UHF radio frequency identification tag sensitivity test method and solution

UHF tags refer to passive radio frequency identification tags from 840M to 960MHz. The label of this band originated from the EPCglobal Class 1 Generation 2 standard. Among them, EPCglobal is the electronic product coding standards organization, and the second-generation RFID standard is often abbreviated as C1G2. This standard specifies a radio frequency identification protocol in the UHF 860M-960MHz range. The feature of this protocol is to realize fast tag reading and writing at a distance of tens of meters through a microsecond reader-tag response and a more scientific anti-collision mechanism. Ideally, it can count up to two to three hundred tags per second, and the reading distance can reach about 30 meters. It was once touted as the standard for the next generation of smart logistics. Afterwards, the ISO organization accepted this standard and converted it to the ISO 18000-6C standard. In recent years, my country has also developed and innovated in this technology, and introduced its own standard GB/T 29768, whose frequency is specified at 840-845MHz and 920M-925MHz, avoiding the nearby GSM service band.

Currently these protocols are collectively referred to as 800-900MHz UHF radio frequency identification. These protocols have inherited the characteristics of high-speed response, fast inventory, and long reading and writing distance. The performance of these popular agreement products has become the key to use. Among them, labels, in particular, are at the center of fierce competition. The unit price of RFID tags is relatively low, but the quantity is large, and the design and manufacturing requirements are higher. Due to the defects and instability of label design technology and production process, it must be checked by performance testing.

Since this label sensitivity test is a non-contact radio frequency measurement, there are various technical problems that need to be overcome. This article focuses on the theory and practice of the methods.

UHF radio frequency tag sensitivity test method

basic settings

UHF tag testing is often carried out in a microwave dark box or dark room, and can also be carried out in a semi-dark room and a field with less interference. However, due to the high frequency of UHF tags, the wavelength is only about 1/3 meters, and the requirements for the size of the darkroom are not too high, and the economy is relatively easy to bear. Regarding the physical settings of the tag test, there are two main methods: dual antennas and single antennas. For performance, EPCglobal and ISO have advocated the dual antenna method. This method uses a pair of left and right circularly polarized antennas, one transmitting and one receiving, to achieve transmission and reception isolation, so that the test system can use high-power transmission and high-sensitivity reception, so as to deal with poorer sensitivity tags. For convenience, a circulator is also used to combine the dual antennas into a single antenna configuration with transceiver duplex. Due to the antenna reflection characteristics, the overall system performance is lower than the dual antenna configuration.

UHF radio frequency identification tag sensitivity test method and solution
Figure 1 Schematic diagram of dual antenna tag test configuration

Representation unit

Tag sensitivity can usually be expressed in terms of power or field strength. EPCglobal is more practical and uses RIPTUT, which is the monopole radiated power received by the tag. In layman's terms, it is the power received by the ideal monopole antenna at the RF field strength at which the tag can just work. Its unit is dBm.

The ISO test is expressed by the field strength, which is the field strength that makes the tag work normally. Its unit is V/m.

The two test results look different, but they are actually calculated by the tester's transmit power.

EPCglobal label receiving monopole power calculation formula:

RIP=EIRP-PL formula 1

EIRP=P+GTx formula 2

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

UHF radio frequency identification tag sensitivity test method and solution
Among them, PRx is the received power, PTx is the generated power, Ae is the antenna 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 give several typical sample frequency points, the free space transmission loss at the typical test distance, the unit is dB.

UHF radio frequency identification tag sensitivity test method and solution
It should be noted that the above calculations are based on the far-field spherical wave model. Too close the sending and receiving distance will cause the calculation result to deviate. EPCglobal specifies a distance of 0.8-1 meters. ISO 18046-3 specifies the close test distance.

UHF radio frequency identification tag sensitivity test method and solution
Among them, R is the test distance, and L is the side length (diameter) of the transmitting antenna. Below we give the typical antenna size and ISO requirements for test distances at typical frequencies.

UHF radio frequency identification tag sensitivity test method and solution
Multiple test items

Forward connection distance

In the label sensitivity test, people often hear the question about the reading and writing distance of the label. The reading and writing distance is related to the sensitivity of the tag and the reflected power of the tag, but it is also related to the performance of the reader in practical applications. So in the test, it is assumed that the reader uses 35dBm power to transmit through the ideal monopole antenna, and the distance that can be read and written. So the question is, the reading and writing distance of UHF tags is very long, should it be equipped with a large radio frequency chamber? No. We measure the working power of the tag under the above far-field conditions and subtract the gain of the transmitting antenna to obtain the equivalent monopole radiation power EIRPTX. Then, based on the principle that the space transmission attenuation is proportional to the square of the distance, the read and write distance can be calculated:

UHF radio frequency identification tag sensitivity test method and solution
The forward link range (forward link range) is also called the reading distance, which depends on the field strength required to open the tag.

Reverse connection distance

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

UHF radio frequency identification tag sensitivity test method and solution
Among them, EIRPTx0 is the transmit equivalent monopole power required for the reverse connection sensitivity, which is defined as the forward connection sensitivity plus 2dB; PRx0 is the tag reflection power received under the EIRPTx0 transmission condition; GRx is the receiving antenna gain.

Sensitivity of different label working modes

The power that the tag needs to consume is different in the working modes of reading ID number, reading register information, and writing register information, that is, the sensitivity of these three working modes is different. This also has three test modes of recognition, reading, and writing sensitivity. The above-mentioned working power, field strength, forward and reverse reading distances all have indicators under these three working modes, and they are all different.

EIRP and ERP

In many standards, the equivalent monopole transmission power is more, but ERP is also useful. ERP refers to the equivalent dipole antenna transmit power in the State Grid Corporation standard released in 2013. The ideal dipole antenna gain is about 2.2, so the difference between the two is such a constant.

Examples of parameters

We assume that the transmit and receive antenna gains are both 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 power PTx, the tag receives power.

PTag=PTx+6-31.7+2=PTx-23.7

Formula 11

Assuming that the reflected power of the tag is 1/3 of the received power, about -5dB. Then the power received by the receiver of the tester is as follows:

PRx=PTag-5+2-31.7+6= PTag-28.7

Formula 12　　 calculates the power received by the chip and receiver corresponding to different transmit powers according to these two formulas:

UHF radio frequency identification tag sensitivity test method and solution
That is to say, in a more ideal situation, the reflected power of the label received by the test UHF label at a distance of 1 meter is about 62dB less than the transmit power. The current tag can reach a turn-on power of about -18dBm, so the tag signal power received by the tester is generally above -47.4dBm. In actual situations, due to the tag antenna design, the gain is less than 2 or the impedance matching brings attenuation, and the tag reflectance is smaller than -5dB. Taking these factors into account, assuming no more than 10dB impact, the received power is above -60dBm.

Therefore, the RFID tag sensitivity test does not require the test instrument to have extremely low sensitivity like a reader. Instead, test accuracy and measurement calibration are key indicators. To put it simply, the instrument is a measurement tool under the condition of ensuring the transmission of the value. It is more accurate than the sensitivity and read/write distance of the measured tag.

Test case

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


UHF radio frequency identification tag sensitivity test method and solution
       (a) EPCUHF sample standard deviation <0.04dBm

(b) National standard sample standard deviation <0.07dBm

Figure 2 Identification of the two types of tags open power

Figure 2 shows the curve of the repeatability test. Among them (a) is the identification power of the EPCglobalC1G2 UHF sample tag, (b) is the identification power of the national standard 800/900M tag sample. It can be seen that in this group of samples, the sensitivity of the national standard label is better than that of the EPC label, and we found that whether the national standard label can be activated under the critical power has greater randomness, so its standard deviation is slightly larger than the EPC sample label. In short, in this experiment, it was shown that the instrument repeatability was better than 0.1dB. And usually the low-end is assembled with reader chip or similar technology.

The repetition accuracy of the test equipment is far worse than the performance of this instrument, which brings a big problem to the measurement accuracy.

In terms of measurement and calibration, the National Institute of Metrology has already provided RFID tester calibration methods and facilities, as well as equipment for antenna gain measurement. The author sent 4 RFID test antennas for inspection, tested their gain, and verified by pairing with the laboratory antennas to achieve high consistency and repeatability.

to sum up

UHF radio frequency identification tag testing is a high-precision traceable test that is achieved through high-precision instruments and antennas under the guarantee of metrological calibration. The instrument responds to the tested tag through the air interface command, and tests the incident power required for tag identification, reading, and writing, and the reflected power of the tag at a relatively close distance. Then calculate the tag's equivalent monopole antenna receiving power sensitivity and forward connection distance based on this working power; calculate the reverse connection distance based on the power sensitivity and reflected power.

For test conditions and measurement units, EPCglobal and ISO have different regulations. EPCglobal uses equivalent power and distance, and ISO uses field strength and reflection radar cross-sectional area change rate. The former is closer to the usage scenario, and the latter is closer to the physical principle, but both are actually the calculation results of the same physical quantity measurement, and there is no difference between good and bad.

According to various standards and specifications, the tag test distance is mostly within 1 meter, the transmit power is 0-30dBm, and the received signal power is mostly above -60dBm.

In terms of measuring instruments, high-precision instruments are the foundation, and measurement and calibration, including the RF transceiver and antenna gain of the instrument, are accuracy guarantees. At present, the measurement accuracy of high-end instruments can reach 0.3dB and the repeatability can be better than 0.1dB.