EEVblog Electronics Community Forum
Electronics => RF, Microwave, Ham Radio => Topic started by: mik4el on August 13, 2018, 07:49:21 am
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I'm working on improving a PCB using the CC1350 dual band MCU for 868 MHz and BLE. I got the chip working but the RF part is performing very bad. It is usable for testing but has very bad performance which requires a redesign. A big reason (I hope the main reason) for the bad performance is that I've used excessively long transmission lines that weren't properly impedance matched.
The RF part on the PCB from the CC1350 consists of first a differential pair (common for sub-GHz and BLE) that goes into an DC bias circuit, then there is an RF switch to switch between sub-GHz and BLE for the antenna path that first goes into a balun and then 50 ohm into the antenna. I think I have the part from the RF switch and forward figured out now with regards to impedance matching and baluns. However from the CC1350 into the DC bias circuit the differential pair is stated by TI in the datasheet (http://www.ti.com/lit/ds/symlink/cc1350.pdf, (http://www.ti.com/lit/ds/symlink/cc1350.pdf,) p51) "On the Texas Instruments CC1350_7XD-Dual Band reference design, the optimal differential impedance
seen from the RF pins into the balun and filter and antenna is 44 + j15."
Therefore my question is: How do I impendace match the transmission line from the CC1350 from RF pins and into the baluns to 44 + j15? Do I need to if I keep the traces really short? All the impedance calculators I find only state the characteristic impedance Z0. I've attached my previous calculations for the 50 ohm transmission lines.
20180814: Changed to DC bias circuit
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You can either use websites such as these: https://home.sandiego.edu/~ekim/e194rfs01/jwmatcher/matcher2.html (https://home.sandiego.edu/~ekim/e194rfs01/jwmatcher/matcher2.html) (most common configurations) or you can impedance match using a software like SimSmith, which is really a handy tool and it's freeware! You can download it at http://www.ae6ty.com/smith_charts.html (http://www.ae6ty.com/smith_charts.html)
You can find tutorials about SimSmith at:
https://www.youtube.com/user/ae6ty (https://www.youtube.com/user/ae6ty)
https://www.youtube.com/channel/UCKSyLSu4fm_1RHoO3Jvk4YQ (https://www.youtube.com/channel/UCKSyLSu4fm_1RHoO3Jvk4YQ)
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Thanks for your comment. I guess I could use an impedance matching circuit as you suggest but I was more after what my transmission line for the 44+15j impedance should look like as a cpwg etc?
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Found the answer myself at https://e2e.ti.com/support/wireless_connectivity/proprietary_sub_1_ghz_simpliciti/f/156/t/717102?jktype=e2e but posting it here for anyone interested.
The traces in question are so short that they don't need to be considered transmission lines and one should instead pick the pcb trace width from the width of the 0402 components.