CC2650 Development Board DIY Part 2 - RF Antenna

1. CC2650 is a SoC that combines RF technology. In many technical solutions, the MCU and RF parts are separated. For CC2650, the RF part is integrated into the chip, which is actually a dual-core, one is the M3 that is open to the outside world, and the other is the M0 chip that specializes in processing RF, but it is not open to the outside world and cannot be accessed by the program.

RF design requires excellent professional technology, test tools, and application experience. It is really a mission impossible for us, but fortunately TI provides many technical solutions and detailed documentation.

There are three types of antennas: PCB antennas, separate antennas, and special antenna chips. Separate wires have good signals, but they take up space; there is a special on-board antenna chip that is very small, about the same size as a crystal oscillator, suitable for more compact designs; the more common use of PCB antennas is done at the same time as the plate making, which is the most economical.

2. CC2650 works in the 2.4GHz ISM band, so all antennas suitable for 2.4GHz can be used in the design. After comparison, the inverted F-type antenna should be the most suitable part for this design.

2.1 The PCB antenna design requires a minimum space of 15.2 x 5.7 mm to achieve 50 ohm impedance matching. See the figure below for details.

When using a PCB antenna in a design, it must be drawn as shown in this figure. Remember, it is a must. As for why, I don't understand. It should be that a slight change has a great impact on the spatial radiation field and transmission effect of the RF, mainly in the 2.4GHz efficient transmission range, the bandwidth is relatively narrow, and if it causes too much deviation from the stable transmission power range, it will seriously affect its performance. The figure below shows a distribution curve of the transmission power under a comparison situation, and you can see that the steepness is very large.

The inverted F antenna is not evenly distributed in the plane space, but is a double dumbbell-shaped distribution space. This space is relatively uniform, and there is no particularly obvious low-emission area at the waist. In other words, it is understandable that the emission energy and direction are still closely related. If it is completely uniform, then this will not be greatly affected.


2.2 About the grounding layer.
There should be no grounding layer on the back of the corresponding PCB antenna. The existence of the grounding layer seriously changes the spatial distribution and emission characteristics of the magnetic field, which should also be copied.
In addition, the thickness of the PCB base layer also has an impact on RF, but compared with the grounding layer, the impact is limited, so a more detailed comparison is generally not made.

2.3 Fine-tuning of antenna design.
In TI's documents, it is suggested that in certain cases, such as installation in a box, the antenna length can be fine-tuned to ensure stable and optimal RF performance. The adjustable length is the L1 part in the above figure. Shortening it can increase the transmission frequency, and increasing it can increase the transmission frequency.

3. Others.
3.1 The onboard antenna needs to pass through a load balancing circuit at the RF output end of the MCU to ensure that the input impedance and antenna impedance match. Some manufacturers have launched dedicated chips (baluns) to achieve this matching function, which is more convenient to use. In the DIY process, this part of the reference design needs to be copied.

3.2 Since the Bluetooth signal transmission distance is short, if you want to enhance the signal and increase the transmission distance, TI has another good option, which is to go through an RF transceiver amplification circuit. This is integrated in CC2590. Therefore, if necessary, you can add another CC2590 to enhance the signal transceiver capability. It is said that the transmission distance can be extended to 100 meters in an open area or the solid wall of a 5-story office.

3.3 Although so much analysis has been done, there is actually only one conclusion, that is, if there is a reference design, then it cannot be modified. It must be copied, must be copied, must be copied. It is important to say it three times. I have said so much here because I know why I cannot play freely and share it with you.

CC2650 development board DIY Part 1 - One thing you need to consider before designing: "How to debug"