by Sanchayan Maity
I don’t know about the education system of other countries, but, in India, it’s funny how most engineering colleges never bother to teach concepts and application of the concepts to practical real world problems. Though it very well may be the same in other countries as well, from what I have been reading about the state of education, even in countries like the US.
During my engineering college days, one of the most difficult subject for me to understand was Electromagnetics. I never thought that, one day, it would be the subject that would interest me the most and I would come across it’s application in such a way, as I am soon going to tell you about.
I was working on a system which used a 10/100 Mbps Ethernet connection for communication with the external world and for exchanging data. I designed the system, it was not a tough one, but, it had this one high speed interface. My understanding prior to first encountering failure on this project, was that on a Printed Circuit Board, a connection between Point A and Point B, is just a simple copper track laid out on the board. I made the schematics and send them to our local third party vendor. The vendor was just a regular run of the mill local guy, who didn’t have any idea about such things, well neither did any of my seniors had any idea nor I. That vendor just tried to do the routing using the auto routing facility available in most PCB Design software these days like the PADS tool by Mentor Graphics. If that didn’t work out properly, he would do the routing himself, trying to fit the tracks on the PCB properly.
So, the first sample prototypes came. I started testing them and as it turned out, the Ethernet interface didn’t work. I started doing research on the net, as to what could be the problem. And alas!! I find out about PCB design and it’s importance. All the PCB’s in the world with high speed interfaces, would have been impossible without PCB design. And as it turns out, it happens to be an application of the principles of Electromagnetics. Now, enough of my background experience, let me tell you why it’s important.
At low speeds the tracks on a PCB act just as normal electrical point to point connections. At high speeds, high frequency effects come into play. Remember lumped parameters and distributed parameters?. At low frequencies, the wavelength of the signal being propagated from one point to other is greater than the length of the track. At high frequencies, the wavelength of the signal is less than the length of the track. The lumped parameter model is considered for the former, while, the distributed parameter model is considered for the latter. Basically, parasitic inductance and capacitance come into play, impedance matching becomes important, length of tracks become important and these effects are just to name a few.
Consider, you are sending a high frequency signal from Point A to Point B. Now, at high frequencies, these tracks start acting as transmission lines. So, if impedance of the track viz. the transmission line is not matched to the load (the point of reception), a part of the signal is transmitted and part of it is reflected. Basic transmission line theory, if Zl != Zo, you get a reflection co-efficient, where Zl is the load impedance and Zo is the characteristic impedance of the transmission line. The reflected signal interferes with the transmitted signal, and depending on the impedance mismatch, the signal can degrade significantly. In the case of Ethernet, a differential impedance of 50 ohms has to be maintained. Also, in case of a differential pair, the’+’ and ‘-’ tracks should be of equal length. To maintain this impedance, the width of the track and the distance of the track from the ground layer has to be set properly. The (W/H) ratio is important, where W is the width of the track and H is the height of the track viz, the layer the track is running on to the ground plane layer, assuming you have a seperate ground layer available.
So, impedance matching is important and one of the parameters to take care of in PCB design. Other important factors to take care of include crosstalk, reducing any spurious EMI emissions, proper clock distribution, proper via placement and sizing, taking care of current return paths and layerstacking.
The above was just a very short explanation of why PCB design is important and an account of my own experience. I will try to cover the topics mentioned above in my future blog posts. In the mean while, if you are like me, who likes to explore things, by reading books and online content doesn’t and never will suffice, below, is a list of books you can refer to for delving deeper into the subject.
If you are good in Electromagnetics or have a decent understanding of it, skip the first two levels.
Level I. Basic Electromagnetics
- Engineering Electromagnetics by William Hayt (Good for beginners)
- Electromagnetics Waves and Radiating Systems by Edward C. Jordan and Keith G. Balmain (Much advanced compared to the first, but, contains a wealth of information. Hats off to you, if you can digest everything in this book)
Level II. Microwaves
Microwave Engineering by David Pozar
Level III. PCB Design
High Speed Digital Design - A Handbook Of Black Magic by Howard Johnson and Martin Graham
Advanced Signal Integrity for High Speed Digital Designs by Stephen Hall and Howard Heck
The first one is for beginners and doesn’t require high level math or calculus. A must read for all embedded engineers. The second one is detailed and dives into PCB design from a mathematical and detailed theorotical point of view and requires knowledge of vectors and calculus.
An advanced book is available for the first one, but, no low price edition available in India. Another book by Eric Bogatin is also available, but, again no low price edition available in India. I mentioned the second book, because, I managed to get hold of a PDF for it. Now, I don’t want to support piracy, but, since I couldn’t lay my hands on any Signal Integrity book for under 5K, I had no other choice. That book is very good and a must read though. All specialised books happen to be so expensive or difficult to find in India. If any of the authors or book publishers come across this post, a sincere request to you, please make a low price edition available. Paying 5 grand for a book is a bit too much, though, I can afford it!