Study On Directional Coupler With High Isolation

Reader as an important part of RFID system, its transceiver isolation performance is one of the key factors affecting the entire performance of RFID system. In the RFID system integrated with transceiver, improving transceiver isolation degree can reduce the same frequency interference between system signal and feedback signal dealt by the radio frequency tag. At present, the most common methods to improve transceiver isolation degree is to add directional coupler in front of the reader antenna feed network, however, the bandwidth of traditional directional coupler is generally narrow and with low in-band isolation degree, which unable to meet the requirements of modern broadband communications. In this article, we focus on the branch line of directional coupler and mainly investigate how to expand the bandwidth and improve the in-band isolation performance.The mainly works and innovations of this article are demonstrated in the following aspects:(1) By analyzing the several common structure of directional coupler, one common method that can expand the bandwidth and increase in-band isolation degree is summed up, that is through increasing coupling paths. Compared with the other methods, this method with simpler principle is easier to implement.(2) According to the analyzing method of odd-even mode, a dual-branch-line 3dB directional coupler model is designed, on the basis of which a triple-branch-line 3 dB directional coupler with coupling degree model is designed. The directional coupler with three branch lines can broaden the bandwidth to a large extent (168 MHZ) and improve the in-band isolation degree (maximum isolation degree is 72.2 dB).(3)The fabrication designed of triple-branch-line 3dB directional coupler with is done, whose mainly performance is actually measured by vector network analyzer. Furthermore, the measure results and simulation results are compared and analyzed respectively.(4) The miniaturization of directional coupler and several methods to realize the multi-frequency operation are innovatively proposed.