Design of WSN node communication module based on CC2531+CC2591

introduction

With the needs of society, wireless sensor networks (WSN) are increasingly widely used in real life due to their advantages of low complexity, low cost, low power consumption, and multiple network nodes. In particular, the emergence of some system-on-chip (SoC) has greatly reduced the difficulty of developing wireless sensor networks. When using these SoCs to develop wireless sensor networks, the transmission power of the wireless communication part of the SoC itself is very small (such as the transmission power of CC2531 is only 4.5 dBm), and its receiving sensitivity is also fixed at a certain level, which limits the communication distance of the wireless sensor network. The common transmission distance ranges from several hundred meters. In some specific application environments, the distance between network nodes is required to be more than one kilometer or even farther.

This paper introduces a wireless sensor network node design based on the core chip CC2531, and uses TI's 2.4 GHz RF front-end chip CC2591 to increase the transmission power of the wireless communication part and further improve its receiving sensitivity, thereby extending the communication distance.

1 Core chip introduction

1.1 CC2531

CC2531 is a system-on-chip (SoC) solution for IEEE 802.15.4, ZigBee and RF4CE applications with USB function launched by TI. It can build a powerful network node with a very low total material cost. CC2531 integrates a 2.4 G. Hz RF transceiver, an enhanced industrial standard 8051 MCU, 256 KB Flash programmable in the system, 8 KB RAM and many other powerful features; the transmit power is 4.5 dBm (adjustable), and the receiver sensitivity is -97 dBm.

1.2 CC2591

CC2591 is a highly integrated RF front-end chip launched by TI that works at 2.4 GHz and is aimed at low-power and low-voltage wireless applications. The gain of the internal integrated power amplifier (PA) of CC2591 is 22 dB, the maximum transmit power is +22 dBm (input +5 dBm), the output 1 dB compression point is +19 dBm, and the internal integrated LNA of the receiving part has high and low receiving gains of 11 dBm and 1 dBm respectively, the noise figure is 4.8 dB, and the receiving sensitivity is improved by 6 dB.

2 Overall design

According to the design requirements, the system hardware structure block diagram is shown in Figure 1. The core chip CC2531 combined with its peripheral circuits (such as crystal oscillator, A/D reference voltage, memory, sensor and debugging interface, etc.), plus the necessary power module and RF front-end chip CC2591, constitute the hardware system of this solution. When applied to different fields, the corresponding sensors, power supplies, A/D reference voltage, memory, etc. can be adjusted accordingly. The hardware design of CC2531 and CC2591 is relatively fixed.

3 Performance parameter budget

3.1 Transmit Power Budget

According to the data sheets of CC2531 and CC2591, the maximum transmit power of CC2531RF port is +4.5 dBm. Modifying the value of TXPOWER register can adjust its transmit power, ranging from -22 to +4.5 dBm. After connecting CC2591, the maximum PA gain of CC2591 in transmit mode is 22 dB, and the corresponding transmit power range is +22 to 10 dBm (the maximum value is determined by PA itself, and the minimum value can be smaller). Considering the 1 dBm compression point (19 dBm) of PA and system power consumption, setting TX-POWER = 0xD5, that is, the output power of CC2531 is 1 dBm, and the transmit power of CC2591 is 19 dBm, which is an ideal high-power output reference setting (for reference only, there may be changes in practice).

3.2 Receiver Sensitivity Estimation

When CC2591 is in high-gain receiving mode, HGM=1, the noise factor NF from the external antenna to the internal T/R selector to the internal LNA is 4.8 dB, the signal-to-noise ratio SNR of the resolvable signal inside the back-end CC2531 is 3 dB (ensuring the bit error rate is at a certain level), and the single-channel transmit and receive bandwidth BW can be set to 1 MHz or 5 MHz. According to the formula, when the normal room temperature T0=290 K, the noise power of 1 Hz bandwidth is N0=-174 dBm, and the receiving sensitivity S=-174 dBm+NF+SNR+10log(BW).

Substituting the parameters into the formula, we can obtain: when the receiving bandwidth is 5 MHz, the receiving sensitivity is S = -99.2 dBm.

3.3 Communication distance estimation

In the real world, when communicating between any two points, the loss caused by the environment to the transmission wave is generally difficult to quantify, and varies greatly depending on the environment. Without considering external influences and transmission losses, and under the condition that electromagnetic waves propagate under ideal conditions, the calculation formula for wireless communication transmission distance is as follows:

Loss=32．44+20lgd+201gFreq

Antenna gain is not considered for now (it needs to be determined based on the actual purchased antenna parameters). The link loss budget Loss is 118.2 dBm (excluding non-ideal loss), and the frequency Freq is calculated based on the RF center frequency of CC2531, 2450 MHz, and the wireless communication transmission distance d=7.93 km can be obtained. This distance cannot be achieved in actual applications, and this value is for reference only. [page]

4 Wireless Communication Module Principle

The main peripheral circuits of CC2531 mentioned in the overall block diagram are not introduced here. The connection circuit of CC2531 and CC2591 is shown in Figure 2. It includes the matching link of the RF differential signal line between chips, the matching circuit of the RF signal to the antenna, the CC2591 control signal line, and the decoupling filter part of the power supply.

Among them, the decoupling filter part of the power supply line, B1 is a magnetic bead, and the frequency range of its effective filtering should be noted when selecting it; at the same time, other components should also be selected for high-frequency capacitors, inductors, etc. The matching circuit part of the RF signal to the antenna, the antenna connection port uses an SMA interface (female head), which can make the access of the spectrum analyzer and vector network analyzer more convenient when used for circuit measurement, and can also connect antennas with corresponding SMA connectors. For the three enable controls of CC2591, the allocation is as follows: P0.1→HGM_EN, P1.1→PA_EN, P1.4→LNA_EN. Among them, P0.1 that controls HGM_EN can be replaced by any GPIO, and the other two cannot be changed. They should be mapped to the internal interface and register of the system protocol stack respectively.

5 PCB circuit implementation

Compared with the design of the schematic diagram, more and more practical problems will be encountered in the actual design and production of PCB circuit boards. According to the schematic diagram given in Figure 2, the wiring of the RF signal line is designed. Figure 3 shows the RF differential routing between chips, and Figure 4 shows the RF routing of CC2591 connecting the antenna. Figures 3 and 4 mainly show the wiring design of the RF routing part, using the most common island wiring in RF. Because its operating frequency is 2.4GHz and the maximum power reaches 20 dBm, there are not many reflections in the schematic diagram, but in the PCB, due to unreasonable wiring, there may be large reflections and energy accumulation in some places, leading to design and production failures. Therefore, all RF signal lines are designed to be straight as much as possible.

The antenna connection part uses an SMA connector. In Figure 4, the part connecting component B8 and the SMA connector is a 50 Ω microstrip line. Impedance control must be performed in actual production. Finally, the surface copper ground is designed to ensure good grounding of the RF signal.

During the actual production process of the PCB design, since parameters such as the dielectric constant and dielectric height cannot be set ideally as expected, there will inevitably be deviations, so it is necessary to always pay attention to the changes in parameters and make timely adjustments.

Set single channel transmission, CC2531 internal TXPOWER = 0xD5 (typical transmission power +1 dBm), and measure the power to be +19.21 dBm when the center frequency is 2.401 GHz. Although affected by other factors, this result is larger than the expected output power, but it is acceptable. The results show that the transmission of the RF link part in this system is relatively ideal, and the reflection and loss in the actual circuit are controlled within the acceptable range of engineering. The corresponding receiving link can also predict that its reflection and loss are acceptable.

Conclusion

According to actual requirements, wireless sensor network nodes were designed and produced to increase the transmission power to extend the transmission distance. From the measured data, it can be seen that the addition of the power amplifier unit has greatly improved the node's transmission power. Due to the limitations of equipment and measurement conditions, some other parameters were not measured. This is an area to be improved in the future. The improvement of the circuit is also one of the focuses of future work.