can be. These articles show why adding some switches and circuit complexity can really pay off. And it’s only CMOS and FETs, so we get ‘em for free from Moore’s law, right?
Microscope objectives – NA, cost, and parabolas
Posted by Michael Beach in math, metrology, optics, tech notes on December 14th, 2009
We were looking at various microscope objectives – those lenses on the turrets that aim toward the slides. Or, if you’re like me, the expensive silver thing that just went ‘crunch’ on the slide while I was trying to focus the image.
Pete noted that there seemed to be a parabolic curve fit – better NA, numerical aperture, better light collection, and the more expensive the objective lens gets. Here’s the curve, and the supporting data.
SPIE Photonics West 2010
Posted by Michael Beach in SPIE, how-to, tech notes on October 13th, 2009
We’ll be teaching a ’short course’ at the Photonics West 2010 show this January.
Titled ‘Fluorescent Detection: System Design and Tradeoffs’, it’s based on work we did to help develop a hand-held chemical detection instrument for Cogniscent Inc., (homepage for Cogniscent Inc.) who have granted us permission to show off some of the things that went into that product’s design.
We also discuss various design options that were not selected for the Cogniscent system, and what motivated those decisions.
Here’s where you can find more information, at the SPIE website:
Radiometry explained by NIST
Posted by Michael Beach in application notes, books we like, tech notes, technical articles on September 14th, 2009
The measurement of light is complicated by a variety of units and concepts that are not used in other fields. For example, the ‘light level’ could be measured in units appropriate to the sensitivity of our eyes (lux), or by the power level (Watts) – but that’s confounded by the wavelength (nano-meters, but sometimes Angstroms) and you need to think in steradians, etendue must be conserved … you get the idea.
We’ve written about some of these issues in earlier posts, but this is one big, complete reference manual – a kind of ‘everything you wanted to know about light, but were afraid to ask’ – and it’s from NIST. They call it a ‘Self-Study Manual’ and it’s a clearly written tutorial on optical radiometry.
And it’s a free download. Enjoy. The test is Tuesday.
link to NIST Self Study Manual on Optical Radiation
The official title is The Self-Study Manual on Optical Radiation Measurements, edited by Fred Nicodemus
Quantization noise
Posted by Michael Beach in books we like, low noise design, math, metrology, tech notes on August 27th, 2009
We notice the assertion that A/D converter quantization noise is equal to ADU/SQRT(12), where ADU is the quantization unit or LSB. We saw this in Hobbs’ excellent book Building Electro-Optical Systems, Making It All Work.
So, we decided to derive this. Took us a while to get the ‘trick’, and to remember how to perform calculus, to get that pesky root-mean-squared function.
Think of the quatization error as a sawtooth function that repeats. Then work out the RMS noise of that sawtooth wave (it happens to be the same as a triangle wave). And, yes, it does work out to that value.
Now the next part is Hobbs’ assertion that this quantization noise is not a Gaussian distribution. Get to work.
Precision Resistors, low tempco
Posted by Michael Beach in low noise design, parts, tech notes, vendors on July 27th, 2009
Here’s a couple sources for precision resistors.
One source seems to provide a great price/performance trade, the other is really the best quality resistor you’ll be able to buy. BUT precision low drift resistor arrays require careful inspection of the data sheet. Here’s a couple examples.
Recently I noticed that Maxim IC, (famous makers of 5V powered RS232 interface chips), have begin selling a pair of precise resistors in a SOT23 package, at an attractive price.
This family of parts, the MAX5491, claims to have 2ppm/degC resistor to resistor drift of the pair in the same package. Take care – sometimes the large print giveth, and the small print taketh away – the absolute tempco is spec’d as 35ppm/degC, so if you compare two different individual SOT23 units with each other, they may drift more that that nice small 2ppm value at the top of the spec sheet!
link to the Maxim-IC precision resistor array MAX5491 data sheet
For better matching, the VFCD1505 parts made by Vishay, also available at DigiKey, are spec’d for 0.2ppm/degC drift, or 10X better than the Maxim parts (and, well, about 5.7X the price, at $20/qnty 1, vs the $3.50 qnty 1 at DigiKey).
link to the Vishay VFCD1505 precision resistor data sheets
I like that Vishay also details subtle values such as ‘voltage coefficient’ and the current noise (or sometimes called ‘excess noise’, noise that is beyond the thermal noise of the resistor value).
The specifications of the Vishay parts are about where the state of the art is for actual resistors you can buy. Better resistors can be found only at NIST.
Light uniformity testing
Posted by Michael Beach in how-to, metrology, optics, photo, prototype, tech notes, tools on July 15th, 2009
For both a clinical test microscope, and a home theater HDTV projection display, the light from the source must be quite uniform.
To test some non-imaging illumination optics, we set up our digital camera, and wrestled with the RAW data files from the camera. Most cameras have some ability to ’see’ infra-red, so we can also test the pattern from the remote control output, or for other purposes.
Here we test the light uniformity of an LED source using a digital camera and some Thor Labs mounts.
These graphs were generated by ImageJ from the RAW data files of the Nikon D1x camera.
Custom front panels
Posted by Michael Beach in how-to, parts, prototype, tech notes, tools, vendors on July 14th, 2009
Here’s a vendor we recommend, and a photo of some parts they made for us.
When you make one or 10 of something it can be difficult to make it look ‘real’, that is, to make it look finished. Sometimes the Dymo-Marker labels are OK, say in a sci-fi movie, for your Custom Flux-Capacitor. But other times, you want the prototype to look clean.
We have enjoyed using the services of FrontPanelExpress to make some custom front panels for our projects. They provide free software, that’s simple and easy to use, and you upload the files to their on line ordering … and you get great parts back. A variety of anodized aluminum options and thicknesses.
http://www.frontpanelexpress.com/
They will engrave text, add paint colors into the text -makes a very nice professional looking prototype or short run of parts.
Just imagine – they can easily make a D-shaped hole for the BNC – so it won’t UNSCREW and FALL OUT !!! That alone is worth the price of admission. I can’t find my D-shaped drill bit …
They can do a lot of things that are a pain to do by hand – countersink holes, nice RS-232 type D-cutouts, square holes, etc. Check out their site for some examples.
We also made a Plexiglass panel – this allows us to compensate for the very annoying differences in height of the switches and knobs – and it allows simple paper graphics to be protected. We also considered making a glowing logo, but have not yet done that.
Low noise NMR design
Posted by Michael Beach in low noise design, metrology, research papers, tech notes, technical articles on July 8th, 2009
Careful consideration of all the elements of a system’s design can lead you to some very improved performance. Imagine improving a benchtop NMR system by making it 60 times lighter (120kg to 2kg), 40 times smaller, and yet 60 times more sensitive!
This article, from the IEEE Journal of Solid State Circuits (Vol. 44, No. 5, May 2009), shows an excellent example of how this occurs.
link to IEEE abstract of ‘CMOS RF Biosensor Utilizing Nuclear Magnetic Resonance’ by Sun, Liu, Lee, Weissleder, and Ham
I recommend reading the article – it’s very well written, it describes how NMR works, and it details their systems approach to their improved design. Much can be learned here. The use of a resonant circuit for gain (they call it ‘passive amplification’) is detailed in Figure 8 of the article. (It reminded me of the old ‘regenerative’ type radio receivers, back when a vacuum tube had a power gain of about 12).
Put another way, this article shows that the ‘building block’ approach, when off-the-shelf 50 Ohm compatible RF modules are used, makes it easy to build a system that works – but that it leaves out some great performance improvements that are only possible when you analyze the basic system operation and theory. The design improves when you ask questions like ‘why 50 Ohms’ or ‘where does that noise originate and how can I maximize the signal’ and ‘how can I make this work with a much smaller and lighter magnet’? The article also answers ‘now that I can use a small magnet, can I make a custom CMOS IC that performs the RF detection, and seriously reduce system cost and size’?
Buying as much stuff off the shelf is not bad – it’s a great way to get a proof of principle working FAST, and it demonstrates that an idea or technique can work. Nothing says ’success’ like working hardware – it allows the investors, managers and engineers to breathe easier.
But that extra performance gain from really digging into the details of how things work can pay off – in this case, it changes a benchtop lab instrument into a battery operated portable clinical test platform – this opens new opportunities and situations where this NMR system can be utilized.