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CC1350: Reference design for antenna and RF circuit

Maneesh EE
Prodigy 10 points
Part Number: CC1350
Other Parts Discussed in Thread: CC-ANTENNA-DK2, CC110L, CC115L, CC1310

I am designing a circuit with CC1350, with 868MHz as the operating frequency. There are a lot of good application / design notes and antenna selection guide, which I have gone though. Before I can make a choice, I need some help.

The preferred antenna is the helical loop antenna, which is used in CC1350 Launch Pad (swrc320a), and in CC115L/CC110L Value Line Reference Design (swrr082a). It is documented in DN038 (swra416) and CC-Antenna-DK2 (swra496a).

Questions:

1. The reference RF differential circuit between the CC1350 and the antenna matching circuit is available in CC115L/CC110L Reference Design schematic, and in Section 3.1 of the Application Note "CC13xx/CC26xx Hardware Configuration and PCB Design Considerations" (swra640g). These are not identical. So, is either circuit okay to use? Or is one of them more advisable? Is this reference circuit same 868 MHz operation with these devices - CC1310, CC110, CC115, CC1350?

2. Assuming that helical antenna reference given in CC115L/CC110L Value Line Reference Design (swrr082a) is the correct choice, then is it okay to reduce the width of the board from 40mm to 22mm (slightly higher than the width of the helical antenna)? The idea is to keep design small. The ground plane will still have the same length. Is this approach okay?

3. The documentation says in some places that 4-layer PCB is preferred for 868MHz antenna designs for sub-GHz devices. However, the gerber files for the CC115L/CC110L Value Line Reference Design (swrr082a) only have two layers, and the board is 0.8mm thick. Is this is a good reference to follow for CC1350 PCB design?

4. The CC1350 Launch Pad is 4-layer and 1.6mm thick. If this launchpad is a better design to refer to, then, is it okay to keep the ground plane to about 25x25mm? If the ground plane is reduced in area, then could you please give a suggestion for approximate increase in the design length of the helical antenna (so that it can be cut to size after the board is back)?

5. Is it completely impossible or not advisable to design the helical antenna in a 2-layer and 1.6mm board? If so, please advise on the reference design to follow.

Thank you!

Regards, Maneesh

  • 0
    Mastermind 6470 points

    Some questions/ comments:

    - Why do you want to use CC1350 if you are only doing a design on 868 MHz? It sounds like CC1310 is a better selection. If you are going to use a single frequency design, follow the ref designs for CC1310. 

    - The helix antenna you are looking for is not ideal. The reason is that the vias connecting the top and bottom layer in the helix are part of the total antenna length. The variation in the PCB thickness will then cause a variation of the antenna length and hence the resonance frequency.

    - It is the area of the ground plane that matters. A smaller groundplane will give lower efficiency and lower bandwidth. 

    - PCB antennas are typically a 1 or 2 layer design. 2 layers are to reduce the loss. 

    - For the CC13x0 part, a 4 layer design is better. With a 2 layer design the radiated 3. harmonic will be higher than for a 4 layer design. The most important is to keep the distance between the signal layer and the ground layer close to what is used on the ref design. The overall board thickness is not that important to copy.   

  • 0 in reply to TheGhostOf
    Prodigy 10 points

    - Yes, agreed, CC1310 is better selection for 868MHz only design. This project has 2.4GHz only variant also. The quantities are such that we need to use same device for both.

    - The helix antenna is the second best recommendation in the DN035 antenna selection guide. It is quite confusing if TI is recommending something that is not likely to work well, or work at all.

    - Yes, agreed, the size of ground plane is important. But I am looking to understand how to make the layout compact, and quantify the reduction in efficiency and bandwidth.

    Both, the CC115L/CC110L Value Line Reference Design (swrr082a) and the CC1350 Launchpad, use the helix antenna. So, I was trying to understand how to reduce the width of the board as the design does not need so much extra space.

    To summarize, the goal is to know the best starting point for a 868MHz antenna design that will help make the smallest possible PCB with reasonable performance.

  • 0 in reply to Maneesh EE
    TI__Mastermind 33330 points

    Hi,

    The PCB helix antenna is still a good antenna and is still recommended but there is just a precaution that this antenna design is sensitive to PCB thickness changes. If you are using one PCB manufacturer for your boards, then this is normally not a problem. If you are using more than one manufacturer, then it is always good to double check the nominal thickness does not deviate too much.