Introduction
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Who controls the impedance?
There are several times when you will want to worry about impedance control on your PCB. The first is when you are connecting to an antenna. The other is when the length of the trace is longer than the rise time or fall time of your signal multiplied by the propagation delay (assume that is 2 inches per nanosecond).
Let’s look at the later condition first. When using high speed logic circuits, if the rise/fall time is 5 ns, then using our rule, 5 ns x 2 in/ns is 10 inches. If your trace length is more than 10 inches you should treat it as a transmission line and terminate it properly to prevent reflections.
That is fine for digital signals, but what if you are using an op amp? Do you need to worry about transmission lines if you are driving an analog signal? The answer is, it depends on the frequency of the transmission. For an analog signal, you can assume the rise time is 0.35 times the maximum frequency you plan to transmit. As an example, if you are transmitting a frequency of 100 MHz, then the rise time is 3.5 ns. The rule of thumb indicates that if the trace length is longer than 7 inches then the signal should be treated as a transmission line.
So, you want to transmit information
In the age of the internet of things, everyone wants to communicate wirelessly. Fortunately, there are rules that govern how things are allowed to use the electromagnetic spectrum. I say fortunately, because if there were no rules then the airwaves would be chaotic and noone would effectively be able to communicate. The flipside of this argument is that if you want to design a new widget you will have to meet certain requirements set forth by the governing agency, in this case the FCC. Meeting, or demonstrating that you are meeting, these requirements can be costly and time consuming.
Many people don’t have the resources to get qualified. Others say they need to get to market fast, presumably before they run out of money. You might wonder if there is a way to get around such regulations. The answer is both yes and no. One way to get around them is to use a radio that has already been certified to meet the FCC regulations. Then if you use that radio in your design, your design automatically meets the FCC regulations, because they manufacturer has gone through the time and expense of meeting those requirements. The downside is that manufacturing costs go up.
The cost of a Bluetooth radio that is certified is any ware from $25-$45. If you build it yourself, it will cost you under $10 for the same thing. Is that unfair? Why would they charge so much for something so cheap? The reality is it costs the manufacturer a lot of money to develop a product (before you consider manufacturing costs). The development costs include the time to design and debug a product and to meet the governmental regulations and the testing cost to demonstrate compliance. Those costs get rolled into the cost of the item they offer for sale.
How is it done?
There are many ways to run a transmission line. These ways are referred to as topologies. Some popular ones are microstrip (two-layer board with a ground plane on the bottom and a trace on the top). The coplanar waveguide with or without a ground (this is when you have a ground plane on either side of your trace, you may or may not have a ground plane below these on the bottom of the board), see Figure 1. One last topology is the symmetric and asymmetric stripline (were your trace is equally or unequally spaced between two ground planes).
Figure 1Coplaner waveguide with ground. Green is the PCB, orange is copper. Width of the trace as well as the gap between the trace and the ground can be manipulated.
There are a host of online calculators that will help you design your transmission line. Most of the parameters will be fixed, for example the geometry of your board and the material will determine many of the distances to the ground plane and the dielectric of your material. This usually brings you down to just a few parameters to play with, the chief one being the trace width of the transmission line, and the gap between the trace and any adjoining ground planes.
Proper termination
There are lots of very complicated geometries possible, but if you stick with the basics, you will be successful. No matter how careful you are, you will not get the impedance matched exactly. When I say matched, you will need to look at the characteristics of the antenna and the transmitter, but usually you are shooting for a characteristic impedance of 50 ohms. This minimizes reflections and maximizes power being transmitted (efficiency). So, what is an engineer to do? You will want to have a series L and a parallel C available near your antenna so that you can hand tune the board when it is made.
Remember to use RF L’s and C’s, low frequency parts will not work. You will want to buy a small kit most likely. Then you will place a particular RC combo, make a measurement, and record it. You are looking for the combination that maximizes your transmission power. We all kid about electronics being part black magic, but RF stuff is the darkest of these magics.
It is not as bad as it sounds. Follow some simple guidelines, be methodical, keep it simple and you will most likely do just fine.
Final thoughts
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