From wireless ergonomic keyboards to hands-free headsets, Bluetooth technology has fundamentally changed the way we work and play. And what’s even better is that we no longer have to waste precious time untangling priorities, and all Bluetooth technologies can instantly pair with any Bluetooth-enabled device without lengthy procedures.

There’s just one small problem, sometimes these Bluetooth devices can interfere with other technologies. In this article, we’ll take a deep dive into why this happens and offer some suggestions to keep all your favorite Bluetooth devices in good shape.

To communicate between devices, Bluetooth sends signals over the 2.4GHz radio frequency, as shown in the figure below, the Bluetooth communication channels (frequencies) are distributed between 2402MHz and 2480MHz:

In theory, if the devices are designed properly, there should be relatively little interference between Bluetooth technologies because their signals are relatively weak and cover a short distance. In addition, most Bluetooth technologies today use so-called spread spectrum frequency hopping technology. That is, they rotate between 70 randomly selected frequencies in the frequency range shown above, changing 1,600 times per second. This makes it unlikely that two devices will share the same frequency. Even if they do, it won't last long. Some Bluetooth technologies also use so-called AFH technology, which can identify "bad" channels (i.e. channels that are already in use) and trigger switching.

In addition to interference between Bluetooth technologies, more wireless interference problems may come from technologies that use the same frequency. Wi-Fi may be the largest and most problematic example, as are other wireless Zigee, Thread technology devices, etc. They may interfere with each other. As shown in the following figure, the channel (frequency) distribution of several major 2.4GHz communication technologies:

As can be seen from the above figure, their operating frequency bands do overlap. The signal in a 20MHz Wi-Fi channel may interfere with the signals in 10 Bluetooth BR/EDR channels. Wi-Fi devices usually work on a fixed channel, and devices working on the same channel will use the channel in a competitive manner. The Wi-Fi signal in a Wi-Fi channel may be intermittent or always occupied. Bluetooth uses frequency hopping technology to jump from one channel to another at a frequency of 1600 times per second according to an agreed random sequence. Bluetooth devices send and receive data in a channel for a fixed time (625us), and then jump to another channel. At this time, all Bluetooth channels may be used. Obviously, from the technical implementation of Wi-Fi and Bluetooth, adjacent Wi-Fi and Bluetooth devices may have overlapping signals, that is, interference may occur.

Because Bluetooth uses frequency hopping technology, its signal will not stay in a Wi-Fi channel for a long time, so the mutual influence here will not last for a long time.

In daily Bluetooth use, if you have been interfered with, it is recommended to eliminate all physical obstacles first. Certain building materials can block weaker signals, such as metal, bulletproof glass, concrete, and so on. Therefore, if you really have trouble dealing with interference, the first step should be to keep your Bluetooth device away from these materials, and absolutely no metal blocking it, which will help improve the quality of Bluetooth communication. 

Secondly, if your Bluetooth is constantly being interfered with by wifi, try restarting your router. After restarting, the router will search for new channels, so that it is on a different channel from your Bluetooth device. Of course, some routers may require you to actively enter the router settings page and try to manually change the channel and verify to see which works best.

Of course, if you are a more in-depth wireless system developer who needs to use wireless products such as Bluetooth modules to build your devices and systems, you need to consider more factors. When laying out the overall module, the Bluetooth module should be kept as far away from DDR, HDMI, USB, LCD circuits, speakers and other easily interfered modules or connectors as possible; when laying out the Bluetooth module, priority should be given to avoiding crystal circuits, staying away from interference sources, and the crystal routing should be as short as possible, and the clock signal needs to be processed throughout the whole process. When laying out the power supply of the Bluetooth module, please note that after the power supply line comes out of the inductor, it first passes through the capacitor and then enters the power pin of the module. The first layer below the module should maintain a complete ground without other signal lines. If the components are really dense, you can use a metal shielding cover, metal cover or metal shielding sheet around the Bluetooth module to strengthen the electromagnetic shielding and reduce the incidence and radiation of interference. Finally, consider the antenna layout of the Bluetooth module. The antenna matching circuit must be close to the antenna base. The antenna line is 50 ohms (it can be used as a reference for the interlayer according to the actual stacking situation) to ensure the integrity of the reference ground. No other signal lines or power supplies are allowed below. If the antenna is placed outside the board, the three sides of the PCB onboard antenna should be kept clear of more than 5mm to reduce the impact on the RF performance. When the antenna is placed inside the board, the bottom and surrounding of the PCB onboard antenna should be hollowed out by more than 5mm to avoid interference from other components to the antenna.