1. The concept of Bluetooth
Bluetooth (Bluetooth) technology is a way of wireless communication, using a specific frequency band (2.4GHz~2.4835GHz) for electromagnetic wave transmission. The Bluetooth transmission principle is a master-slave relationship, and a master device can be paired with 7 slave devices.
2. Bluetooth channel introduction
Bluetooth (Bluetooth) works in the 2.4G ISM (Industrial, Science and Medicine) frequency band, specifically 2400-2483.5MHz, with a channel bandwidth of 1MHz and 79 channels. The first channel starts from 2.402; the channel frequency is 2402+K, K ranges from 0-78, a total of 79 channels. You can find that there is a section of protection bandwidth in front of and behind the frequency band, which are 2.4~2.4019G and 2.481~2.4835M respectively.
Channel Regions by Country
Frequency Range RF Channels
USA, Europe and rest of the world 2.4 to 2.4835 GHz f=2.402+n, MHz (n=0 to 78)
Japan 2.471 to 2.497 GHz f=2.473+n, MHz (n=0 to 22)
Spain 2.445 to 2.475 GHz f=2.449+n, MHz (n=0 to 22)
France 2.4465 to 2.4835 GHz f=2.454+n, MHz (n=0 to 22)
3. Bluetooth power introduction
Bluetooth power is divided into 3 levels:
power level power limit coverage distance
Class 1: 0dBm~20dBm; 100 meters
Class2: -6dBm~4dBm; 10 meters
Class3: less than 0dBm; 1 meter
we usually use Class2, and the power is generally around 0dBm. But it is not a standard value, the power is between Class1-2.
4. Bluetooth transmission mode
vco launch mode. Let Bluetooth transmit an unmodulated radio frequency signal at a fixed frequency point.
continues mode. Let Bluetooth continuously transmit modulation signals at a fixed frequency point.
The burst mode allows Bluetooth to transmit the modulated signal of the corresponding packet type at a certain frequency point or frequency hopping.
5. The packet type transmitted by Bluetooth
Classic Bluetooth transmitter package types:
DH1, DH3, DH5, 2DH1, 2DH3, 2DH5, 3DH1, 3DH3, 3DH5.
Classified by modulation method:
DH1, DH3, DH5 belong to the packet type of BDR, and the modulation method is GFSK; 2DH1, 2DH3, 2DH5 belong to the packet type of EDR, and the modulation method is π/4-DQPSK; 3DH1, 3DH3, 3DH5 belong to the packet of EDR Type, the modulation method is 8DPSK.
BLE Bluetooth transmission package type: generally BLE_1M, BLE_2M:
DH1, DH3, DH5, 2DH1, 2DH3, 2DH5, 3DH1, 3DH3, 3DH5 should be selected when doing BQB certification as a classic Bluetooth fixed-frequency test.
When doing BLE certification, the fixed frequency test should be selected: BLE_1M, BLE_2M;
6. Bluetooth version
Bluetooth version Release time Maximum transmission speed
Bluetooth 1.0 1998 723.1Kbps
Bluetooth 1.1 2002 810Kbps
Bluetooth 1.2 2003 1Mbps Bluetooth
2.0+EDR 2004 2.1Mbps Bluetooth 2.1+EDR 2007
3Mbps Bluetooth
3.0+HS 2009 24Mbps
Bluetooth 4.0 2010 24Mbps
Bluetooth 4.1
2013 24Mbps Bluetooth 4.2 2014 24Mbps
Bluetooth 5.0 2016 48Mbps
Bluetooth 5.1 2019 48Mbps
Bluetooth 5.2 2020 48Mbps
7. Bluetooth transmission principle
The master Bluetooth device sends electromagnetic waves with a wavelength of about 121~124mm 1 million times per second. 121mm is defined as 1; Switch back and forth between the Bluetooth slave device (Bluetooth headset/speaker) and itself, so as to achieve the transmission of information. In order to achieve stable high-speed transmission in a complex environment, the Bluetooth electromagnetic wave is divided into 79 different channels. At any time, our Bluetooth master device can communicate wirelessly with the slave device, and the connected channel is one of the 79 channels. one. If there are multiple mobile phones and headphones/speakers in the same space, and they use the same channel at the same time, how can our device only receive the binary information sent by the connected mobile phone? Don't know how to cross the stage? In order to avoid this problem, this long string of binary information will be packaged into a data packet. In each data packet, the first 72 bits are the access code for synchronizing the mobile phone and the headset, so that the occurrence of channel crossing can be avoided. After these 72 bits are 54 bits of information, which provide the details of the information being sent and become the header; there are 500 bits at the end, which represent the real content of the sent information; when the mobile phone communicates with the Bluetooth headset/audio, they will not only Stay on one channel, but switch at high speed on these 79 channels. This kind of switching is called frequency hopping, spread spectrum, and the switching speed is 1600 times per second; Send directly to the earphone/speaker; if the channel where the interference is great, it will automatically jump to other channels until the interference disappears;
1. The first 72 digits: represent the access code for synchronizing mobile phones and earphones: it can be understood as the address for express delivery;
2, 54 digits in the middle: the representative provides the details of the delivery information, which can be understood as the size and color of the courier, etc.
3, the last 500 digits: represent the specific content of the sent message, such as a song, a picture, etc.;
8. Bluetooth indicator parameters
Specifications for wireless transmission
1. SBC is the most basic universal decoding method. It is supported by Bluetooth headsets, supports 44khz/16bit audio, the highest bit rate is 328kbps, and the delay is >220ms, so the sound quality is average.
2. AAC is a common decoding method for Apple products. It is similar to sbc. It supports 44khz/16bit audio, the highest bit rate is 512kbps, and the delay is about 120ms. The sound quality is slightly better than sbc.
3. aptx supports 48khz/16bit audio, the highest bit rate is 352kbps, and the delay is about 70ms. The sound quality is better than sbc, but the improvement is not much compared with sbc. 4. aptx hd is an enhanced high-bit rate version of aptx, which supports 48khz/24bit audio, the highest bit rate is 576kbps, and the delay is >200ms.
5. aptx_LL is the low-latency version of aptx, which supports 48khz/16bit audio, the highest bit rate is 352kbps, and the delay is <40ms. The sound quality is similar to aptx.
6. ldac is a real high-quality audio decoding product produced by Sony. It supports 96khz/24bit audio and has a maximum bit rate of 990kbps. It is known as lossless decoding.
7. lhac or HWA has been upgraded to L2HC. It is also lossless decoding, which is mainly promoted by Huawei, supports 96khz/24bit audio, and the highest bit rate is 900kbps, which is comparable to ldac, but currently supports relatively few devices, mainly supported by Huawei mobile phones. Although AirPods uses AAC, it adopts a dual-channel mode, which increases the utilization rate by 50%. The actual effect is basically the same as that of most aptX earphones.
Antenna type
PCB antenna: extremely low cost, simple debugging, single frequency band. Ceramic antenna: It can be divided into block ceramic antenna and chip ceramic antenna, which are characterized by small size, low cost and good performance. FPC: low cost, good performance, complex debugging, a relatively common antenna. LDS: It is an evolutionary version of FPC, with high cost, high space utilization, and good effect. Generally, many TWS earphones with a price of 400 yuan use LDS antennas.
9. RF parameters of the antenna
1. Output power: Test the Bluetooth output power by conduction. The headphone/audio output power is generally around 6dBm. The output power varies according to different chips, but the difference is not big;
2. Sensitivity: Test the sensitivity of Bluetooth by conduction. Generally, the sensitivity of RF debugging is better at about -90dBm;
3. Antenna efficiency: test the antenna efficiency. Install the whole machine, and test it in the dark room in passive mode. Generally, the antenna efficiency is 50% which is a good effect;
4. OTA data: The theoretical value of OTA's TRP (dBm) and TIS (dBm) is consistent with the power and sensitivity tested by the conduction method, and the closer they are, the better the effect;
5. Standing wave ratio: The full name is voltage standing wave ratio, also known as VSWR and SWR, which is the abbreviation of Voltage Standing Wave Ratio in English. Refers to the ratio of the antinode voltage of the transmission line to the valley voltage amplitude, also known as standing wave coefficient and standing wave ratio. When the standing wave ratio is equal to 1, it means that the impedance of the feeder and the antenna are completely matched. At this time, all high-frequency energy is radiated by the antenna, and there is no energy reflection loss; when the standing wave ratio is infinite, it means total reflection, and the energy is not radiated at all.
6. Return loss (Return Loss):
Return loss is the ratio of reflected wave power to incident wave power (Return Loss=-10lg [(reflected power)/(incident power)]), the absolute value of this ratio is higher The bigger the antenna, the better. The goal of impedance matching is generally to control the absolute value of the return loss to be greater than 10dB. When the return loss is equal to 10dB, it means that 90% of the incident power is loaded to the load, and only 10% of the incident power is Reflection; for example, 10% of the reflected power and 1% of the reflected power are brought into the formula respectively, and their absolute values can be obtained as 10dB and 20dB respectively; the higher the decision value, the smaller the reflected power, the smaller the return loss, and the better the antenna effect. the better;
7. Smith chart (Smith chart)
Smith chart is a chart used in electronic engineering, mainly used for impedance matching of transmission lines. The impedance of a transmission line will change with its physical length. To design a circuit with impedance matching, many complicated calculation procedures are required. The characteristic of the Smith chart is that some calculation procedures are omitted. The antenna impedance matching of the Bluetooth antenna is 50Ω, so we need to adjust it to close to 50Ω; for example, the three points in the figure below are the middle, high and low frequency bands of Bluetooth, and when the three points are close to the center point, it means that the antenna impedance is close to 50Ω;