Editorial Note: Today’s design article comes from the book “Signal Integrity Issues and Printed Circuit Board Design” by Douglas Brooks. More information about the book can be found in this posting. This is the first half of Chapter 10: Reflections and Transmission Lines. The second half of this chapter will be made available in the coming weeks. The first half and second half of Chapter 9: Electromagnetic Interference (EMI) were published here earlier this year.
Chapter 10: Reflections and Transmission Lines
Figure 10-1 illustrates what we sometimes call a general communications model. Any time there is a communication, we can think in terms of there being (a) a sender of that communication, (b) a message that is being sent, (c) the media over which the message is being sent, and (d) the (intended) receiver of the message.
Figure 10-1: A general communications model.
For example, at this moment you are reading something “sent” by Doug Brooks, author of this book. The message is what is written on this page. The media is the page you are reading, and the receiver is you, the reader.
On PCBs, the sender is the driver, the message is typically the change in state from a high to a low signal or from a low to a high signal, the media is the trace, and the receiver is the receiving circuit.
The evening news is “sent” by the news anchor (and his or her team behind the scenes). The message is what you see on TV, and might be designed to be entertaining as well as informative. The media can be thought of in simple terms as the television system. In reality it can be a much more complex system of microphones, cameras, switchers, satellites, retransmitters, and so on. The receivers are those watching the program.
In advertising, we think of the sender being the advertiser (e.g., Nike). The message is the advertising message. The media is the media chosen. The receivers are those targeted to receive the message. When we think about advertising in this context, we begin to think about some of the issues that advertisers face:
- How do you select which receivers to target?
- Where are they?
- What media will reach those targeted receivers most efficiently?
- How do we structure our message so that the receivers “hear” it while being bombarded with thousands of other messages at the same time from other senders trying to get their attention?
- How do we make sure the receiver “hears” the same message the sender thinks he or she is sending? It is fairly common for targets of advertising messages to perceive a different message than that which the advertiser intends to send.
Think about this communication problem: Picture yourself sitting down at the end of a high school gym at assembly time. The football coach is at the other end handing out the team awards. He is speaking through a handheld public address (PA) system. The floor is made of hardwood, the walls are concrete, the ceiling is metal, and the echoes are awful. It is almost impossible to understand what the coach is saying. The media is the primary cause of this particular communications problem.
Now, this time, picture yourself positioned along a PCB trace. The driver is sending a signal down the trace and you are trying to “hear” it. But here is the problem: Any time a signal travels down a wire or a trace, it reflects. Every time—(well, almost). Notice what I said and what I didn’t say. I did not say “often” reflects. I did not say “sometimes” reflects. I did not say “under certain circumstances” it reflects. It always reflects. (Well, it turns out there is one very special case when it doesn’t. But this special case doesn’t happen by accident, as you will see. It is this special case that is, in fact, the point of this chapter.)
So, in both our examples, the high school gym and the PCB trace, there are reflections and echoes that can interfere with the receiver understanding the message being sent. Here are some solutions to these communications problems:
1. We can encode the message, so we can pull it from the noise level. There is an entire science related to encoding and decoding messages in noisy environments, and this certainly a legitimate solution. It’s not particularly practical in the gym environment, however, and probably not in our PCB environment, either.
2. We can listen “harder,” or become better listeners. In the gym, we can concentrate harder on what the coach is saying. On our boards we can use better, more selective, but probably more expensive receivers. Again, these are legitimate solutions, but perhaps not desirable.
3. We can shorten the distance the message travels. In the gym, we can get up and move closer to the coach. We can shorten the trace on the board. On the circuit board, this is usually a highly desirable alternative, but we probably have already designed the traces about as short as they can be, so further shortening them is usually no longer an alternative.
4. A particularly interesting solution is to slow down the message. If the coach will speak more slowly it will clearly be easier to understand him. The analogy on our boards is to use signals with slower rise and fall times. This, too, is a desirable solution. Many people recommend using circuits with the slowest rise and fall times we can get away with to avoid reflection (and most other high-speed design) problems. The problem is (a) we may not have control over the rise time of the circuits we are using and (b) we may have already chosen the fastest rise time circuits available to meet other circuit requirements.
These are four legitimate solutions to our communications problem, but none of them may be practical for our specific situation. But there is one more:
5. We can acoustically engineer the gym to absorb reflections. Next time you are in a hotel or convention center conference room or hall, look closely at the surroundings. Chances are very good the room has been acoustically engineered to absorb echoes and reflections. It will have a soft carpet, cloth-paneled walls, and sound-absorbing ceiling tiles. Speakers are probably placed where there will be no phase shift between the sound coming from the presenter (wherever he or she is) and where you are sitting.
Similarly, we can (not acoustically, but) electrically engineer our trace to absorb reflections. Then there will be no reflections going back and forth on the trace to interfere with the receiver’s ability to receive and understand the message the sender is sending.
The problem is this: In general, we don’t know how to do that. We do know how to acoustically engineer a room, but we don’t know how to electrically engineer a wire or trace to absorb reflections—with one very special exception. We do know what to do if we are dealing with what we call a transmission line.