Testing methods and solutions for the sensitivity of ultra-high frequency RFID tags

　　Ultra-high frequency tags refer to passive RFID tags from 840M to 960MHz. The labels for this band originate from the EPCglobal Class 1 Generation 2 standard. Among them, EPCglobal is the organization for electronic product coding standards, and the first-class, second-generation RFID standards are often abbreviated as C1G2. This standard specifies the radio frequency identification protocol in the ultra-high frequency range of 860M-960MHz. This protocol features microsecond-level reader-to-tag response and a scientific collision prevention mechanism to enable fast tag reading and writing over tens of meters. Ideally, it can count two to three hundred labels per second, with a reading distance of about 30 meters, once highly regarded as the standard for next-generation intelligent logistics. Later, the ISO organization accepted this standard and transitioned it to ISO 18000-6C. In recent years, China has also innovated in this technology, launching its own standard GB/T 29768, which specifies frequencies at 840-845MHz and 920M-925MHz, avoiding the adjacent GSM service bands.

　　Currently, these protocols are collectively referred to as 800-900MHz ultra-high frequency RFID (UHF). These protocols all inherit features such as high-speed response, rapid inventory, and long read/write distances. The performance of these popular protocol products is key to their use. Among them, labels are at the center of fierce competition. RFID tags have a relatively low unit price but are used in large quantities, which places higher demands on design and manufacturing. Due to defects and instability in label design technology and production processes, performance testing is essential for control.

　　However, since this tag sensitivity test involves non-contact RF measurement, various technical challenges need to be overcome. This article focuses on introducing the theoretical methods and practical aspects of these methods.

　　Sensitivity testing method for ultra-high frequency RF tags

　　Basic setup

　　UHF tag testing is often conducted in microwave anechoic chambers or darkrooms, but can also be conducted in semi-darkrooms or field sites with less interference. However, because UHF tags have relatively high frequencies and wavelengths of only about 1/3 meter, the requirements for anechoic chamber size are not very high, making them economically easier to bear. Regarding the physical setup of label testing, there are two main methods: dual-antenna and single-antenna. For maximum performance, EPCglobal and ISO advocate the dual-antenna method. This method uses a pair of left-right circular polarization antennas, one transmitting and one receiving, achieving maximum transmission and reception isolation, allowing the test system to transmit at high power and receive with high sensitivity, thus handling labels with lower sensitivity. For convenience, a looper is also used to combine dual antennas into a single antenna configuration with transmit/receive duplex. Due to antenna reflection characteristics, the overall system performance is lower than that of dual-antenna configurations.

　　Figure 1 Schematic diagram of dual-antenna tag test configuration

　　Indicates the unit

　　Label sensitivity is usually expressed in terms of power or field strength. EPCglobal is more practical, using RIPTUT, which is the unipolar radiation power received by the tag. Simply put, it is the RF field strength that the tag can operate at is the power received by an ideal monopole antenna. Its unit is dBm.

　　ISO testing field strength is expressed as the minimum field strength required for the label to function properly. Its unit is V/m.

　　These two test results may look different, but in fact, both are calculated using the tester's transmitted power.

　　EPCglobal tag receives monopole power calculation formula:

　　RIP=EIRP-PL Formula 1

　　EIRP = P + GTx Formula 2

　　Where EIRP is the instrument's equivalent monopole radiation power (dBm), PL is the free-space transmission loss from the antenna transmitting to the tag (dB), P is the input power of the transmitting antenna (dBm), and GTx is the gain of the transmitting antenna (dB).

　　Here, PRx is the received power, PTx is the generated power, Ae is the antenna's equivalent aperture area, and R is the distance between the transmitting and receiving antennas. This formula describes the relationship between far-field transmission loss and distance between ideal monopole antennas. Below, we present several typical sample frequency points with free-space transmission loss at typical test distances, measured in dB.

　　Note that the above calculations are based on the far-field spherical wave model. If the transmit/receive distance is too short, the calculation results will deviate. EPCglobal specifies a distance of 0.8-1 meter. ISO 18046-3 specifies the nearest test distance.

　　Here, R is the test distance, and L is the maximum side length (diameter) of the transmitting antenna. Below, we provide ISO requirements for test distances at typical antenna sizes and frequencies.

　　Multiple test items

　　Forward connection distance

　　In tag sensitivity tests, people often hear questions about tag read/write distance. Read/write distance is related to tag sensitivity and reflection power, but in practical applications, it also relates to the performance of the reader. Therefore, in testing, it is assumed that the reader/writer transmits at 35dBm power through an ideal monopole antenna, achieving a read/write distance. So here's the question: the ultra-high frequency tag has a very long read/write distance. Should we equip an ultra-large RF chamber? SenseTech is not. We measure the minimum operating power of the tag under the above far-field conditions, subtract the transmitting antenna gain, and obtain the equivalent monopole radiation power EIRPTX. Then, based on the principle that spatial transmission attenuation is proportional to the square of the distance, we can estimate the read/write distance:

　　Forward link range, also known as read distance, depends on the field strength required for tag activation.

　　Reverse connection distance

　　The magnitude of the tag reflected power determines how far the reader can read, so the reverse link range can be estimated from the tag reflection power. The reverse connection distance is the distance at which the reflected power is read by a reader with antenna gain of 5dBil and reception sensitivity of -70dBm. The EPCglobal standard [2] provides a calculation method, and the results are usually greater than the forward connection distance.

　　Here, EIRPTx0 is the transmitting equivalent monopole power required for reverse connection sensitivity, defined as forward connection sensitivity plus 2dB; PRx0 is the tag reflection power received under EIRPTx0 transmission conditions; GRx is the gain of the receiving antenna.

　　Sensitivity of different tag operating modes

　　The power consumption required for tags varies in the operating modes of reading ID numbers, reading register information, and writing register information, meaning the sensitivity of these three modes is different. This results in three testing modes: recognition, read, and write sensitivity. The above minimum operating power, minimum field strength, forward and reverse read distances all have indicators under these three operating modes, and each differs.

　　EIRP and ERP

　　Among many standards, equivalent monopole transmission power is more common, but ERP is also used. In the State Grid Corporation standard released in 2013, ERP refers to the transmitting power of an equivalent dipole antenna. The ideal dipole antenna gain is about 2.2, so the difference between the two is just one constant.

　　Parameter examples

　　We assume both the transmitting and receiving antennas have gains of 6dBi, the test distance is 1 meter, the tag antenna gain is 2dB, and the tag reflection loss is 5dB. When the instrument transmits at 915MHz and the power is PTx, the tag receives the power.

　　PTag=PTx+6-31.7+2=PTx-23.7

　　Formula 11

　　Assuming the tag's reflected power is one-third of the received power, about -5dB. The power received by the tester receiver is as follows:

　　PRx=PTag-5+2-31.7+6= PTag-28.7

　　Formula 12 Calculates the power received by the chip and receiver for different transmission powers according to these two formulas:

　　In other words, under ideal conditions, the reflected power of the ultra-high frequency tag received from a 1-meter distance is about 62dB less than the transmitted power. Currently, the best labels can reach an opening power of around -18dBm, so the tag signal received by the tester generally has a power above -47.4dBm. In practice, due to the tag antenna design, its gain is less than 2 or attenuation caused by impedance matching, resulting in a tag reflection ratio of -5dB. Taking these factors into account, assuming the impact does not exceed 10dB, the received power is above -60dBm.

　　Therefore, RFID tag sensitivity testing does not require the instrument to have extremely low sensitivity like a reader; rather, test accuracy and calibration are the most critical indicators. Simply put, an instrument is a tool for precise measurement while ensuring the transmission of measured values. The comparison is precision, unlike the measured label which focuses on sensitivity and read/write distance.

　　Test example

　　The author used Juxing Instruments' second-generation RFID comprehensive tester to test the sensitivity of two ultra-high frequency tags in a dark box environment. One of the tested tags is EPC C1G2, and the other is a national standard 800/900MHz tag. Each label is tested 10 times to obtain repeatability.

　　(a) The standard deviation of the EPCUHF sample < 0.04dBm

　　(b) Standard deviation of the national standard sample < 0.07dBm

　　Figure 2 Minimum Opening Power for Identification of Two Tags

　　Figure 2 shows the curve of the repeatability test. Where (a) is the recognition power of the EPCglobalC1G2 UHF sample label, and (b) is the recognition power of the national standard 800/900M label sample. It can be seen that in this sample set, the national standard tag has better sensitivity than the EPC tag, and we found that the national standard tag has greater randomness in whether it can be activated at critical power, so its standard deviation is slightly larger than that of the EPC sample label. In summary, this experiment demonstrated an instrument repeatability better than 0.1dB. Typically, low-end testers assemble using reader chips or similar technologies

　　The repeatability of the test equipment is far inferior to the performance of this instrument, which poses significant issues with measurement accuracy.

　　In terms of metrology calibration, the National Metrology Institute system already has RFID tester calibration methods and facilities, as well as equipment for antenna gain measurement. The author sent four RFID test antennas for inspection to assess their gain, and verified them by cross-firing them with the laboratory antennas, achieving very high consistency and repeatability.

　　Summary

　　Ultra-high frequency RFID tag testing is a high-precision, traceable test achieved through high-precision instruments and antennas, with metrology calibration guaranteed. The instrument responds to the test tag via air interface commands, testing the minimum incident power and tag reflection power required for tag recognition, reading, and writing at close range. Then, based on this minimum operating power, calculate the tag's equivalent monopole antenna reception power sensitivity and forward connection distance; Calculate the reverse connection distance based on power sensitivity and reflected power.

　　EPCglobal and ISO have different regulations regarding test conditions and measurement units. EPCglobal uses equivalent power and distance, ISO uses field strength and reflection radar cross-sectional area rate of change. The former is closer to the usage scenario, the latter closer to physical principles, but in reality, both are calculated results from the same physical quantity measurement, with no clear advantage or inferiority.

　　According to various standards and specifications, the test distance for tags is mostly within 1 meter, with transmission power ranging from 0 to 30 dBm and received signal power mostly above -60 dBm.

　　In terms of measuring instruments, high-precision instruments are fundamental. Accurate measurement and calibration, including RF transceiver and antenna gain, are key to ensuring accuracy. Currently, high-end instruments can achieve measurement accuracy up to 0.3dB, while repeatability can be better than 0.1dB.