Design of six-port technology based on power divider coupler

The six-port technology was originally used in microwave and millimeter wave measurements and network analyzers. Therefore, under the guidance of the best conditions and empirical criteria, microwave components such as directional couplers and power dividers can be used to form different six-port structures. The most common form is Engen's quasi-ideal six-port circuit consisting of one power divider and three directional couplers.

1 Design Purpose

The six-port measurement technology is based on the principle of vector analysis and uses amplitude measurement instead of phase measurement to measure the amplitude and phase of the complex reflection coefficient. The six-port measurement technology can accurately measure the phase of the reflection coefficient. It is applied to the receiver, and the received signal is considered to be the reflected wave of the transmitted signal. The phase relationship between the received signal and the transmitted signal is measured, and the echo or communication information contained in the received signal is obtained.

This topic originated from a horizontal cooperation project. The main task is to complete the overall design of the RF front end of a phase measurement receiver. This microwave front end has both the advantages of a general direct conversion receiver and improves its disadvantages. First, its circuit structure is very simple, and there is no image interference, which avoids the use of expensive high-performance filters and image suppression mixers. Second, the isolation of the six ports is very good, which effectively suppresses the local oscillator leakage during direct frequency conversion. Third, the baseband signal output by the detection must be processed by a differential op amp circuit, which effectively reduces the interference of the DC bias caused by self-mixing.

2 Design ideas

This paper designs a six-port circuit consisting of a GP2X+ power divider from Mini and three QCS-722+ couplers from Mini, and uses a square-rate detector as the detection circuit. The principle is shown in Figure 1.

In this way, the high-frequency signal is converted into a baseband signal, digitized by the ADC, and then sent to the DSP signal processing unit. Through some digital calculations, the phase difference φ of the two signals can be calculated.

3 Related circuits

In this paper, a power divider and three couplers are designed to form a six-port circuit, and a square-rate detector is used as the detection circuit. Considering the symmetry of the six-port circuit and the accuracy of the test results, the shapes of the left and right input microstrip lines are different. With the help of HFSS software simulation, the physical lengths of the two input microstrip lines should be calculated when the electrical lengths are the same. Rogers RO4003 board is selected as the substrate, with a relative dielectric constant of 3.55 and a thickness of 0.508mm. As shown in Figure 2.

The power divider in this article uses the GP2X+ chip from mini company, which has wide bandwidth (2.8-7.2GHz), small size, excellent amplitude imbalance (0.1dB), and good phase imbalance (3°). The coupler also uses the QCS-722+ chip from mini company, with a frequency range of 4-7.2GHz, high power, and low imbalance (0.2dB and 2°), leading the industry in both size and bandwidth.

4 Actual pictures

After processing, we got the real object. Each port is connected with SMA connector, and the matching load is connected to the matching end. The vector network analyzer is used for testing. The real object is shown in Figure 3 and Figure 4.

5 Conclusion

The phase difference between port 5 and 3, and port 6 and 4 can be considered as a constant within a certain frequency range. Then the influence of the six-port on the signal phase can be compensated by digital calibration, which is a key point for improving accuracy and an important advantage of the six-port technology. According to formula (8), it can be judged that the six-port itself has little influence on the phase difference of the two input signals. Therefore, it is proved that this six-port junction can be used to measure the signal phase difference.