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cc2541-pcb design

sheetal verma1
Intellectual 600 points
Other Parts Discussed in Thread: CC2541

We are using FR4 material of dielectric 4.6 and thickness 0.8mm. The copper thickness for both planes is 0.035mm. We need to understand the following issues before we can finalize the PCB design:

1. Impedance matching line between Balun and Antenna: The width and length of the track from the Balun to the Antenna. The TI design has a width of 0.3mm while the online calculators give a value of 1.41mm for an impedance of 50 ohm. Also will the length have an effect on the performance due to a phase shift
2. Resistance in path of Transmission Line: Utility of R251 and also how to calculate the impedance with it in place
3. Clearance between Transmission line and other Nets
4. can one of the crystals be on the bottom plane while the CC2541 is on the top plane
5. Max distance of decoupling capacitors from CC2541
6.Is RF_N and RF_P required to be routed as differential pair, if so then here Is it drawn correctly?

Please find attached documents details :-

Reference design - tidc156a.zip and tidr203a.pdf

Thermometer Design - BLE_V2_Top.pdf,BLE_V2.pdf and BLE_V2_Bottom.pdf


kphcs.zipBLE_V2_Top.PDF

  • 0
    TI__Mastermind 18780 points

    HI, 

    1) The balun and antenna are both matched for 50 ohm, so the transmission line between the balun output and antenna should be 50 ohm. Note however that if the length of the line between the balun and antenna components is less than wavelength/10, it will not act as a transmission line and the width is not as important. 

    2) the resistor in our reference design R251 is together with the two shunt cap C251 and C252 placeholders for potensial antenna matching components.In our ref design this is 0 ohm. The antenna performance and resonance will be affected of several factors like size of the ground plane, plastic etc. and needs tuning. We recommend to add placeholders for antenna matching for potensial tuning. More information regarding this is found for instant in the Antenna selection guide,

    Also the antenna producer might share more information of antenna performance and potensial need for tuning,.

    3)Other external components like sensors etc should be placed as far away as possible from the antenna  as possible, but we do not have a general rule on min distance. Note that the distance to the GND plane on the top layer from the transmission line affects the impedance and should be taken into consideration when calculating the width of the transmission line (it is a parameter you add when using the transmission line calculators). 

    4) Yes, but keep the distance short. and also check that the freq of the xtal is correct afterwards, and potensially adjuist the value of the external xtal load capacitors. 

    5) the decoupling capacitors should be placed as close as possible to the pin it is meant to decouple. The optimal order for the decoupling should be VDD to decoupling cap to pin. The decoupling cap gnd should have separate vias to the GND plane below and there should be no routing between the chip GND vias and these vias. There should be a complete GND plane underneath the chip, radio part (balun/filter, transmission line to antenna, antenna matching etc) and deocupling cap GND vias. All RF components connected to GND should have separate GND vias to the GND layer below close. 

    6) It is very important to place the balun close to the RF pins and keep the routing symmetrical. I recommend that the balun in your design is moved closer to the chip and routing kept more symmetrical. 

     

    Based on this feedback, I recommend that you do more changes for your design, (move decoupling caps, power routing, balun, add more antenna matching comp if possible, more GND vias).