IoT Industry Trends and Design Challenges
With the development of modern sensor technology and wireless communication technology, the Internet of Things has begun to enter people’s daily life. IoT applications represented by technologies such as RFID, ZigBee technology and NFC near field communication are becoming the direction of research and development and innovation for many enterprises and universities. Although semiconductor manufacturers offer a variety of specialized chips and even highly integrated solutions for these technologies, engineers still face many challenges when designing an actual IoT device. One of the most important factors is how to measure time-dependent time- and frequency-domain signals in the system. Although the RF signal applied in RFID and ZigBee technology is not very complicated, the quality, power and timing relationship of the signal determine whether the system can work normally. And these RF parameters themselves are not only related to the RF transmit/receive circuit, but also affected by the baseband circuit and the control circuit. The reading and writing of internal registers, the working conditions of the power supply, and even the size of the system delay time will determine the working state of the entire system. Traditional oscilloscopes or spectrum analyzers cannot complete this time-correlated time-domain and frequency-domain signal synthesis debugging.
Innovative Design Concept of MDO Mixed Domain Oscilloscope
The unique innovative concept of Tektronix MDO4000 series mixed domain oscilloscope provides a unique tool for debugging cross-domain time-frequency related systems. On the basis of a full-featured mixed-signal oscilloscope, MDO4000 adds a 3GHz or 6GHz spectrum analyzer, which can complete various frequency domain measurement functions of ordinary spectrum analyzers. The completely independent oscilloscope time-domain acquisition system and spectrum analyzer frequency-domain acquisition system can work independently or work together through triggering. By moving the spectrum time, the user can observe the spectrum of the RF signal collected at any point in the RF channel within the time window collected by the oscilloscope. MDO also provides modulation-domain analysis of RF signal amplitude, frequency, and phase versus time. These exclusive functions help users measure various modulation information of RF signals. A problem often faced by engineers working with spectrum analyzers is how to accurately trigger on and capture the RF signals of interest. Due to the limited triggering capabilities of traditional spectrum analyzers, it is difficult for users to do so. MDO4000 can not only trigger through various characteristics of RF signals, but also use the trigger system of the oscilloscope to complete the trigger acquisition of RF signals through baseband or control signals. This function greatly reduces the difficulty of modulating IoT devices.
When debugging an RFID system, an important difficulty an engineer faces is how to measure the return signal from the tag. Due to the small amplitude of the signal returned by the tag, it is often difficult to capture this signal with a normal oscilloscope, let alone further analyze its amplitude and frequency. The main reason is that the dynamic range of ordinary oscilloscopes is only 40dB, which cannot capture weak label signals. With a dynamic range of 60dB and a noise floor as low as -152dB/Hz, the MDO4000 is well suited for the task of capturing both reader and tag signals simultaneously. Its unique time domain waveform function of AvsT RF signal amplitude can even Display the change process of label signal amplitude.
Let’s take a 13.56MHz RFID reader system as an example to introduce the cross-domain debugging application of MDO4000.
Application of MDO Mixed Domain Oscilloscope in RIFD System Development
Figure 1 RFID reader using NXPCLRC632 chip
Test RF signal quality parameters of 13.56MHz RFID reader
The 13.56MHz high-frequency RFID system is currently the most widely used and mature RFID system in China. Relevant international standards have clear requirements for parameters such as RF transmission frequency, channel bandwidth, and transmission power, especially when the amplitude (power) of the RF signal changes with time, the standards have strict regulations. Taking the reading and writing equipment as an example, the amplitude change time of the carrier signal sent by the reading and writing equipment must meet the time limit of t1-t4 in the ISO18000-3 standard.
Figure 2 ISO18000-3 13.56MHz RFID air interface time parameter specification
By using the unique trigger function of the MDO4000, users can easily and stably capture RFID time and frequency domain signals. As shown in the figure, it is difficult to measure the length of time for the RF signal to drop from 90% to 5% of T1 using traditional means due to the changing amplitude of the carrier signal. We can turn on the AvsT modulation curve, which represents the trajectory of the RF signal’s amplitude versus time. With automatic measurement or manual cursor measurement, we can easily get the exact time of T1. Similarly, tests of other time parameters can be completed.
Figure 3 Time domain and AvsT modulation domain waveforms of 13.56MHz RFID PCD to PICC signal
Figure 4 Measure the delay time between the PCD’s transmitted signal and the tag’s return signal
Test PCD to PICC read and write time
Another time that needs to be strictly guaranteed is the time from the reader to send the card reading signal to the time that the tag returns the signal. Too long or too short time will be regarded as read and write failure. Measuring these signals with traditional instruments is difficult. The MDO4000 can Display the AvsT trace of the RF signal on the screen, and the user only needs to use the cursor to locate the corresponding position, and then the delay time can be obtained.
Figure 5 Time domain waveform, modulation domain waveform and spectrum display of 13.56MHz RFID RF signal
RFID systems using ASK modulation transmit data information through subcarriers. In the spectrum part of the above figure, we can clearly see that the carrier of the RF signal is 13.56MHz, and the sub-carrier signal is about ±800KHz. Meet the requirements of relevant regulations. If you need to measure the RF parameters of the RF signal, such as channel power, adjacent channel power ratio or occupied bandwidth, etc., you can directly display these measurement results on the screen by selecting the automatic measurement function of the MDO4000.
If the designer wants to understand the data situation that the RFID system transmits, MDO4000 can also provide strong support. The MDO4000 can provide IQ data for RF signals. After importing these data into Tektronix’ RSAVu software, the decoding of RFID data and the calculation of RF index can be completed. As shown in the figure below, use the RSAVu software to read the .TIQ data provided by the MDO4000. The software can calculate the amplitude and time domain waveform of the RF signal, and calculate the parameters such as EVM, modulation depth, modulation coefficient, frequency deviation, and code rate. And the data represented by these RF signals can be decoded and displayed. Simplifies the debugging difficulty of designers.
Figure 6 RSAVu automatic test and decoding function
System-level debugging and analysis capabilities of MDO
An RFID reader is a complex RF embedded system that includes a baseband microcontroller, RF transmit and receive modules, and power and control buses. The state of the baseband control signal and the internal register of the system directly affects the working state of the system. Taking the reader we tested as an example, the NXP CLRC632 read-write control chip includes a voltage-controlled oscillator, a phase-lock circuit, encoding, decoding, frequency mixing, and transmit/receive functions. The work of the chip is controlled by the single-chip STC 90c58RD+.
Test the timing relationship between system control signals and TX and RX signals
Figure 7 Time domain relationship between Rx signal and RF signal
The relevant pins of the NXP CLPC632 RF chip can measure the control signal of the RF transmission, as shown in the CH2 blue waveform shown in the figure above, we can measure these control signals, the time domain waveform of the RF signal, and the AvsT waveform of the RF signal at the same time, In this way, we can easily observe the effect of various control commands on the RF emission.
Figure 8 Capture register status data through SPI bus
The communication between the single-chip microcomputer chip and the read-write control chip is through the SPI bus. The actual working function of the read-write control chip is managed by changing the value of the internal register. For example, the register at address 14 is the codercontrol register, which controls the encoding clock and mode. When the value of the third to fifth bits of the register is 000, the encoding rate is 848KB, when the value is 011, it is a typical ISO1443A encoding standard, the code rate is 106KB, and the value is 100. ISO1443 TYPE B encoding rate. In this debugging practice, if we find that the frequency of the spectrum subcarrier signal is inconsistent with the transmission code rate we designed, we can capture the SPI bus data of the corresponding address and check the value of the corresponding register to determine the cause of such a problem. Bits 0-2 of the Codercontrol register control the encoding form of the transmitted data. If there is a problem that data communication cannot be established during design debugging, you can check the values of these three bits to check whether the actual coding form is correct. “000” represents the NRZ non-return-to-zero encoding of ISO14443-B, “001” represents the Miller encoding of ISO14443A, and “110” and “111” represent the corresponding encoding forms of the ISO15693 standard.
Summarize
The unique time-correlated spanning analysis function of the MDO4000 mixed signal oscilloscope provides a powerful tool for the development and debugging of IoT devices represented by RFID. Using MDO4000 can not only easily measure the analog waveform, spectrum condition and various frequency domain parameters of the signal, but also easily verify whether the product meets the requirements of international and industry standards through modulation domain traces such as AvsT, FvsT and ΦvsT. More importantly, since the analog signals, digital signals and bus signals are related to the radio frequency signal in time, we can not only verify the actual working process of the system through the timing relationship of these signals, but also verify the actual working process of the system by comparing the bus signals and register data. analysis to find out the cause of the failure. The MDO4000 is currently the only test instrument on the market that can provide such functionality. We hope that MDO4000 can accelerate the design of IoT products and contribute to the development of the entire industry.
Main indicators of MDO4000 mixed domain oscilloscope:
Typical configuration:
MDO4054-3 mixed signal oscilloscope (if designing a 2.4G RFID system, it is recommended to choose MDO4104-6)
DPO4EMBD bus analysis module (I2C+SPI bus)
MDO4TRG Advanced RF Trigger Module
Special label antenna tooling
Tektronix Near Field Probe Kit (Model: 119414600, 100KHz-1GHz)
BNC cable
BNC Tee Adapter
The Links: VI-J61-CY LTM10C209A
