Category Archives: Electromagnetics

IC Friday: Atmega’s 8535

Today’s IC Friday entry is the 8bit AVR micro from Atmega. This chip was sent in by a reader, all the way from Iceland.

HOWTO: make a simple and cheap Yagi antenna for wifi applications

small.jpg

I have been playing around with some wifi networking lately, mostly with the La Fonera, and finally decided to build a directional wifi antenna. Although the cantenna, however, I don’t really like Pringles chips and wanted to make something more interesting. I decided to try and make a simple Yagi antenna with a magnetic dipole as the driving element.

From a construction standpoint, the Yagi antenna is made by spacing conducting rods along the directionality axis with a driving element near one end. It is assumed that the incoming radiation is a TEM plane wave, so the direction of the electric field component should be parallel to the conducting rod orientation. The magnetic component is then perpendicular to the rods and to the directionality of the antenna. The rods spacing is then configured so that the coupled EM field generates a magnetic field component (and a curling electric field component) along the directionality axis of the antenna which has constructive interference at the driving loop. Proper spacing then determines the antenna’s gain and directionality in the band of interest (2.4-2.5GHz).

As a first step, I decided to reproduce the Yagi design made available by Andrew Hakman who reproduced the dimensions of a commercial antenna. This first implementation will test the basic operation and is still missing fine tuning and optimization. I am pretty happy with the initial results which demonstrate a 10dBi gain, which is pretty nice given that it took roughly half an hour to assemble. I will use a more precise construction technique (EGX-300 to mill the main beam) and will work out the optimal metal rod length to magnetic dipole ratio. The main idea is that the loop length needs to support one of the resonant transmission modes for the given frequency while the rods should be as long as possible to increase gain, but shorter than the length of the loop. If anyone wants more info on Yagi theory of operation, please post a comment and I will try to write up a post about it.

To construct this, I used a 0.5×0.5 inch piece of wood for the main beam, and 0.125 inch zinc rods for the conductors. I cut the rods to match the lengths in the above design and sanded the ends to remove any pointy spots. I measured out the positions for the rods on a piece of tape and used a small drill press to make the holes. I then gently tapped the rods into place and removed the tape. I cut the loop out of a sheet of bronze, mainly because that is what I had around. It is better to use a strip (versus a round wire) here to make the loop more sensitive to magnetic field components along the directionality axis. Finally, I decided to minimize transmission losses and mounted a USB 802.11g adapter directly onto the loop. I hot-glued everything into place and went to a large set of windows to test out the contraption.

To benchmark the devices performance, I compared signal strengths to the internal wifi adapter on my Lenovo T60. The signal strengths for the same APs were comparable between the internal adapter and the intact USB adapter so any improvement that I saw here was likely due to the Yagi. Although it was sometimes challenging to find the right direction to point the Yagi, I noted a substantial increase in signal power when I switched Netstumbler between the internal and external wifi adapters.  Over all, I consider this to be a success since I got better performance from the USB adapter by investing a few dollars and a hour of my time. The next version will be forthcoming in the next weeks and will hopefully display even better performance.

0531081229a.jpg 0531081319a.jpg 0531081302.jpg

0531081329.jpg 0531081335a.jpg 0531081341.jpg

0531081350.jpg 0531081351a.jpg 0531081406a.jpg

0531081411.jpg 0531081430.jpg 0531081431.jpg

netstumbler-comparison1.jpg

FRAM guide from Fujitsu

pzt.JPG

When looking at the application notes section of Fujitsu’s site, I came across their FRAM memory guide book. I was surprised as I did not know what FRAM really was and so I flipped throug. Basically, a film deposition process was developed, which is compatible with standard CMOS processing, that introduces films who can maintain their polarization after the applied electric field is removed. We are all familiar with ferromagnetic devices, these are the pieces of metal that can be magnetized when placed in a constant magnetic field. Thanks to some nice electromagnetic research, we can do something similar with thin films and thereby create ferroelectric capacitors that are capable of retaining data without applied power while being as fast as SDRAM. It is clear that half of the Fujitsu guide is a sales pitch for their ICs, however, the other good is a fairly good introduction to the FRAM technologies. The basic technology is discussed along with some typical ferroelectric substrates. A reference list is also attached.

( mn05-00009-5e.pdf )

More free textbooks

books.jpg

While looking for ways to escape muti-variate calculus purgatory in the final weeks of the semester, click I came across Open Math Text.  These are a collection of math books (in PDF and LaTeX) that are openly available for distribution and are aimed at general scholars. A quick look at the collection will show that most of the books are authored by Dr. David Santos, a professor a the Community College of Philadelphia.  It seems that he has written and made available more books, in multiple languages, than the number of scholarly papers that most researchers publish at full universities.

While looking at his personal page, I found another open textbook collection called Textbook Revolution. The obvious downside is that these publications may not go through the same levels of review as textbooks printed at conventional publishers, however, it is nice to know that there is a group of people actively working to make affordable textbooks available.

High-speed transmission line termination techniques

resistors.jpg

For almost every digital circuit designer out there signal integrity problems often come up as frequencies increase, board sizes decrease and IC pin impedances change. (Every designer except for myself, I work with very low frequency analog circuits!) When signal integrity becomes so poor that unacceptable levels of transmission errors are reached, the efficient digital designer may venture into the analog domain and start looking at transmission line models of their digital traces. For those who prefer to think of everything as analog, this could be the point of argument to “prove” that ones and zeros only exist in the digital designer’s mind and do not represent physical reality (although thinking of voltages as high/low is sometimes more efficient).

Sometime in the 1970s, Motorola introduced digital emitter coupled logic (M ECL) circuits. I don’t know if Motorola was first, however, they had plenty of expertise on the subject. The made ECL useful in those days was the incredibly fast switching rate for these types of logic circuits. ECL works very similarly to standard bi-polar junction transistor based designs, however, the transistors in ECL are always partially conducting. The high and low logic levels are determined by different points along the devices’ load curves which made them faster than BJT devices which had to go from completely off to completely on to switch logic. ECL devices were (and still are) much faster than comparable CMOS devices since CMOS depends on relatively slow thermal generation of carriers to create the conduction region. The point is that fast digital circuits are not that new, and we are facing some of the same transmission line problems as thirty years ago when we scale dimensions and voltages down and increase the operating frequency. If the operating frequencies of interest are such that wave lengths (in the conducting metal trace) are comparable to the length of the trace, transmission line models must be employed. This matter of wavelength can be a tricky question to answer as it can be readily shown that the wavelength of a 60Hz signal in a thick copper conductor is about 5cm (with a phase velocity of only 3.22m/s).

Now that we believe that our traces can act like transmission lines, we are faced with a problem of matching impedance. In the simplest of cases, we have only the driving logic (generator), the trace and the receiving logic (termination). From a driving perspective, the output impedance of the device should closely match the trace impedance, typically something like 30-70Ohms. If the output logic is not matched to the trace, a reflection will not occur at the driving logic in the strict sense, however, the signal traveling down the conducting trace will already be deformed. Now that we have a packet of current traveling down the trace, as specified by the generator, any mismatch in impedance between the trace the termination logic will result in a reflection which will further deform the other current packets traveling down the conductor. This problem can easily happen when CMOS logic (infinite input impedance) is coupled with low output impedance logic and the transmission frequency is gradually increased. The problem becomes more complicated when there are multiple terminations on a given conductor segment as each impedance mismatch generates a reflection and so forth.

Besides the MECL Design Handbook,  Altera provides a few application notes [1][2][3] on signal integrity and high speed design which include termination practices. Typically, introducing a resistor in series or in parallel (to ground) is all that is required to mostly match impedances and give adequate performance, the most important concept is knowing when and where to use these terminating resistors. Although some devices come with various termination options built into the die, most still don’t, so it is good to know when a properly placed resistor network can save a lot of shielding attempts and speed up the debugging process.

Impedance measurement handbook from Agilent

agilent.JPG

To follow up the last post on resistor selection, seek here is a the Agilent Technologies Impedance Measurement Handbook. I found this handbook to be quite useful and well written as it covers everything from the basics of measurement problems to examples of both low frequency and RF frequency impedance measurements. The authors focused on the often overlooked parasitic properties of common system components as well as the measurement systems themselves. They go on to outline methods to construct test structures and procedures to minimize these parasitics and go on to give practical examples. For obvious reasons, sovaldi sale all of the test equipment in the handbook is made by Agilent, however, other brands can be used just as well.

( 5950-3000.pdf )

How to duplicate certain RF proximity cards

prox-assembled-standing.jpg

With RFID-type devices becoming more and more ubiquitous in our society, it is good to know some of the advances being made in the security research fields so as to avoid a false sense of security. I came across Jonathan Westhues‘ site which outlines his experiences duplicating certain identification devices. It is important to note that the duplicated devices are of the identification-only type and do not have any built in security mechanisms, however, these are accessible initial steps. Hopefully this will motivate me to do something with the TMS3705A-based RFID reader I built following a sample design.

A response to the problem of two moving electrons

electrons-img.jpg

The following physics problem has been proposed on Momes (via  Perfectly Reasonable Deviations):

Two adjacent electrons move parallel to each other, in the same direction in a vacuum. What happens? Consider the situation at varying speeds from 0 to c.

The most intuitive response is to say that they attract since it has been taught to us that two parallel wires carrying electric current in the same direction will attract each-other. The question is somewhat ill-posed until the frames of reference are defined. Do the electrons move at the same speed? What speed does the observer move? If we assume that the electrons are stationary with a fixed distance between then and the reference frame of the observer moves at some v that varies from 0 to c, we can write the Lorentz force equation to describe the force that one electron exerts on the other as observed in the moving reference frame. The thing to remember here is that the magnetic field is the Lorentz transform of the electric field, or rather that it is the result of a charge moving with respect to the observer. Since the Lorentz force equation, F=qE + v x B, has both an electric and magnetic component, it can be shown that the force is repulsive when the observer is stationary with respect to the electrons and becomes attractive after the speed (I know this should be velocity, but we are assuming that the direction is away from the electrons) becomes a certain v’ such that F = 0. This is different from parallel cables that attract for any measurable current in the same direction because the metal cables are electrically neutral for any macroscopic volume. That is, for every electron, there is a proton, so there is no net radial electric field and the total force component is due to the magnetic field only.

One final thought to keep in mind is that two electrons stationary in vacuum without external fields is nothing more than a mathematical construct. Like plane waves, this configuration would not be static in time without some external force keeping the electrons a fixed distance from each-other. The reason that I bring this up is that, although they are good exercises, one must not try to infer laws of nature from mathematical constructs.

[Image is of density measurements of an electron cloud from Physics Central.]

Setup wizard for Linksys WGA54G wireless game adapter

wga54g.JPG

A couple times a year, medical I have to reconfigure my Linksys wireless game adapter. Every time I figure that the setup utility is online and realize that it is nowhere to be found. Consequently, help I spend about an hour every time trying to find my original install CD and cursing Linksys. I am guessing that some other people might be going through the same process a couple times a year, so for you, here is the setup utility.

( wga54g-setup.zip )