Commonly used wireless modules for the Internet of Things
Simplified Measurement Method of Receive Sensitivity and Transmit Power
1. Overview of common wireless standard test requirements
At present, the commonly used wireless modules in the IoT industry are mainly Wifi, Bluetooth, 433M, LORA, NB-lot, etc. The above descriptions are not rigorous descriptions, and 433M itself is not a standard or protocol.
In terms of system and frequency band, Wifi generally works at 2.4GHz or 5.8GHz, the modulation format is generally OFDM, and Bluetooth generally works at 2.4GHz, and the modulation format is generally GFSK , PSK, π/4- DQPSK , 8 DPSK . Wifi and Bluetooth technologies themselves are designed for audio-visual consumer electronics. They are characterized by high transmission rates and short distances. However, because both Wifi and Bluetooth functions are currently available on smartphones, IoT products that use Wifi and Bluetooth as means of communication are There are many applications in the field of smart home. The Internet of Things products using these two standards generally do not have too high requirements on the communication distance, and can cover the space of 10m*10m in the general family. Because the requirements for the communication distance are not very high, but the The test requirements for transmitting power and receiving sensitivity are not very large, and because of the complex modulation methods of Wifi and Bluetooth, there is no low-cost test solution on the market at present.
In wireless meter reading, industrial sites, etc. where communication distance is required and power consumption is expected to be as low as possible, 433M is widely used. 433M is not a rigorous expression. MHz ~350 MHz, 420 MHz ~525 MHz, 850 MHz ~1050 MHz, but the frequency bands near 433MHz and 460MHz are most used in China. It should be reminded that 150~160MHz is the maritime frequency band. If there are rivers, seas and lakes nearby , the frequency is easy to conflict with the local maritime department, causing unnecessary trouble, it is best to avoid this paragraph. In the 433 frequency band, or the sub-GHz frequency band, the communication system is generally simple OOK, 2(G)FSK, 4(G)FSK, etc. The equipment using the sub-GHz frequency band is generally industrial and commercial equipment. is a key indicator, so RF power and receiving sensitivity are key indicators. At present, Agilent and R&S are the mainstream in the market, and the test cost is still relatively high.
The newly developed sub-GHz frequency band equipment begins to use the new LORA system. The frequency band is the same as the previous frequency band. Due to the special spread spectrum modulation method, the communication distance has been greatly improved. The instruments used by Agilent and R&S also use the waveform files provided by Semtech, so high-end vector signal sources must be used to test, so the test cost is higher.
As for the latest NB-lot technology, this technology is completely different from the above technologies. It is more like an upgrade of traditional GPRS. Compared with GPRS, it reduces power consumption and optimizes the usage scenarios of the Internet of Things. The larger the communication distance depends on the coverage of the operator's base station, and the current hardware of NB-lot is also the most complex. Most IoT product companies will not manufacture modules by themselves, and will basically use simcom, Quectel, etc. Large module manufacturers purchase finished modules, and the measurement of module indicators is only a matter for module manufacturers to consider, and has nothing to do with most IoT companies.
Wifi and Bluetooth can be untested because of their short distances. NB-lot technology is more important in terms of distance from near and far base stations, so it is not necessary to test. For sub-GHz OOK, FSK, and LORA, which have the greatest test requirements, how to test?
2. General test method for sensitivity and power
Let’s first look at the definitions of RF power and receiver sensitivity:
Definition of power: The maximum output power refers to the average power that the transmitter supplies to the transmission line in one RF cycle.
Definition of sensitivity: Under the specified test frequency and modulation mode, when the receiver bit error rate is less than or equal to the specified value, the input signal power of the receiver antenna port.
Figure 1 Using a spectrum analyzer, a power meter, and a comprehensive tester with a power meter function to measure the signal power
Conventional power tests generally use a power meter or spectrum analyzer. The advantage of power meter measurement is accuracy, especially in other RF application scenarios, such as high power (greater than 100W) measurement, standing wave ratio measurement, etc. It is more convenient and accurate. However, the power meter can only measure the continuous wave, and the pulse wave can only measure its peak value or average value, which cannot fully reflect the real transmit power level. Therefore, the power meter cannot be used for OOK modulation.
Spectrum analyzer is the most commonly used RF measurement instrument. By adjusting the appropriate bandwidth (RBW, VBW) and trigger mode, it can measure the power of various adjustment methods. In addition, it can also observe the signal quality, such as spurious, harmonics, etc. Richer information, but the spectrum analyzer is suitable for research and development, it is not convenient to apply on the production line, and it is not suitable for automated testing (not to expand the discussion).
Figure 2 Spectrum analyzers are also commonly used to analyze signal quality
The comprehensive tester with power meter function is more suitable for use on the production line, and the programming and reading are more intuitive, which is convenient for the production line to use with automated testing software. Of course, even without automation software, it is very simple for workers to operate.
The sub-GHz frequency band commonly used in the IoT industry is relatively low, and the test can be programmed into continuous wave mode. The detector chip for this kind of measurement is relatively mature, and a detector can be designed for power measurement. The difficulty lies in the detector's performance. Calibration is cumbersome.
For the definition and measurement method of receiving sensitivity, we will study it again according to the script:
Overview
Under the specified test frequency and modulation mode, when the bit error rate of the receiver is less than or equal to the specified value, the input signal power of the receiver antenna port.
Measurement methods
The useful signal generator should be a signal generator that can generate a specified signal, and can output a data sequence for comparison by the error evaluation equipment. The equipment under test should provide a demodulation output data interface.
The test steps are as follows:
a) Set the receiver receiving frequency as the test frequency, turn on the useful signal generator 1, and set the signal generator to output the standard test signal according to the selected test frequency
b) When the output power of the useful signal generator is adjusted so that the bit error rate of the receiver is less than or equal to 5×10-2, note down the input signal power of the antenna port of the receiver at this time;
c) The power recorded in step b) is the static sensitivity, expressed in dBm
Figure 3 Schematic block diagram of sensitivity test
Generally, the data to be sent by the signal source is preset and known by us, so the judgment of the bit error rate is also made by the processor in the receiver. Generally, the data received and preset by the microcontroller on the receiving module Compare the known data of the tester, and send the results to the PC through the serial port (network port) for display. The tester judges the error situation according to the results displayed on the PC. When it is not strict, the operation will be simplified. The situation is represented by LED flashing, the correct data received will flash, and the data will not flash.
The key to sensitive testing is the signal source, generally a signal source with modulation function, and a vector signal source to be more flexible. The advantages of the vector signal generator are high precision, the most flexible, and various modulation information and parameters of the signal source can be changed through programming revision, such as simple shaping filter parameters, code rate, code pattern, and even can simulate multipath effects, multiple Puller effect, simulate the most realistic communication situation, find the optimal reception method through various settings. The disadvantage is that it is expensive and complicated to operate, so it is not suitable for production line applications.
Figure 4 Sensitivity test using vector signal source
Figure 4 Set the vector signal source
3. Sensitivity bottom cost simplifies the test method
For the Internet of Things industry, Agilent (Keysight) also provides a low-cost solution. This solution is lower in cost, but it is quite troublesome. Generally, those who are not familiar with it need to be adjusted several times to get it right, especially if there is no other signal source to do it. When comparing, it is not clear whether it is adjusted properly or not. That is to say, after you set up the test environment, the signal comes out. You may not know whether the signal you come out is the signal you want. For example, you want to generate a FSK signal with a frequency offset of 1k, but the actual signal generated is a frequency offset of 1k. Or 0.5k frequency offset? It is not easy to guarantee, because the modulated baseband is an analog signal, and the final signal is related to several parameter settings. It is not an engineer who is familiar with the use of the instrument, and it is easy to make mistakes. Of course, if there is a signal analyzer (spectrum analyzer or comprehensive tester with analyzer function) to analyze the generated signal, we will know whether it is the signal we want, but this process is also somewhat troublesome, and with Signal analyzers are also not cheap.
Of course, if the LORA format is used, this solution is not supported.
Of course, the most direct is to choose the instrument designed for this application, and use the wireless comprehensive tester.
Figure 5 Agilent's low-cost solution
I have also used the above scheme to build a test environment. Of course, the signal source used is not exactly the same as the above, but using this method, after repeatedly adjusting the parameters and comparing with standard instruments, the sensitivity test error can be less than 2dbm. The detailed test data is not expanded. If you are interested, please contact me for discussion.
Figure 6 Comprehensive tester for IoT wireless testing
The simplest and low-cost solution is to use a dedicated comprehensive tester, which will be simpler to operate and lower in cost, and its performance is a little worse than that of a vector signal source. Measuring power is very accurate because measuring power is relatively easy.
Figure 7 Comprehensive tester to test the transmit power
RF transmit power test
Operation method:
1. Connect the device under test to the RF2 port of the comprehensive test instrument through a coaxial cable
2. Menu selection power test interface
3. Make the device under test in the transmitting state (with modulation)
4. The comprehensive tester reads out the test results
--There is a programming interface on the back of the comprehensive tester, which supports automated testing of production lines
Figure 8 Comprehensive tester to test the receiving sensitivity
Receive Sensitivity Test
Operation method:
1. Connect the device under test to the RF1 port of the comprehensive test instrument through a coaxial cable
2. Menu selection sensitivity test interface, set signal frequency, modulation parameters, data source
3. First set a strong power value (for example: -50dbm) to ensure that the device under test can receive data and indicate it, and gradually reduce the power value until the device under test cannot receive data correctly.
4. The comprehensive tester reads out the test results (the minimum signal power value that enables the device to be tested to receive normally)
Note 1: The device to be tested needs to be programmed in advance, and an indication will be issued when valid data is received (it can be a simple indicator light flashing, or the bit error rate result can be printed out)
--There is a programming interface on the back of the comprehensive tester, which supports automated testing of production lines
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