I have followed quite a few RF MCU based designs from Texas Instruments like CC3200, CC2650 etc and in all of them I did not have to worry about the trace impedance between the MCU pins and the antenna.
Their application notes mentioned that as long as the antenna is not far away from the MCU, there is no need to bother with impedance matching.
However now I am designing a RF (2.4GHz) based MCU from NXP, MKW21 to be precise. If you look at the hardware design guidelines, they mention that the trace impedance should be 50 Ω (page 6ff). Here is the pdf link .
Now, my design is space constrained, so the antenna will be kept as close as possible to the MCU. In this case do I still need to monitor the impedance of the track between the tuning circuit and the antenna ?
Best Answer
Short answer: Yes, you should.
Long answer: For digital signals, it's really true that signals on traces don't behave like waves much if the trace length is sufficiently shorter than the wave length. Now, a) at 2.4 GHz, \$\lambda = \frac cf \approx \frac{3\cdot10^8\frac{\text m}{\text s}}{2.4\cdot 10^9\,\text{Hz}}\approx 12.5\,\text{cm}\$, so the "\$\frac1{10}\lambda\$" rule of thumb might already be broken, and b):
An antenna is nothing but an impedance matching circuit between the impedance of the transmission line and free space. If you don't match your transmission line to both the antenna and the IC, you will have reflections on the interfaces between antenna and trace, and trace and IC, and hence, lose RX power, and hence, lose signal quality (in TX this typically isn't that critical, but still relevant).
Now, a short piece of mismatched will a) be pretty much inevitable and b) will not kill you, but really: you're only doing one RF signal on your board, most probably, so simply do yourself a favor and match that as good as possible. It's really not that hard – there's hundreds of stripline calculators on the internet, KiCAD integrates one, too, and you just have to enter your frequency, your board thickness, and your board's substrate relative permittivity (which you can look up, too – FR4 has 4.7, IIRC), and you'll get a trace width spit out.
For more info on matching lines, see Maxim's tutorial/app note on that topic.