Lora is a kind of LPWAN communication technology that is an ultra-remote distance wireless transmission scheme based on spread spectrum techniques using and promoted by Semtech. This program has changed its compromise on transmission distance and power consumption, providing users with a simple system that achieves long-distance, long battery life, and large-capacity systems, and extended sensing networks.
Currently, LORA is mainly operated in a global free frequency band, including 433, 868, 915 MHz.
LORA technology has long distance, low power (battery life), multi-node, low cost characteristics.
The LORA network is mainly composed of a terminal (can built-in LORA module), gateway (or known base station), Server, and clouds. Application data can be transmitted two-way.
With the continuous heating of LORA technology, plus its unique and superior transmission performance, the population of LORA technology is explosing, because large part of the group is first contact, in the process of doing products In the middle, it is usually encountered with a tricky RF circuit design problem. In fact, as long as you master a few points, you can basically play the best performance of LORA.
Key point one, matching circuit design
When the schematic design is designed, a π-type matching circuit needs to be reserved between the antenna connector and the module's antenna pin. The impedance of the antenna is affected by factors such as the laying of the circuit board, the outer casing and mounting angle, and the π-type matching circuit is reserved to correct it to 50 ohms when the antenna is severely deviated from 50 ohms. By default, the antenna impedance is relatively close to 50 ohms, and the C17 and C18 in the figure below are not soldered; while L2 uses 220PF capacitance, or 1 nH inductance, or 0 euros, three can be. When encountering special circumstances, such as antenna mounting mold inside, the volume of the antenna is small or requires strengthening high-hard harmonic suppression, etc., these three matching elements need to match the matching adjustment.
In theory, it can be matched to 50 ohms from the π-type matching circuit whether the antenna impedance is in any value. However, the actual inductor capacitor is internal resistance, this internal resistance will absorb energy, if the antenna impedance is too small (several ohm) or big (on thousands), the matching circuit matches 50 ohms to lose meaning . The reason is that most of the energy is consumed on the internal resistance of the matching element.
Tone 2, microstrip line line rules
The microstrip lines mentioned here refers to the PCB trace between the antenna pin of the LORA module to the antenna connector. The following figure is a microstrip line example on the LORA module ZM470SX-M evaluation board. Since the internal impedance of the module is designed with a 50 ohm standard, it is also 50 ohms when the microstrip line characteristic impedance is also 50 ohms. Best match.
In order to get a microstrip line of 50 ohms, on the one hand, it can improve the impedance processing requirements to the PCB manufacturer. The ability PCB manufacturers can control the line impedance according to the line parameters; on the other hand, they can obtain sheets from the PCB manufacturer. After the parameter (the main is the dielectric constant), the line is calculated by software, thereby controlling the impedance in our expectations.
According to experience, when the sheet material (between 4.2 to 4.6) is used in the line, the characteristic impedance is close to 50 ohms when the line is wide as the microstrip line to the reference layer distance. For example, when using double-panels, the plate thickness is 0.8 mm, the wire width is 1.7 mm. However, it must be noted that the laying below the microstrip line must be complete, the microstrip line and the copper spacing are set depending on the impedance computing software. The floor pads on both sides of the module antenna pin must be well grounded.
Since the wavelength of 470 MHz electromagnetic waves is longer, if the microstrip line length does not exceed 20mm, the trailing feature impedance is not much affected by the performance of 25 ~ 75 ohms, and it is recommended to use a 25mil line width in this case.
To point three, PCB spread requirements
We have encountered a lot of this situation: Users use our modules to use the product, using the same configuration parameters as our evaluation board, and use the antenna on our evaluation board, the communication effect is obviously better than our evaluation board A lot of difference. There are three key parameters related to the transmission power, receive sensitivity, and antenna associated with the communication distance, where the front two parameters have been tested in our module, unqualified products are treated by waste, and the antenna is Due to different users' design. The distance affecting the communication is mainly the parameters of the antenna, and the other two parameters will not change significantly due to the different users' boards.
In air, the electromagnetic wave wavelength of 470 MHz is 63 cm, and the antenna is at least half wavelength, i.e., 31.5 cm if the standard half-wave dipolar antenna is designed. In practical applications, most products do not have such a large space to the antenna design, so spring antennas are commonly employed. When using this spring antenna, the antenna is connected to different boards, and its performance parameters are different. This is because of this type of antenna, spring is only part of the entire antenna, and the other is the ground on the board. Therefore, it has to be passionate when the circuit is paved, and the general principle is: First, it is necessary to make the antenna vertical The board is mounted, and the second is to make the copper blocks of the ground as large and continuous, and relied on dense via holes to make the positive opposite sides continuously.
To point four, antenna installation specification
After all hardware parameters are adjusted, the installation of the antenna is also a key step. The antenna radiation is directional, not the energy of each direction radiation, like us, the sound is strong, some directions Weak. When installing antenna, you need to align the antenna of the antenna radiation to the received antenna. The received antenna may get the strongest reception signal. To do this, you must first know the radiation direction of the antenna. In the case of a professional antenna test laboratory such as a dark room, how do you test the radiation direction of the antenna? We can continue to send data continuously after finalizing the product, and test the signal strength of a certain distance from the product with spectrum analyzer. Rotate the product being tested and record the signal strength in each direction, so that the antenna radiation of the product can be plotted.
When installing reflective surfaces such as walls, doors, and metal, it is necessary to pay attention to the impact of reflection, the theory and test results have proven the following conclusions:
Best location:
Worst location:
The best and the worst position is alternate from λ / 4;
RX1 is better than RX2, for example 470 MHz, the effect of 16 cm from the reflective surface is excellent in 32 cm.
Original link: https://www.eeboard.com/news/lora/
Search for the panel network, pay attention, daily update development board, intelligent hardware, open source hardware, activity and other information can make you master. Recommended attention!
[WeChat scanning picture can be directly paid]
Technology early know:
Two horses, encounter pig teammates: iPhone 8 is equipped with Intel baseband performance, Qualcomm
Interference suppression scheme of several effective switching power supplies (EMI)
QLC arrival, or will end large capacity SSD high price!
What makes Amazon so angry?
Brand new small computer makes Arduino to equip Windows10
Our other product:
