How Can the Directivity of Coupler in RFID System Be Improved
0 IntroductionRFID system has been more and more widely used in the world. It is known as a new technology that will develop rapidly in the 21st century. RFID system can be applied to multiple frequency bands. Different frequency bands have different characteristics. RFID system in UHF frequency band has fast reading speed and long recognition distance. It has developed rapidly in recent years. This paper will focus on the improvement of microstrip directional coupler design in RFID system in UHF band.In many RFID systems, some microwave multi port devices are placed between the reader antenna and the signal processing module to separate the output reader signal and the tag scattered signal, such as circulator, directional coupler and so on. The toroidal coupler has a large volume and needs ferrite material, so the manufacturing cost is high. However, the microstrip directional coupler is usually small and easy to process, so it has been widely used in these systems. Microstrip coupler is generally composed of a 1 / 4 wavelength microstrip coupling line, and two ports are added at both ends of two parallel conduction bands to form a four port network of directional coupler.However, because the transmission mode of microstrip line is not strict TEM wave and has a small amount of longitudinal field components, the phase velocity imbalance of odd and even mode transmission is caused, which directly leads to the reduction of the directivity of microstrip coupler. As shown in formula (1):In this formula, I = e, O. It can be seen from the above formula that the phase velocity of odd and even modes is different, which will not only affect the coupling performance and orientation performance of microstrip coupler, but also narrow the frequency band. At this point, the strip line is better than the microstrip line, because the filling medium around the strip coupling line is uniform, the phase velocity of odd and even modes is the same, and the transmission of TEM wave itself has advantages over the microstrip line, but the processing is more troublesome, and other errors will be introduced in the bonding.For the above reasons, the isolation of directional couplers on the market is only about - 30dB, and the directivity usually does not exceed 20dB. A new improved scheme introduced in this paper is to add a high impedance line at the coupling end, so that the coupling end does not match and there is a certain amount of reflection. The reflected energy is transmitted to the isolation end through the microstrip line, so as to offset part of the leakage energy at the isolation end, which greatly improves the directivity. In the following experiments, it can be seen that at the specified frequency point, the isolation can reach below - 50dB and the directivity can reach - 30dB.Theoretical analysis and Simulation of coupler modelThe model of microstrip directional coupler in ads is shown in Figure 1. It is a four port device with a coupling line in the middle. The four ports are respectively connected to the external 50 Ω port. From point 5 to point 7 is the high impedance line, and the 7 ends are grounded. This length is a variable. The microstrip line connecting each port (except 3 ports) is 2.25mm wide and 14.4mm long, and the microstrip line connecting 3 ports is 1.4mm wide and 5mm long. The length of the coupling line is 57.7mm, the width of the conduction band is 2.1mm, and the spacing of the conduction band is o.45mm. This paper mainly discusses the function of high impedance line, so first set the length of high impedance line to zero. The PCB is made of PTFE with a dielectric constant of 2.5 and a thickness of 0.5cm.Firstly, the directional coupler is analyzed by theoretical analysis method. Using the line calculation tool in ads, the characteristic impedance and electrical length of each line can be obtained. The characteristic impedance of the microstrip line connecting ports 1, 2 and 4 is 36.24 Ω and the electrical length is 22.6 °. The characteristic impedance of the microstrip line connecting ports 3 is 50 Ω and the electrical length is 8.2 °. The characteristic impedance of the coupling line is 37.5 Ω, the odd mode impedance is 33.72 Ω, the even mode impedance is 41.73 Ω, the coupling degree is -19.48db, and the electrical length is 92.4 °. Theoretically, if the length of the coupling line is 90 °, the maximum coupling energy and the maximum coupling terminal voltage can be seen from formula (2). In ads, the simulation results of such a directional coupler are shown in Figure 2.Where: C is the coupling degree; V3 is the output voltage of the coupling end; V0 is the input voltage of the coupler. Obviously, when the length is 90 °, Tan θ= 0, V3 = CV0, maximum signal at coupling end. The length of the coupler is 92.4 °, which basically conforms to the basic theory of the coupler, and the value of this coupling degree should be close to that of S31. It can be seen from the following analysis that this value is basically consistent with the results calculated by the method of moments. Another important parameter is S41, which is 0 in the ideal coupler theory, but it is obviously not 0 in practice. Because of the imbalance of odd and even modes, its performance may become worse or even very poor. In addition, the input S11 parameters are analyzed by the transmission line equivalent theory analysis method, but this method can only be a rough analysis, because the phase velocity of odd and even modes transmitted by microstrip lines is unbalanced, and the odd and even mode components are difficult to calculate. However, because the coupling degree is relatively low, it can be calculated by assuming that the coupling line from port 1 to port 2 is an independent lossless transmission line. The impedance of Ports 1 and 2 is 50 Ω. According to formula (3), zin1 = 44.25-j10.24 Ω, zin2 = 30.7 j7.71 Ω, zin3 = 37.12 j10.65 Ω. Here 1, 2 and 3 refer to the points marked on the graph. It can be calculated by formula (4) Γ= 0.13 j0.138, S11 = -14.4db. From this data, the impedance matching is not very good. The present theoretical analysis results are compared with the later calculation results of the moment method.Figure 2 (a) is a diagram of S31 and S41. At 900MHz, S31 is -19.116db and S41 is -25.589db. Fig. 2 (b) is a diagram of S11. Between 800MHz and 1GHz, S11 is between - 12dB and - 20dB. As can be seen from Figure 2, the performance of this coupler is not good. Firstly, S11 is only -15.39db at 900MHz. The directional coupler is a straight through device. Generally speaking, S11 must be below - 30dB, otherwise the insertion loss is too large, which will damage the system. In addition, the directivity is too low. At 900MHz, the isolation is -25.589db, the coupling is -19.116db, the directivity is only about 6dB, and the directivity does not exceed 8dB in the whole frequency band. This result is obviously in line with the results calculated above. S11 = -15.39db is close to -14.42db above, while S31 = -19.116db is also close to the theoretical value of maximum coupling power -19.48db.Such performance obviously does not meet the requirements. Because the power difference between the signal scattered by tag tag and the signal transmitted by reader is more than 40 50 dB. The directivity of the coupler is only 8dB, so it is difficult to separate tag signal and reader signal. At the output of tag signal, the amplitude of reader signal is much larger than that of tag signal. Especially at the output of the amplifier, the tag is superimposed on the continuous wave signal of the reader. It is likely that the amplifier is saturated before the tag signal is amplified enough to be detected, which is very harmful. Next, the size of the high impedance line of the directional coupler will be adjusted to make the coupler reach a better index.2 improvement method and effect of couplerIn this section, we mainly describe an improved scheme of coupler, that is, adding high impedance line method. As shown in Figure 1, the terminal grounding of high impedance line belongs to short route, and most of the energy will be reflected back. Theoretically, the reflected energy is used to offset the energy of the coupler at the isolation end (port4) to improve its isolation. In addition to the poor isolation of the coupler itself, the energy leaked by the 4 port also includes the coupling of the signal reflected from the 2 port on the 4 port in practical application. This reflected signal is mainly caused by the mismatch of the antenna. Here, only 2 ports are considered as the ideal matching load. In an ideal coupler, the signal leaked at the isolation end is 90 ° later than the signal at the coupling end, and the offset signal should be exactly 180 ° different from the signal at the isolation end. Since the offset signal is mainly reflected by the terminal of the high impedance line, in Figure 1, the electrical length from point 4 to point 7 should be about 90 °. In this way, when the reflected signal is transmitted to point 4, it will be reversed, and then transmitted to point 6, and the signal at the isolation end is exactly reversed. Adjusting the width of the high impedance line can control the power of the reflected signal; By adjusting its length, the phase of the reflected signal can be controlled. After adjustment, the length of the high impedance line is 53.7mm and the width is 0.4mm. This length plus the 5mm short microstrip line connecting three ports, the electrical length is close to 90 ° (91.2 °). The simulation results of S11, S31 and S41 are shown in Figure 3.It is obvious from Figure 3 that S11 has only a small change because the energy of the coupling line coupled to ports 3 and 4 is relatively small and has little impact on the input reflection coefficient. In the improved model, the parameters other than the high impedance line are not changed. Both S31 and S41 change, especially S41 changes obviously from - 25dB to - 51dB, while S31 also changes from - 19dB to - 21db. The change of S31 is mainly due to the addition of high impedance lines, the matching condition of 3 ports changes, and the reflection increases. Therefore, the energy of 3 ports decreases slightly. S41 decreased significantly to nearly - 51dB, resulting in directivity of more than 30dB, which is due to reflection cancellation of high impedance line. This directivity has been very high, exceeding the index of most directional couplers in the market. Such directional couplers are very useful in the application of RFID system. It is worth noting that although such a high-performance coupler is applied, the reader signal is still much larger than the tag signal, but the system resolution is increased, and the tag scattering signal with smaller power can be recognized. If the amplitude difference between the two signals is not very large, the tag scattering signal can be obtained under the condition of unsaturated amplifier.However, it can also be seen from Figure 3 that the disadvantage of the coupler is that the bandwidth of high directivity is very narrow, and 20dB is only about 20MHz, because the performance of the coupler itself is poor. If it is a coupler with good performance and adjusted by high impedance line, a satisfactory frequency characteristic can be obtained. The lowest 903mhz in the middle can show such high directivity, obviously because at this frequency, the leakage signal at the isolation end is just opposite to the reflection cancellation signal.3 conclusionIn RFID system, multi port networks such as coupler and circulator are very important components, which are mainly used to separate reader and tag signals. However, the general directional coupler in the market can only reach the directivity of 20dB at most, which is very limited. When applied to RFID system, the ability of separating tag signal is relatively weak, or it can be separated from the channel only when the tag signal is relatively strong. Therefore, its structure needs to be improved.Theoretically, the signal strength of the directional coupler at the isolation end is 0, but in practice, due to the imbalance of the phase velocity of odd and even modes, the components of odd and even modes often change in the process of transmission, and the signal at the isolation end is not 0, or even very large. In the model proposed in this paper, the signal strength leaked at the isolation end is very large, only about 6dB less than that at the coupling end. In order to improve the directivity, the method of adding high impedance line is proposed. This method uses the reflected signal at the terminal of high impedance line to offset the leakage signal at the isolation terminal.An important conclusion of the high impedance line is that the electrical length from the end to the coupling end is about 90 °. According to the microstrip coupler theory, in order to achieve the best coupling effect, the length of the coupling end and the isolation end is about 90 °, and the signal phase difference is 90 °. The reflected signal shall be 180 ° different from the signal at the isolation end. The signal reflected back to the coupling end at the high impedance line terminal must be reversed from the original signal at the coupling end, so that it can be isolated after 90 ° transmission
