IoT testing from baseband to RF (1)

The Internet of Things (IoT) has been a hot word for several years, and the development of smart home products has also begun to take off vigorously at home and abroad. Whether it is the Internet of Things or smart home, in addition to promoting the development of technologies such as sensors, the most critical point is how to achieve "connection", or more precisely, "wireless connection". In order to achieve the "wireless connection" required for these applications, there are many types of short-range wireless communication technologies and new standards related to RFID, NFC, WiFi, BlueTooth, ZigBee, Z-Wave, etc., which are emerging in an endless stream. We have collected and summarized the application fields and characteristics of some of the main wireless connection technologies today, and briefly outlined them below:

 

l ZigBee is a low-speed, short-distance wireless network protocol, which consists of the physical layer (PHY), media access control layer (MAC), transport layer (TL), network layer (NWK), and application layer (APL) from bottom to top. The physical layer and media access control layer follow the IEEE 802.15.4 standard. This is an economical, efficient, low data rate (<250kbps) wireless technology that works at 2.4GHz (global) and 868 (Europe)/915MHz (North America). The number of channels and channel bandwidths in different frequency bands are also different; the modulation technology is also different: the 868MHz and 915MHz frequency bands use BPSK modulation technology, and the 2.4GHz frequency band uses OQPSK modulation technology.

 

l Other standards similar to ZigBee include Z-wave, ANT, EnOcean, etc., which are incompatible with each other. Z-wave has a strong position in smart home, and mainstream manufacturers have joined this camp, which is difficult to shake. It has low cost, low power consumption, high reliability, and short-range wireless communication technology suitable for the network. The working frequency band is 908.42MHz (USA) and 868.42MHz (Europe), using FSK (BFSK/GFSK) modulation, and the data transmission rate is 9.6 kbps and 40 kbps. If you want to build a smart home, it seems that you have to follow the "rules" of this industry?

 

l EnOcean is the only wireless international standard in the world that uses energy harvesting technology. By harvesting energy from the surrounding environment, such as mechanical energy, indoor light energy, temperature difference energy, etc., these energies are processed and supplied to the EnOcean ultra-low power wireless communication module, realizing a truly data cable-free, power cable-free, and battery-free communication system. Compared with similar technologies, it has the lowest power consumption and the longest transmission distance, and can be networked and support relaying. EnOcean operates in the following frequency bands: 868 MHz, 315 MHz, and 902 MHz. It uses ASK modulation technology. Each radio signal occupies the channel for 1 millisecond and has a transmission rate of 125KB/s.

 

In the fields of energy and industrial control, there are also standards such as Wi-SUN and WirelessHART.

 

There are many kinds of standards, and different regulations are set for several layers from the physical layer to the 7-layer protocol. High-level tests can be performed through relevant protocol analyzers or price-sensitive users can test through software. Here we focus on the physical layer tests. Even for the physical layer of wireless connections, these different standards use different frequencies. The frequencies they commonly use are 315/433/868/915MHz, 2.4GHz and even 5.8GHz. They use different modulation methods, such as ASK, FSK, OQPSK, etc. Of course, the baseband processing is also different. Next, let's talk about the IoT test from baseband to RF.

 

RF transceiver modules with different frequencies plus baseband processing are the main components of this type of product, and have been widely used in these fields, such as: wireless alarm, wireless meter reading, security system, industrial monitoring and control, smart wearables, smart home, smart logistics, smart parking, remote control, toys and other IoT applications. In China, there are also many manufacturers that develop and produce such products. The following takes the 2.4GHz frequency band, which has become increasingly popular in recent years, as an example to explain the test methods from baseband to RF for such products.

 

RF transceiver modules are indispensable in these products. Companies such as TI and NORDIC provide a wide range of RF transceiver chips, such as TI's CC2520 and NORDIC's nRF24L01, which are well-known chips widely used in wireless transceiver modules. These chips can be easily combined with MCU or FPGA to form various products that meet different applications. Its main features are as follows:

l 2.4GHz global open ISM band license-free use

l The working speed is adjustable, the highest working speed is about 2Mbps

l Adopt FSK, MSK, GFSK and other modulations, with strong anti-interference ability, especially suitable for industrial control occasions

l Support multiple channels, some with more than 100 channels, to meet the needs of multi-point communication and frequency hopping communication

l Built-in hardware CRC error detection and point-to-multipoint communication address control

l Output power can be controlled programmably

l High receiving sensitivity, up to -80dBm or even lower

l Complete data exchange through interfaces such as SPI, including data sending and data receiving.

 

 

The internal structure diagram of this type of chip is shown in Figure 1:

Figure 1: Schematic diagram of the internal structure of the transceiver chip (from TI data sheet)

 

 

Figure 2 shows a 2.4GHz wireless transceiver module based on TI and NORDIC chips. Next, we will discuss the testing methods for this type of product.

 

 

Figure 2: Common 2.4GHz wireless transceiver modules

 

For the research and development and production testing of such products, the following measuring instruments are usually required:

 

l Spectrum analyzer, to measure and analyze the spectrum of the transmitted signal, such as DSA832 or DSA875;

l RF signal source with digital modulation function, which can simulate the signal with GFSK modulation and test the receiving performance of the module, such as DSG3030-IQ or DSG3060-IQ;

l Four-channel digital oscilloscope, used to test SPI bus and baseband signals, such as DS/MSO4000 series;

l DC power supply provides DC power supply, such as DP800 series

 

The setup for measuring the transceiver performance of this type of product is shown in Figure 3. If all these things are gathered together, we can start micro-testing this type of product from baseband to RF, from digital to analog.

 

 

 

Figure 3: Setup for measuring the module’s transmit and receive performance

 

The products of this type of application cannot do without RF transceiver modules. The RF transceiver module and the MCU or FPGA usually use the SPI bus to configure, control, and transmit the transmitted or received data. We can use a digital oscilloscope with SPI bus trigger and decoding functions, such as DS4054 or MSO4054, to test the SPI bus to verify whether the actual communication signal is correct. The timing of the read and write operations defined by the SPI specification is as follows:

 

 

Figure 4: SPI bus read operation

 

 

Figure 5: SPI bus write operation

 

 

Next, we will test a product using DS4054. First, set the trigger condition of the SPI bus on the oscilloscope. If your oscilloscope does not have this special trigger function, it will be difficult. You can set it to trigger when the SPI MOSI or MISO is transmitting a specific data, for example, the trigger is triggered when 00010111 is transmitted. If it does not appear, there is no display on the screen, and the oscilloscope is in the "waiting for trigger" state. Once it appears, the display shown in Figure 6 will appear:

 

 

Figure 6: Setting up the SPI bus trigger on the DS4054

 

Once the data set in the trigger condition appears on the SPI bus, the DS4054 will capture it. Not only can you see the waveform display, but you also need to know the specific content of the transmitted data. Engineers who are familiar with the frame structure can count and "decode" by themselves, which is a laborious task. It is better to use the automatic decoding function of the oscilloscope. Through the SPI decoding function, the specific content of each frame can be automatically displayed in different base formats. By directly looking at the results, you can judge whether the transmitted data is wrong. Once an error occurs, you can analyze whether it is a software error or an error caused by signal distortion or interference. Figure 7 shows the specific content of the frame through the automatic decoding function of the SPI bus.

 

 

Figure 7: DS4054 decoding settings and display of SPI bus

 

 

Through the SPI bus, we can obtain the data sent and received by the entire product and then analyze it.