![]() So now we know what all of these physical effects are and that they can (will!) act to mess up our signal when frequencies are high. Another thing to consider, especially in bus topologies, is that we want to ‘stop’ the wave at the receiver and not have it bounce back again – this is usually done with a terminating resistor, which absorbs the energy of the wave at the end of the bus (such as in RS485). For high-speed design, we therefore want to try and keep the impedance of our tracks as consistent as possible along their length. In electrical terms, this is known as “impedance”, which is a function of the resistance, capacitance and inductance of the conductor. Multiple changes in width degrading the signal. It is therefore important that the width/height of the channel stay as constant as possible along its length, to avoid reflections. Most of the energy of the signal would also not be reaching the receiver (at least not at the correct time). If there were multiple narrowed sections within the canal / channel, then there would be multiple reflections bouncing around, interfering with the signal. Imagine that our channel was 100cm wide, but at a certain point it was suddenly narrowed to being only 1cm wide – when our wave reached the suddenly narrowed section (essentially a wall with a small gap in it), most of the wave would bounce off the narrowed section (wall) and back towards the transmitter. A wave is generated at one end of the channel, and makes its way along the channel (at close to the speed of light) to the other end. Let’s imagine that our conductors are like water-filled canals / channels. ![]() Waves are subject to various phenomenon, the most important of which to us is “reflection”. In our 3GHz / 30cm trace example, there are 3 waves – peaks and troughs – within the conductor at any given time. ImpedanceĪs described under the ‘Speed’ section, our electrical signals are not instantaneous, and actually travel as waves in our conductors. Each track on the PCB can also be viewed as a small radio antenna, capable of both generating and receiving radio signals, which can distort the signals meant to be carried by the tracks. All of the conductors in the circuit – typically tracks on the PCB – are thus capable of both generating and receiving electromagnetic interference, which can distort the signals meant to be carried by the tracks. Conversely, when a magnetic field passes through a conductor, then it generates a voltage within that conductor (via the electromotive force). Whenever electricity travels through a conductor, it generates a magnetic field around that conductor – however small that may be. ![]() This is certainly not instantaneous on/off from one end of the conductor to the other! Signal Transmission can no longer be thought of as instantaneous! Reliability Make that 3GHz, and the source of the clock signal is already generating the 3rd pulse by the time the 1st pulse is reaching the other end of the conductor! Thinking about that (3GHz, 30cm conductor), this means that our single 30cm conductor ‘contains’ 3 pulses, 3 high and low states, within its length. Considering however that a 1GHz signal has a period of 1ns (1 nanosecond), light only travels at approximately 0.3 m/ns, or 30 cm/ns meaning that over a 30cm long conductor, the first clock pulse of a 1GHz signal is only just reaching the other end of the conductor by the time the next clock pulse is being generated at the start of it. The speed of light is 299 792 458 m/s, which is very fast. SpeedĮlectrical signals also have a speed limit – the speed of light. The speed at which the voltage transitions from its low state to its high state is called the slew rate. 3.3V), passing through all the voltages inbetween, even if it does so very quickly at some point in time during the transition it is 1.8V, and at another point in time it is 2.5V, and so on. We need to understand that there is no such thing as an instantaneous transition from 'off' to 'on' – the voltage has to transition from the low level (e.g. In this article we will look at some of the factors which make high-speed design more complex, as well as some of the techniques used to address them. Working with faster and faster electronics however, as clock speeds move from the MHz to the GHz, it becomes obvious that things are just not that simple. Certainly from a programmer’s viewpoint, this level of abstraction is assumed at the software level! If, like me, you come from a firmware background you probably prefer to think of digital electronics as on and off or high and low those on and off signals travelling instantaneously and reliably through the electrical circuitry.
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |